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In unit 2 we will learn about 
the physics of electricity and 
electronics. 
This includes circuits, Ohm’s 
law, resistance, electrical 
energy and power, 
electromagnetism and 
electronic components.
What is 
electricity?
Key words: electrons, conductors, 
insulators, charge, current 
By the end of this lesson you will be able 
to: 
State that electrons are free to move in a 
conductor 
Describe the electrical current in terms of 
movement of charges around a circuit 
Distinguish between conductors and insulators 
and give examples of each. 
Carry out calculations involving Q = It
Thomson’s Plum pudding model 
Rutherford Bohr model 
What is inside an atom? 
Rutherford model 
Quantum model of the nucleus 
Charge cloud model
The atom 
An atom is a fundamental unit of matter 
made up of 
protons (with a positive charge) 
neutrons (neutral – no charge) 
electrons (with a negative charge)
What is electricity? 
Everything is made of atoms which contain 
POSITIVE particles called PROTONS and 
NEGATIVE particles called ELECTRONS. 
Proton (+) 
Neutron 
Electron (-)
An atom will usually have the same number of 
positives and negatives 
This makes the atom NEUTRAL. 
Proton (+) 
Neutron 
Electron (-)
Electrical Charge 
Electric charge is given the symbol 
Q 
Electrons are the charge carriers 
that flow in an electrical circuit – 
from the negative to positive 
terminals.
Electrical Charge 
Charge is measured in 
Coulombs 
which is given the symbol 
C
Electrical Charge 
The charge on a proton is 
1.6 x 10-19C 
which is the same size as the charge on an 
electron.
What is electricity? 
Electrons have a negative charge 
(Q) measured in coulombs (C). 
Electrons move round a circuit from 
negative to positive (remember like 
charges repel, opposites attract) 
giving rise to an electric current.
Electricity
What is a conductor? 
Name some conductors and insulators 
What is an insulator? 
What makes them effective conductors / 
insulators?
Conductors & Insulators 
What makes something a good conductor? 
Good conductors allow electrons to move 
through them easily. Insulators do not 
allow electrons to move easily.
What is electricity? 
So electricity is… 
movement of charge round a 
circuit. 
We call this electric current.
Electricity
Charge, Current & Time 
Electric current is given the symbol 
I 
Electric current is the movement of 
negative charges (electrons) in a 
circuit
Charge, Current & Time 
Current is the amount of charge flowing 
per second and is given the unit 
Amps (A)
Charge, Current & Time 
If current is charge flowing per second then 
so a current of 1 A is 1 C of charge transferred 
in 1 s. 
I = Q 
t 
time in seconds (s) 
Current in Amps (A) 
Charge transferred 
in coulombs (C)
Charge, Current & Time 
This can be rearranged as 
or 
Q = It 
t = Q 
I
Key words: series, current, ammeter, voltmeter, 
battery, resistor, variable resistor, fuse, switch, lamp, 
voltage 
By the end of this lesson you will be able 
to: 
Draw circuit diagrams to show the correct positions of 
an ammeter in a series circuit. 
Draw and identify the circuit symbols for an 
ammeter, voltmeter, battery, resistor, variable 
resistor, fuse, switch and lamp. 
State that in a series circuit, the current is the same at 
all positions.
Different types of circuit 
There are different ways in which you can 
connect cells and components (such as 
lamps) to create a circuit: 
series 
parallel 
a mixture of both
Series Circuit 
A series circuit has only one electrical 
path. 
You can trace from one side of the 
battery to the other, through each 
component, without lifting your finger 
from the page.
Different types of circuit 
There are different ways in which you can 
connect cells and components (such as 
lamps) to create a circuit: 
series 
parallel 
a mixture of both
Series Circuit 
A series circuit has only one electrical 
path. 
You can trace from one side of the 
battery to the other, through each 
component, without lifting your finger 
from the page. 
Physics Animations – Series Circuits
Name that component 
Resistor Ammeter 
Fuse 
Battery 
On the back of p2 carefully draw each symbol and 
label – in pencil! 
Voltmeter Switch 
Lamp 
Cell 
Variable resistor
Build a series circuit 
On the worksheet you will find four 
building circuit activities. 
Follow the instructions carefully! 
Answer each question as you go. 
Make careful observations. 
Lesson 2 build a series circuit.pub
Build a series circuit 
Build a series circuit which contains a 
6V battery pack, three 3.5 V lamps in 
lamp holders, and a meter used for 
measuring current. 
What is the meter called? 
Where is it positioned in the circuit?
Activity 1
Activity 2 
Bulbs are much dimmer!
Activity 3 - Change your circuit… 
Move your ammeter to different positions 
in the series circuit. 
Make a note of the positions each time, 
and of the current at each position. 
What can you say about the current in a 
series circuit?
Successful Circuit Diagrams 
On your worksheet you have drawn a circuit 
diagram. 
To be successful at circuit diagrams: 
• use a ruler and pencil 
• draw components carefully 
• draw wires as straight lines (with corners as 
• right angles!) 
• make sure all components are correctly draw 
• and joined in the circuit.
Your circuit diagram… 
should look like this:
Notice in this circuit, current is the same at all points
Notice in this circuit, current is the same at all points
Series Circuits and Current 
We are measuring the current I in a series circuit. 
What have we observed? 
We find that the current is the same at 
all points. 
How can this be written mathematically? 
I1 = I2 = I3 = I4 and so on 
Virtual Int 2 Physics – Electricity & Electronics – Circuits – Series Circuits
Think… 
How could you make use of a series circuit 
to investigate which materials are 
conductors and which materials are 
insulators? 
Which components would you need? 
What would you observe?
…and learn 
components and names 
formulae and symbols 
what is a series circuit? 
current in series circuit 
drawing a series circuit diagram
What have I learned?
Key words: series, parallel, ammeter, current, 
By the end of this lesson you will be able 
to: 
Draw circuit diagrams to show the correct positions of an 
ammeter in a parallel circuit. 
Draw and identify the circuit symbols for an ammeter, and 
lamp. 
State that in a series circuit, the current is the same at 
all positions. 
State that in a parallel circuit, the sum of the current in 
the branches adds up to the current drawn from the 
supply.
Quick Quiz 
What is a series circuit? 
What is the symbol for current? 
What are the units of current? 
What is the relationship between current and 
time? 
What do we know about the current in a series 
circuit? 
How do we measure current? 
Draw the symbol for this. 
Describe how to measure current in a series 
circuit.
Build another circuit 
Build a series circuit which includes a 6V 
battery, a 6V lamp and an ammeter. 
Draw the circuit diagram for your circuit:
Build another circuit 
We will now take one of your series 
circuits, and “add it” to someone else’s. 
Another ammeter has been added. 
What do you notice about the readings on 
the ammeter?
Build another circuit 
We will now “add” another series circuit. 
What do you notice about the readings on 
the ammeter?
What sort of circuit is this? 
We have constructed a parallel circuit. 
What does the circuit diagram look like? 
Try drawing it on Crocodile Physics.
Draw the circuit diagram below
Parallel Circuit 
We have constructed a parallel circuit. 
This is a circuit with different branches. 
When it reaches a junction, the current 
can divide and take different branches.
Parallel Circuits and Current 
We are measuring the current I in a 
parallel circuit. 
What have we observed? 
We find that the current in each of the 
branches adds up to the total current. 
How can this be written mathematically? 
IT = I1 + I2 + I3 and so on
Electric Circuits 
How many ways can you make 
two light bulbs work?
A SIMPLE CIRCUIT 
CELL 
SWITCH 
LIGHT BULB 
Close the switch, what happens?
A SIMPLE CIRCUIT 
2A 2A 
2A 
2A 2A 2A
A Series Circuit 
1A 
1A 
What happens now? 
1A 
1A 
1A 1A 1A
A Parallel 
Circuit 
What happens now?
2A 
2A 
4A 22AA 4A 
2A 
2A 
4A 4A 
4A 
4A 
A Parallel 
Circuit
What have you learned today?
Key words: voltage, potential difference, 
voltmeter, series, parallel 
By the end of this lesson you will be able 
to: 
Draw and identify the circuit symbols for a 
voltmeter, battery, and lamp 
State that the voltage of a supply is a measure 
of the energy given to the charges in a circuit. 
Draw circuit diagrams to show the correct positions of a 
voltmeter in a circuit. 
State that the sum of potential differences across the 
components in series is equal to the voltage of the 
supply. 
State that the potential difference across components 
in parallel is the same for each component.
What is electricity? 
What is a voltage? 
What is a volt? 
Discussion 
Demonstration 
Voltage in series and parallel
What is the energy change which 
takes place in a battery? 
Chemical to Electrical
When a battery is in a circuit… 
The electrical energy is carried by the 
electrons that move round the circuit. 
It is converted into others forms of 
energy.
If there is a bulb in the circuit, it is 
converted from 
to 
http://www.members.shaw.ca/len92/current_animation.gif
The amount of electrical energy the 
electrons have at any point in a circuit is 
known as their “potential”. 
As they move the electrons transfer energy 
into other forms. 
This means at any two points the electron has 
different amounts of energy.
Electrons start with (for example) 6J of energy. They have “potential”. 
As they pass through 
the bulb, some of the 
energy is converted 
to light. 
Electrons which have 
passed through the 
bulb have less 
energy. Or less 
“potential”. 
There is a “potential” difference 
in the circuit
What has “potential difference” 
got to do with voltage? 
It is the same thing! 
The potential difference (p.d.), or voltage, 
of a battery is a measure of the electrical 
energy given to one coulomb of charge 
passing through the battery.
Potential Difference or Voltage (V) 
A 9 V battery will give how much energy 
to each coulomb of charge passing 
through the battery? 
9 J
Potential Difference or Voltage (V) 
A 1.5 V battery will give how much energy 
to each coulomb of charge passing 
through the battery? 
1.5 J
Potential Difference or Voltage (V) 
A battery with a p.d. of 6V will give how 
much energy to each coulomb of charge 
passing through the battery? 
6 J
Voltage or p.d. 
Voltage (or p.d.) is measured in 
volts 
and is given the symbol 
V
Summary of Units 
Quantity Symbol Units Symbol 
charge Q coulombs C 
time t seconds s 
current I amperes A 
voltage V volts V
How can we measure voltage? 
Voltage (or p.d.) can be measured using a 
voltmeter. 
V 
An ammeter is connected in the circuit 
but a voltmeter must be connected across 
the component.
You can’t measure voltage… 
in a circuit 
through a circuit 
through a component 
flowing
Build a series circuit 
Build a series circuit which contains a 
6V battery, two 6V lamps, and a meter 
used for measuring potential difference 
across each lamp. 
What is the meter called? 
Where is it positioned in the circuit?
Drawing a circuit diagram 
Now draw a circuit diagram of the series 
circuit which you built. 
Remember to use a ruler and pencil, draw 
components carefully, draw wires as 
straight lines (with corners as right 
angles!), and make sure all components are 
correctly draw and joined in the circuit.
Series Circuits and Voltage 
We are measuring the potential difference (V) in a series circuit. 
What have we observed? 
We find that the 
How can this be written mathematically?
Parallel Circuit 
Now use the same components to 
construct a parallel circuit. 
This is a circuit with different branches.
Parallel Circuits and Voltage 
We are measuring the potential 
differences in a parallel circuit. 
What have we observed? 
How can this be written mathematically?
Tasks & Homework 
Yellow Practice Questions: 2.10, 2.11 
Numerical Questions: p33-36 qu 5-14 
Complete for homework for Tuesday 27th 
November
What have you learned today?
Quick Quiz 
What have we learned?
What have you learned today?
Key words: electrical resistance, voltage, 
current, Ohm’s law, ohms, resistor, 
variable power supply 
By the end of this lesson you will be able to: 
State that V/I for a resistor remains 
approximately constant for different currents. 
State that an increase in resistance of a circuit 
leads to a decrease in the current in that 
circuit. 
draw the symbol or a variable power supply and 
resistor.
Key words: electrical resistance, voltage, 
current, Ohm’s law, ohms, resistor, 
variable power supply 
By the end of this lesson you will have 
practised: 
building a series circuit 
using an ammeter and a voltmeter to find 
current and voltage. 
graphing results
Resistors 
The symbol for a resistor is
Resistors 
Resistors oppose (or resist) the 
flow of electric current. They have a 
property called resistance (R) which 
is measured in ohms (Ω).
What is the relationship between 
current and voltage in a resistor? 
Current is measured using an ammeter. 
Voltage is measured using a voltmeter. 
Investigation: relationship between 
current and voltage in a resistor.
Relationship between current and 
voltage in a resistor 
I / Amps 
p.d. / 
Volts 
Straight line through 
the origin tells us that 
current is 
directly proportional to 
voltage 
The ratio V/I is constant 
and is equal to resistance 
in the circuit.
Relationship between current and 
voltage in a resistor 
 is approximately constant 
The  constant is resistance R 
R 
V 
I 
V 
= 
I
Relationship between current and 
voltage in a resistor 
R V =I 
Ohm’s Law 
V = IR
Resistors 
A 
What do you expect 
to happen to the current 
if you increase the value 
of the resistor in the 
circuit shown? 
lamp 
cell 
resistor Demonstration
Calculate 
For a voltage of 12V, calculate the 
current for a resistant of 
(i) 1 Ω 
(ii) 2 Ω 
(iii)4 Ω 
(iv)24 Ω 
(v)1 k Ω
What can you say about current and 
resistance for a fixed voltage? Complete 
the sentences. 
As resistance increases, the current 
As resistance decreases, the current
Varying Resistance 
The opposition to current or resistance 
of a material (measured in Ω) depends 
on several things. 
Think and discuss what some of these 
might be.
Varying Resistance 
The opposition to current or resistance of 
a material (measured in Ω) depends on 
- type of material (the better the conductor, 
the lower the resistance) 
- length of material (the longer the material, 
the higher the resistance) 
- thickness of material (the thinner the 
material, the higher the resistance) 
- temperature of material (the higher the 
temperature, the higher the resistance)
Varying Resistance 
The relationship between length of the 
material and resistance allows us to make 
a 
variable resistor (or rheostat).
Variable Resistor 
Incoming 
current 
Outgoing 
current 
A B 
Demonstration
Variable Resistors 
In the above diagram, if the 
slider is moved in the direction 
A→B the resistance will 
increase because the length of 
wire through which the current 
passes increases.
Uses of Variable Resistors? 
Variable resistors can be used 
• as volume or brightness controls on 
televisions 
• volume control on MP3 players 
• light dimmer switches.
Key words: resistance, series, parallel, 
ohms, ohmmeter 
By the end of this lesson you will be able 
to: 
State the relationships between total 
resistance and individual resistances in 
series and parallel circuits 
Carry out calculations involving the 
relationships between resistors in series 
and in parallel
Key words: resistance, series, parallel, 
ohms, ohmmeter 
By the end of this lesson you will have 
practised: 
building a series circuit 
building a parallel circuit 
drawing circuit diagrams 
using an ohmmeter to measure resistance 
in a circuit
Variation of Resistance and 
Current for a Lamp Filament 
Look at the circuit diagram below: 
Handout
Name each of the components 
Is this a series or parallel circuit? 
As the voltage across the lamp increases, what do 
you expect to happen to the current? 
Sketch a graph of your prediction of the 
relationship between current and voltage.
In the resistor, current and voltage are 
directly proportional. 
But in a filament lamp, heat is generated. 
We know that resistance increases as 
temperature increases. So we see that as 
voltage increases, temperature increases, 
resistance increases and current 
increases – but more slowly than we might 
predict.
Measuring Resistance 
We can find the resistance of a 
component by measuring 
voltage across the component using 
a voltmeter 
current through the component using 
an ammeter
Measuring Resistance 
or we can measure it directly using an 
ohmmeter 
Ω 
Demonstration & experiment
Series and Parallel Circuits 
Voltage, Current and Resistance 
Vs 
I3 
V1 V2 V3 
I2 
I1 
R3 
- + 
R2 R1 
What type of circuit is this?
Vs 
I3 
V1 V2 V3 
I2 
I1 
R3 
- + 
R2 R1 
One electrical path from negative 
to positive therefore series.
Vs 
I3 
V1 V2 V3 
I2 
I1 
R3 
- + 
R2 R1 
What is the relationship between the three currents? 
The current is the same at each point. 
1 2 3 I = I = I
Vs 
I3 
V1 V2 V3 
I2 
I1 
R3 
- + 
R2 R1 
What is the relationship between the four voltages? 
They add to equal the supply voltage. 
1 2 3 V V V V s = + +
Disadvantages of Series Circuits? 
When one component fails the whole circuit 
fails. 
The current is the same at all points and the 
voltage is divided between the bulbs. The 
more bulbs added the dimmer each one is.
Vs 
I3 
V1 V2 V3 
I2 
I1 
R3 
- + 
R2 R1 
How do you find total resistance in series? 
Add each resistance together. 
1 2 3 R R R R total = + +
Vs 
- + 
IT IT 
I1 
I2 
I3 
V1 
R1 
V2 
R2 
V3 
R3 
What typ e of circuit is this?
Vs 
- + 
IT IT 
I1 
I2 
I3 
V1 
R1 
V2 
R2 
V3 
R3 
More than one electrical path – components connected on different 
branches therefore parallel.
What is the 
relationship 
between the 
four 
currents? 
The four currents 
add to give the 
total current. 
Vs 
- + 
IT IT 
I1 
I2 
I3 
V1 
R1 
V2 
R2 
V3 
R3 
1 2 3 I I I I T = + +
What is the 
relationship 
between the 
four 
voltages? 
Each voltage is 
equal to the 
supply voltage. 
Vs 
- + 
IT IT 
I1 
I2 
I3 
V1 
R1 
V2 
R2 
V3 
R3 
1 2 3 V V V V S = = =
- + 
IT IT 
I1 
I2 
V1 
R1 
V2 
R2 
V3 
1 1 1 1 
= + + 
R R R RT 
1 2 3 
Vs 
I3 
R3 
The 
resistance 
in parallel?
If more resistors are connected in 
parallel the total resistance will 
always 
decrease 
This is because there are more 
branches through which the 
electricity can flow.
Advantages of the Parallel 
Circuit? 
When one bulb fails the rest of the circuit 
continues to work. 
The more components, the lower the 
resistance. The total current drawn 
increases. Voltage in each branch is the same as 
the supply voltage therefore bulbs in parallel 
will each be as bright as a single bulb.What have 
you learned today?
Handout 3 
Key words: resistor, resistance, series, 
potential, potential divider 
By the end of this lesson you will be able 
to: 
State that a potential divider circuit 
consists of a number of resistors, or a 
variable resistor, connected across a 
power supply. 
Carry out calculations involving potential 
differences and resistance in a potential 
divider.
Name each component. 
What type of circuit is this? 
V V
The supply voltage is 6V. What is voltage V1? V2? 
10Ω 10Ω 
V1 V2
The supply voltage is 10V. What is voltage V1? V2? 
10Ω 10Ω 
V1 V2
The supply voltage is 5V. What is voltage V1? V2? 
10Ω 10Ω 
V1 V2
The supply voltage is 6V. What is voltage V1? V2? 
5Ω 10Ω 
V1 V2
A series circuit with two resistor and a power 
supply is known as a potential divider. 
V1 V2 
Why is it 
called a 
potential 
divider?
The potential difference of the supply is 
divided between the two resistors. 
When the two resistors are identical (i.e. 
have the same value of resistance), the 
potential difference is split equally.
Investigating Potential Dividers
Potential Divider Circuits 
A voltage divider consists of two devices, 
usually resistors, connected in series. 
V1 
V2 
R1=100 Ω 
6V 
R2=100 Ω 
V1 
V2 
R1=4.5 kΩ 
6V 
R2=9 kΩ
The current in each resistor is calculated 
using Ohm’s Law: 
I = V 
R
What can we say about the current in a 
series circuit? 
It stays the same throughout the circuit. 
I2 = V 2 
R2 
I1 = V = 1 R1
In a voltage divider circuit 
I2 = 
V 2 
R2 
I1 = V = 1 R1
This can also be written 
V2 
R2 
V1 R1 
=
If the resistance of one resistor 
is increased, the voltage across this 
resistor will 
This means the other voltage must
Potential Dividers 
1 
2 
S 
V R 
V R 
V R 
V 
R 
= 
V R 
T 
S 
T 
S 
S 
V 
R 
V 
R R 
V 
R R 
2 
2 
1 
1 
1 2 
2 
1 2 
1 
= 
+ 
= 
+ 
= 
What do the symbols mean? 
V1 is the voltage across resistor R1 
V2 is the voltage across resistor R2 
VS is the supply voltage 
RT is the total resistance
Potential Dividers 
= 1 
Look again at the worksheet. 
2 
S 
V R 
V R 
V R 
V 
R 
= 
V R 
T 
S 
T 
S 
S 
V 
R 
V 
R R 
V 
R R 
2 
2 
1 
1 
1 2 
2 
1 2 
1 
= 
+ 
= 
+ 
Use the formula to calculate 
V1 and V2 for each circuit. 
The answers found using the 
formula match the values 
measured using the 
voltmeter.
Potentiometer 
The potentiometer is a special type of 
voltage divider. 
It is a variable resistor with a sliding 
contact.
What range of output is it possible to 
obtain from a potentiometer? 
Range of output voltages 0V to supply 
voltage.
Key words: electrical energy, power, 
voltage, current, resistance 
By the end of this lesson you will be able 
to: 
State that when there is an electrical current 
in a component there is an energy 
transformation and give some examples. 
State the relationship between energy and 
power. 
Carry out calculations using E = Pt 
State that in a lamp electrical energy is 
transformed into heat and light. 
State that the energy transformation in an 
electrical heater occurs in the resistance wire.
What is electricity? 
What is a voltage? 
What is a volt?
What is “potential difference” ? 
What is voltage? 
It is the same thing! 
The potential difference (p.d.), or voltage, 
of a battery is a measure of the electrical 
energy given to one coulomb of charge 
passing through the battery.
What is the energy change which 
takes place in a battery? 
Chemical to Electrical
When a battery is in a circuit… 
The electrical energy is carried by the 
electrons that move round the circuit. 
It is converted into others forms of 
energy.
If there is a bulb in the circuit, it is 
converted from 
to 
Virtual Int 2 Physics -> Electricity ->Electrical Energy & Power ->Energy Transformation in a Lamp
Filament lamps 
Filament of 
tungsten wire 
Glass 
How does it work?
Filament Lamp 
Tungsten (metal) filament becomes 
so hot it glows. 
Why isn’t oxygen used 
inside the bulb?
Filament lamps 
Electric current 
passes through the 
resistance wire which 
is made of tungsten. 
Electrical energy is 
changed into heat 
energy and the 
wire glows white hot. 
Filament lamps 
produce both heat and 
light.
In an electric fire, energy is converted 
from 
to
Resistance in a wire 
We have learned that when a 
voltage is applied across a 
lamp, the resistance 
increases. 
What happens to the 
temperature?
Resistance in a wire 
As current passes through a resistance wire, 
the wire gets hot. 
This is how electric fires and filament lights 
work. 
The filament becomes hot enough to glow and 
emit light. The bar of the electric fire is a 
length of wire which also glows when hot.
Electrical appliances change 
electrical energy into other 
forms. 
What are the energy changes taking place in 
these appliances?
Power and Energy 
Electrical energy has the symbol 
and is measured in
Power 
The power rating of an appliance or a 
component is defined as 
the amount of energy used by the 
component / appliance in one 
second
Power 
The power rating tells us the rate at 
which energy is transformed, that is the 
energy transformed each second.
Power 
For example, an appliance with a power 
rating of 250 W converts 250 Joules of 
electrical energy into another form each 
second.
Power 
How can this be written as a formula? 
Power in Watts (W) 
time in seconds (s) t E 
Energy in Joules (J) 
P = 
Demonstration / experiment
Investigating Energy and Power 
Connect the joule meter to the voltage supply and a ray box bulb to the 
joule meter. 
Set the supply voltage at 6V and switch on. You’ll see the counter on the 
joule meter increasing (note each time the counter increases by 1, this is 
100J of energy). 
Record the number of joules used in 50s and 100s. Calculate the number of 
joules used per second. 
Power is energy used per second, in watts. Write the formula: 
If the supply voltage was increased to 12V, what would you expect to 
happen? 
Increase supply voltage to 12V and repeat the experiment. 
Worksheet / experiment
Power and Energy 
Ray box bulb, 6V supply Ray box bulb, 12V supply 
Number of joules used in 50 s? Number of joules used in 50 
s? 
Number of joules used in 100 s? Number of joules used in 100 
s? 
Number of joules used each 
second? 
Number of joules used each 
second? 
Power (W) Power (W) 
Were your results as expected? 
1 watt is equivalent to the transfer of 1 joule per second.
Power & Energy Example 
If an electric fire uses 1.8 
MJ of energy in a time of 
10 minutes, calculate the 
power output of the fire.
Power & Energy Example 
P = ? 
E = 1.8 MJ = 1.8x106 J 
t=10 minutes = 600 s
Formula? 
t E P =
Power Ratings of Appliances 
Different appliances have different 
power ratings. 
What is meant by power?
Watt’s my power rating? 
500 W, 150 W, 1200 W, 
100 W, 3000 W, 300 W, 
800 W, 1500 W, 30 W, 60 W, 
11 W
Watt’s my power rating? 
60 W, 
11 W 
1200 W 
3000 W 
30 W 
150 W 
800 W 
1500 W 
500 W 
100 W 
300 W
What have you learned today?
Key words: electrical energy, power, 
voltage, current, resistance 
By the end of this lesson you will be able 
to: 
State that the electrical energy 
transformed each second = VI 
Carry out calculations using P=IV and E=Pt 
Explain the equivalence between VI, I2R 
and V2/R. 
Carry out calculations involving the 
relationships between power, current, 
voltage and resistance.
Watt’s my power rating? 
500 W, 150 W, 1200 W, 
100 W, 3000 W, 300 W, 
800 W, 1500 W, 30 W, 60 W, 
11 W
Watt’s my power rating? 
60 W, 
11 W 
1200 W 
3000 W 
30 W 
150 W 
800 W 
1500 W 
500 W 
100 W 
300 W
Current through Appliances 
Different appliances have different 
power ratings. 
P = IV 
For appliances which use the mains supply 
V =
Current through Appliances 
As power increases for a fixed voltage, 
what happens to the current? 
As power increases the current 
increases
Red flag 
indicates 
9V. 
Live Neutral
Even with 
the switch 
open and 
zero current 
the lamp is 
still at 9V. 
Live Neutral
This time, 
when the 
switch is 
open, the 
lamp is at 
0V and is 
safe to 
touch. 
Live Neutral
The red flags indicate that 
voltage at these points is 9V.
Closing the third switch results in a 
current greater than 1A, blowing the 
fuse.
Inserting a voltmeter across a bulb shows that 
the bulbs are at zero volts. 
If you touch them, you won’t receive an electric 
shock as they are isolated from the voltage 
supply.
The red flags indicate that 
voltage at these points is 9V. 
The fuse is now in the neutral 
wire.
Closing the third switch results in a current 
greater than 1A, blowing the fuse. 
The red flags show that at these points the 
voltage is still at 9V. 
If you touch this now, you’ll complete the circuit 
and receive an electric shock – you become the 
“neutral wire” and allow electricity to flow 
through you.
Why can a bird 
sit safely on this 
high voltage 
power line? 
What will 
happen if the 
bird spreads its 
wings and 
touches the 
pylon?
Electricity
Which fuse to use? 
How would you calculate which fuse is 
required for an appliance? 
An appliance operating from the mains 
supply has a supply voltage of 230V. 
The rating plate gives you information 
on the power of the appliance.
The formula which links voltage, power 
and current: 
P = VI
The general rule for fuses 
The fuse value needs to be just above the 
normal operating current 
If the appliance 
has a power rating 
of: 
Fuse value should 
be: 
Less than 700W 3A 
More than 700W 13A
Example 
What is the appropriate choice of fuse for a 
mains appliance with a power rating of 
330 W? 
V = 
V 
230 
P = 
W 
= ? 
I 
330 
P IV 
= 
I P 
V 
=
I 
330 
230 
I = 1 . 
44 
A 
= 
Example 
What is the appropriate choice of fuse for a 
mains appliance with a power rating of 
330 W? 
V = 
V 
230 
P = 
W 
= ? 
I 
330
Power Ratings of Appliances 
Which type of appliances tend to have the 
highest power ratings? 
Generally, appliances which produce heat.
Power Ratings of Appliances 
Which type of appliances draw the 
highest current? 
Generally, appliances which produce heat.
Power Ratings of Appliances 
Which type of appliances need the largest 
value of fuse? 
Generally, appliances which produce heat.
Examples of 
rating plates
What have you learned today?
Key words: electrical energy, power, 
voltage, current, resistance 
By the end of this lesson you will be able 
to: 
State that the electrical energy 
transformed each second = VI 
Carry out calculations using P=IV and E=Pt 
Explain the equivalence between VI, I2R 
and V2/R. 
Carry out calculations involving the 
relationships between power, current, 
voltage and resistance.
Investigating… 
power, voltage, current and 
resistance. 
What do you notice about 
IV I R V 
2 
R 
, 2 , 
Worksheet / experiment
Power can be calculated from the 
voltage across the appliance and the 
current flowing through it. Written as 
an equation: 
P = IV
Relationship between power, 
current, voltage and resistance 
Our experiments showed that 
IV I R V 
2 
R 
= 2 =
Relationship between power, 
current, voltage and resistance 
P IV I R V 
2 
R 
= = 2 =
Equations for Power 
P = VI and 
V = 
IR 
Substituting 
P IxRxI 
2 
= 
P = 
I R
Equations for Power 
P = VI V = 
IR 
I = 
V 
Substituting 
P = 
VxV 
P V 
R 
R 
R 
2 
and 
=
What have you learned today?
Key words: alternating current, direct 
current, mains supply, frequency 
By the end of this lesson you will be able 
to: 
Explain in terms of current the terms a.c. and d.c. 
State that the frequency of the mains supply is 50Hz. 
State that the quoted value of an alternating voltage is 
less than its peak value. 
State that a d.c. supply and an a.c. supply of the 
same quoted value will supply the same power to 
a given resistor.
Direct Current (d.c.) 
• The voltage drives a steady 
or direct current. 
• The electrons move in one 
direction. 
• The current (or voltage) 
does not change with time.
Direct Current
Alternating Current (AC) 
•An alternating 
current is continually 
changing direction 
•The alternating 
voltage and current 
has a distinctive 
waveform
Alternating Current 
Using the oscilloscope, we can measure 
the peak voltage of the a.c. supply. 
The declared, quoted or 
“effective”, voltage is always less 
than the peak voltage.
Calculating Declared Voltage 
The declared (or effective) voltage can be 
calculated from the peak voltage. 
The quoted voltage is ~ 0.7 x 
peak voltage. 
The declared voltage is the value of a.c. 
voltage which gives the same heating or 
lighting effect as d.c. voltage.
Mains Supply 
What is the frequency of the mains 
supply? 
50 Hz
Mains Supply 
What is meant by the frequency of the 
supply? 
Alternating current flows one way then 
the other. It is continually changing 
direction. The rate of the changing 
direction is called the frequency and it is 
measured in Hertz (Hz) which is the 
number of forward-backward cycles in 
one second.
Mains Supply 
Why does the current change 
direction? 
Voltage pushes the current. The voltage 
changes polarity causing the current to 
change direction.
Mains Supply 
What is the declared value of the mains 
supply voltage? 
230V 
What is meant by the voltage of the 
supply? 
The voltage of a power supply or battery 
is a measure of how much “push” it can 
provide and how much energy it can give 
to the electrical charge.
Measuring effective voltage / 
current in an a.c. circuit 
The effective voltage or current in an a.c. 
circuit can be measured using a.c. 
voltmeter or ammeter.
Measuring peak a.c. voltage using 
an oscilloscope 
1. Adjust the position so the trace is 
central on the screen. 
2. Adjust the volts/div so the trace fills 
the screen. 
3. Count the number of boxes from the 
axis to the peak. 
4. Multiply the number of boxes by the 
volts / div.
What have you learned today?
Key words: electromagnetism, induced 
voltage, field strength, turns. 
By the end of this lesson you will be able to: 
State that a magnetic field exists around 
a current carrying wire. 
Identify circumstances in which a voltage 
will be induced in a conductor. 
State the factors which affect the size 
of the induced voltage i.e. field strength, 
number of turns on a coil, relative 
movement.
Permanent Magnets 
A magnetic field is the region around a 
magnet in which a magnetic force can be 
detected.
Electricity
Magnetic Field Around a Current 
Carry Wire 
What happens when the direction of the 
current is reversed? 
The direction of the magnetic field is 
reversed.
Electromagnets 
When an electric current passes through a 
wire which is coiled around an iron core, the 
core becomes magnetised and an 
electromagnet is produced. 
When an a.c. current is used, the current 
changed direction and so the magnetic field 
changes direction. 
e-m demo
Electromagnets 
Strength of electromagnet with/without iron 
core? 
Effect of increasing current through the coil? 
Effect of increasing number of turns in the 
coil (while keeping current constant)?
How is an electromagnet 
constructed? 
A current through a wire can be used to 
create an electromagnet. 
http://micro.magnet.fsu.edu/electromag/java/compass/index.html
How is an electromagnet 
constructed? 
A conducting wire is wound round an iron core. 
When a current passes through the 
conductor there is a magnetic field around 
the conductor. By wrapping it round a soft 
iron core, the magnetic field is concentrated.
Electricity
How can the strength of an 
electromagnet be increased? 
By increasing the current through the 
coil. 
By increasing the number of turns on the 
coil of wire.
What are the advantages of an 
electromagnet over a permanent 
magnet? 
The electromagnet can be switched off. 
The magnetic field strength can be varied 
(how?) 
The electromagnet provides a much stronger 
magnet field for the same size than a 
permanent magnet.
Electromagnetic Induction 
What happens when a wire is moved in a 
magnetic field? 
A voltage is created – or induced. For this 
reason we call this electromagnetic 
induction.
Electromagnetic Induction 
http://micro.magnet.fsu.edu/electromag/java/faraday2/ 
What happens when a permanent magnet 
is moved towards or away from a coil of 
wire?
Electricity
What do we know so far? 
When a current passes through a coil of 
wire, there is a magnetic field around the 
wire. 
Changing direction of the current changes 
the direction of the magnetic field.
What do we know so far? 
When we move a wire in a magnetic field, 
voltage is induced. 
When we move a magnet in a coil of wire, 
a voltage is induced. 
What do we have in common? Changing 
magnetic field leading to electricity!
Electricity
FARADAY’S EXPERIMENT 1832 
I 
A B
FARADAY’S EXPERIMENT 1832 
I 
A current in B is 
only present when 
the current in A is 
changing. 
A B
I HAVE DISCOVERED 
ELECTROMAGNETIC INDUCTION
Now I understand! 
VOLTAGE IS ONLY 
INDUCED WHEN 
THERE IS RELATIVE 
MOTION BETWEEN A 
CONDUCTOR AND A 
MAGNETIC FIELD
S N
S N
STRONGER FIELD (B) 
S N
STRONGER FIELD (B) 
S N
S N 
FASTER
S N 
FASTER
THE INDUCED VOLTAGE IS 
DIRECTLY PROPORTIONAL TO 
THE RATE OF CHANGE OF 
MAGNETIC FIELD
What is observed when… 
the magnet is stationary next to the coil? 
Nothing! No voltage is induced. 
The magnet is moved in the opposite 
direction (towards the coil instead of 
away from it)? 
The voltage produced has opposite 
polarity.
What is observed when… 
the magnet is moved backwards and 
forwards? 
Voltage induced which has a changing 
polarity. 
What does this mean for the current? 
The current will change direction – it is 
a.c.!
Generating Electricity 
A voltage can be induced in a coil of wire 
if a magnet is moved towards (or away 
from the coil). 
This effect is known as induction. 
What does the induced voltage depend 
on?
Generating Electricity 
Induced voltage depends on: 
strength of the magnetic field (the stronger 
the greater the induced voltage) 
speed of movement (the faster the greater the 
induced voltage). 
number of turns in the coil (the more turns of 
wire on the coil the greater the induced 
voltage). 
Virtual Int 2 Physics – Electricity & Electronics – em induction
Generating Electricity 
To summarise: 
A voltage is induced across the ends of a wire 
coil is the coil experiences a changing magnetic 
field.
Electricity
I HAVE DISCOVERED 
ELECTROMAGNETIC INDUCTION
Now I understand! 
VOLTAGE IS ONLY 
INDUCED WHEN 
THERE IS RELATIVE 
MOTION BETWEEN A 
CONDUCTOR AND A 
MAGNETIC FIELD
THE INDUCED VOLTAGE IS 
DIRECTLY PROPORTIONAL TO 
THE RATE OF CHANGE OF 
MAGNETIC FIELD
Generating Electricity 
How do we “create” electricity?
A Simple Generator 
A current can be passed through a wire to 
result in movement (a motor!). 
Electrical energy was changed to kinetic 
energy.
A Simple Generator 
The motor can work “in reverse”. 
Kinetic energy can be used to create 
electricity in a dynamo or simple 
generator.
Transformers 
What is a transformer? 
Demonstration.
Transformers 
A transformer consists of two separate coils of 
wire wound on the same iron core. 
The first coil, the primary, is connected to an a.c. 
voltage supply. There is therefore a changing 
magnetic field around the core. 
This changing field induces a voltage across the 
other coil, the secondary. A current flows as a 
result of the induced voltage.
Transformer Terms 
We talk about 
Primary coil (the first one – connected to 
a.c. voltage) 
Secondary coil (the second one – voltage 
is induced) 
Number of turns – number of “loops” of 
wire in coil
Transformer Terms 
We talk about 
Np – the number of turns on the primary coil 
Ns – the number of turns on the secondary coil 
Vp – the voltage applied to the primary coil 
Vs – the voltage induced across secondary coil 
Ip – the current in the primary coil 
Is – the current in the secondary coil
PRIMARY Np Turns 
AC input VP Volts 
THE TRANSFORMER 
Laminated soft 
iron core 
SECONDARY NS Turns 
AC output VS Volts 
S 
P 
V S 
= 
P 
N 
N 
V
Equipment 
2 coils 
1 x a.c. voltmeter 
Four wires 
A variable power supply. 
Set your power supply to 2V. YOU 
MUST NOT EXCEED 2V as the 
primary voltage.
Measure the output voltage for a 2V input 
for each of the combinations of number 
of turns in the primary and secondary. 
Record your results in your table.
V 
Calculate and 
S 
V 
p 
N 
S 
N 
p 
and record your results in your table.
Investigating Transformers 
V 
s V P N s N P V p 
2V 125 125 
2V 125 500 
2V 125 625 
2V 500 125 
2V 500 500 
2V 500 625 
s 
V 
s 
N 
p 
N
Transformers 
V 
V N V s N s 
P P s V 
p 
N 
s 
N 
p 
2 V 125 2 V 125 1 1 
2 V 125 8 V 500 4 4 
2 V 125 10 V 625 5 5 
2 V 500 0.5 V 125 0.25 0.25 
2 V 500 2 V 500 1 1 
2 V 500 2.5 V 625 1.25 1.25
Transformers 
A step-up transformer is one in which the 
secondary voltage is greater than the primary. 
A step-up transformer has more turns on the 
secondary coil than the primary coil. 
Which of the transformers are step-up?
Transformers 
A step-down transformer is one in which the 
secondary voltage is less than the primary. 
A step-down transformer has fewer turns on 
the secondary coil than the primary coil. 
Which are step-down transformers?
What would happen if a d.c. supply 
was connected to a transformer? 
At the moment of switching on, there is a 
changing magnetic field which would induce 
a voltage in the secondary coil. The same at 
the moment of switching off. 
Once switched on, no changing magnetic 
field (since steady current) and therefore no 
induced voltage.
Step-Up Transformer 
What is a step up transformer? 
What can you say about the relationship 
between the number of turns in the secondary 
and primary? 
> NP s N 
and the voltage in the secondary and 
primary? 
P > V s V
Step-Down Transformer 
What is a step down transformer? 
What can you say about the relationship 
between the number of turns in the secondary 
and primary? 
< NP s N 
and the voltage in the secondary and 
primary? 
P < V s V
Energy Losses in Transformers 
For calculations, we often assume that 
the transformer is 100% efficient. 
however in reality they are about 95% 
efficient. 
What causes the energy losses?
Energy Losses in Transformers 
- Heating effect of current in coils (coils 
are long length of wire with resistance 
hence electrical energy changed to 
heat) 
- Iron core being magnetised and 
demagnetised 
- Transformer vibrating -> sound 
- Magnetic field “leakage”
Voltage and Current in Transformers 
Assuming an ideal transformer with no 
energy losses total energy input must 
equal total energy output. 
Since rate of energy input is power: 
power input = power output
Voltage and Current in 
Transformers 
Power is given as 
P = V I 
so 
p p S S V I =V I 
which can be rearranged as 
P 
S 
V s 
= 
P 
I 
I 
V
Voltage and Current in Transformers 
P 
S 
V s 
= 
P 
I 
I 
V 
In a step-up transformer, the 
voltage in the secondary is 
greater than the primary. 
What happens to the current?
Voltage and Current in Transformers 
P 
S 
V s 
= 
P 
I 
I 
V 
The current in the coils is in 
the reverse ratio to the 
voltage therefore as voltage 
increases, current decreases.
Voltage and Current in Transformers 
P 
S 
V s 
= 
P 
I 
I 
V 
In a step-down transformer, 
the voltage in the secondary is 
less than the primary. What 
happens to the current?
Voltage and Current in Transformers 
P 
S 
V s 
= 
P 
I 
I 
V 
The current in the coils is in 
the reverse ratio to the 
voltage therefore as voltage 
decreases, current increases.
Transformers 
I 
s 
p 
n 
V 
= p 
= 
s 
p 
s 
I 
V 
n 
np = number of turns on primary coil 
ns = number of turns on secondary coil 
Vp = voltage across primary coil 
Vs = voltage across secondary coil 
Ip = current in primary coil 
Is = current in secondary coil
Type of 
transformer 
Turns 
ratio? 
Effect on 
VOLTAGE? 
Effect on 
CURRENT? 
Step-up 
Step-down
What have you learned today?
Key words: electromagnetism, induced 
voltage, field strength, turns. 
By the end of this lesson you will be able to: 
State that high voltages are used in the 
transmission of electricity to reduce 
power loss. 
Carry out calculations involving power loss 
in transmission lines.
Transmitting Electrical Energy 
Transformers are used by the National 
Grid system through which electrical 
energy is transmitted. 
Demonstration
Electricity Transmission 
Electrical energy is transferred from the power station to 
the consumer via the National Grid. 
• Electricity is sent for many kilometres along transmission 
lines on pylons.
Transformers in Electrical 
Transmission 
What happens as current flows through 
the wires? 
The length of the wires means large 
resistance and hence heating in the wires.
Transformers in Electrical Transmission 
Energy is changed from electrical to heat 
resulting in large power losses in the wires. 
Relationship between power, current and 
resistance? 
P = I 2R
Transformers in Electrical 
Transmission 
At the power station, a step-up transformer 
is used to increase the voltage. 
Why?
Transformers in Electrical 
Transmission 
P 
S 
V s 
= 
P 
I 
I 
V 
As voltage stepped up, current stepped down 
by the same factor. And since P = I 2R 
by 
reducing current the power losses due to 
heating are reduced.
Transformers in Electrical Transmission 
This stepping up of the 
voltage and hence stepping 
down of the current makes 
the transfer much more 
efficient. The losses due 
to heating are reduced.
Transformers in Electrical 
Transmission 
At the consumer end, a 
step-down transformer 
reduces the voltage to 
230V, increasing the 
current.

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Electricity

  • 1. In unit 2 we will learn about the physics of electricity and electronics. This includes circuits, Ohm’s law, resistance, electrical energy and power, electromagnetism and electronic components.
  • 3. Key words: electrons, conductors, insulators, charge, current By the end of this lesson you will be able to: State that electrons are free to move in a conductor Describe the electrical current in terms of movement of charges around a circuit Distinguish between conductors and insulators and give examples of each. Carry out calculations involving Q = It
  • 4. Thomson’s Plum pudding model Rutherford Bohr model What is inside an atom? Rutherford model Quantum model of the nucleus Charge cloud model
  • 5. The atom An atom is a fundamental unit of matter made up of protons (with a positive charge) neutrons (neutral – no charge) electrons (with a negative charge)
  • 6. What is electricity? Everything is made of atoms which contain POSITIVE particles called PROTONS and NEGATIVE particles called ELECTRONS. Proton (+) Neutron Electron (-)
  • 7. An atom will usually have the same number of positives and negatives This makes the atom NEUTRAL. Proton (+) Neutron Electron (-)
  • 8. Electrical Charge Electric charge is given the symbol Q Electrons are the charge carriers that flow in an electrical circuit – from the negative to positive terminals.
  • 9. Electrical Charge Charge is measured in Coulombs which is given the symbol C
  • 10. Electrical Charge The charge on a proton is 1.6 x 10-19C which is the same size as the charge on an electron.
  • 11. What is electricity? Electrons have a negative charge (Q) measured in coulombs (C). Electrons move round a circuit from negative to positive (remember like charges repel, opposites attract) giving rise to an electric current.
  • 13. What is a conductor? Name some conductors and insulators What is an insulator? What makes them effective conductors / insulators?
  • 14. Conductors & Insulators What makes something a good conductor? Good conductors allow electrons to move through them easily. Insulators do not allow electrons to move easily.
  • 15. What is electricity? So electricity is… movement of charge round a circuit. We call this electric current.
  • 17. Charge, Current & Time Electric current is given the symbol I Electric current is the movement of negative charges (electrons) in a circuit
  • 18. Charge, Current & Time Current is the amount of charge flowing per second and is given the unit Amps (A)
  • 19. Charge, Current & Time If current is charge flowing per second then so a current of 1 A is 1 C of charge transferred in 1 s. I = Q t time in seconds (s) Current in Amps (A) Charge transferred in coulombs (C)
  • 20. Charge, Current & Time This can be rearranged as or Q = It t = Q I
  • 21. Key words: series, current, ammeter, voltmeter, battery, resistor, variable resistor, fuse, switch, lamp, voltage By the end of this lesson you will be able to: Draw circuit diagrams to show the correct positions of an ammeter in a series circuit. Draw and identify the circuit symbols for an ammeter, voltmeter, battery, resistor, variable resistor, fuse, switch and lamp. State that in a series circuit, the current is the same at all positions.
  • 22. Different types of circuit There are different ways in which you can connect cells and components (such as lamps) to create a circuit: series parallel a mixture of both
  • 23. Series Circuit A series circuit has only one electrical path. You can trace from one side of the battery to the other, through each component, without lifting your finger from the page.
  • 24. Different types of circuit There are different ways in which you can connect cells and components (such as lamps) to create a circuit: series parallel a mixture of both
  • 25. Series Circuit A series circuit has only one electrical path. You can trace from one side of the battery to the other, through each component, without lifting your finger from the page. Physics Animations – Series Circuits
  • 26. Name that component Resistor Ammeter Fuse Battery On the back of p2 carefully draw each symbol and label – in pencil! Voltmeter Switch Lamp Cell Variable resistor
  • 27. Build a series circuit On the worksheet you will find four building circuit activities. Follow the instructions carefully! Answer each question as you go. Make careful observations. Lesson 2 build a series circuit.pub
  • 28. Build a series circuit Build a series circuit which contains a 6V battery pack, three 3.5 V lamps in lamp holders, and a meter used for measuring current. What is the meter called? Where is it positioned in the circuit?
  • 30. Activity 2 Bulbs are much dimmer!
  • 31. Activity 3 - Change your circuit… Move your ammeter to different positions in the series circuit. Make a note of the positions each time, and of the current at each position. What can you say about the current in a series circuit?
  • 32. Successful Circuit Diagrams On your worksheet you have drawn a circuit diagram. To be successful at circuit diagrams: • use a ruler and pencil • draw components carefully • draw wires as straight lines (with corners as • right angles!) • make sure all components are correctly draw • and joined in the circuit.
  • 33. Your circuit diagram… should look like this:
  • 34. Notice in this circuit, current is the same at all points
  • 35. Notice in this circuit, current is the same at all points
  • 36. Series Circuits and Current We are measuring the current I in a series circuit. What have we observed? We find that the current is the same at all points. How can this be written mathematically? I1 = I2 = I3 = I4 and so on Virtual Int 2 Physics – Electricity & Electronics – Circuits – Series Circuits
  • 37. Think… How could you make use of a series circuit to investigate which materials are conductors and which materials are insulators? Which components would you need? What would you observe?
  • 38. …and learn components and names formulae and symbols what is a series circuit? current in series circuit drawing a series circuit diagram
  • 39. What have I learned?
  • 40. Key words: series, parallel, ammeter, current, By the end of this lesson you will be able to: Draw circuit diagrams to show the correct positions of an ammeter in a parallel circuit. Draw and identify the circuit symbols for an ammeter, and lamp. State that in a series circuit, the current is the same at all positions. State that in a parallel circuit, the sum of the current in the branches adds up to the current drawn from the supply.
  • 41. Quick Quiz What is a series circuit? What is the symbol for current? What are the units of current? What is the relationship between current and time? What do we know about the current in a series circuit? How do we measure current? Draw the symbol for this. Describe how to measure current in a series circuit.
  • 42. Build another circuit Build a series circuit which includes a 6V battery, a 6V lamp and an ammeter. Draw the circuit diagram for your circuit:
  • 43. Build another circuit We will now take one of your series circuits, and “add it” to someone else’s. Another ammeter has been added. What do you notice about the readings on the ammeter?
  • 44. Build another circuit We will now “add” another series circuit. What do you notice about the readings on the ammeter?
  • 45. What sort of circuit is this? We have constructed a parallel circuit. What does the circuit diagram look like? Try drawing it on Crocodile Physics.
  • 46. Draw the circuit diagram below
  • 47. Parallel Circuit We have constructed a parallel circuit. This is a circuit with different branches. When it reaches a junction, the current can divide and take different branches.
  • 48. Parallel Circuits and Current We are measuring the current I in a parallel circuit. What have we observed? We find that the current in each of the branches adds up to the total current. How can this be written mathematically? IT = I1 + I2 + I3 and so on
  • 49. Electric Circuits How many ways can you make two light bulbs work?
  • 50. A SIMPLE CIRCUIT CELL SWITCH LIGHT BULB Close the switch, what happens?
  • 51. A SIMPLE CIRCUIT 2A 2A 2A 2A 2A 2A
  • 52. A Series Circuit 1A 1A What happens now? 1A 1A 1A 1A 1A
  • 53. A Parallel Circuit What happens now?
  • 54. 2A 2A 4A 22AA 4A 2A 2A 4A 4A 4A 4A A Parallel Circuit
  • 55. What have you learned today?
  • 56. Key words: voltage, potential difference, voltmeter, series, parallel By the end of this lesson you will be able to: Draw and identify the circuit symbols for a voltmeter, battery, and lamp State that the voltage of a supply is a measure of the energy given to the charges in a circuit. Draw circuit diagrams to show the correct positions of a voltmeter in a circuit. State that the sum of potential differences across the components in series is equal to the voltage of the supply. State that the potential difference across components in parallel is the same for each component.
  • 57. What is electricity? What is a voltage? What is a volt? Discussion Demonstration Voltage in series and parallel
  • 58. What is the energy change which takes place in a battery? Chemical to Electrical
  • 59. When a battery is in a circuit… The electrical energy is carried by the electrons that move round the circuit. It is converted into others forms of energy.
  • 60. If there is a bulb in the circuit, it is converted from to http://www.members.shaw.ca/len92/current_animation.gif
  • 61. The amount of electrical energy the electrons have at any point in a circuit is known as their “potential”. As they move the electrons transfer energy into other forms. This means at any two points the electron has different amounts of energy.
  • 62. Electrons start with (for example) 6J of energy. They have “potential”. As they pass through the bulb, some of the energy is converted to light. Electrons which have passed through the bulb have less energy. Or less “potential”. There is a “potential” difference in the circuit
  • 63. What has “potential difference” got to do with voltage? It is the same thing! The potential difference (p.d.), or voltage, of a battery is a measure of the electrical energy given to one coulomb of charge passing through the battery.
  • 64. Potential Difference or Voltage (V) A 9 V battery will give how much energy to each coulomb of charge passing through the battery? 9 J
  • 65. Potential Difference or Voltage (V) A 1.5 V battery will give how much energy to each coulomb of charge passing through the battery? 1.5 J
  • 66. Potential Difference or Voltage (V) A battery with a p.d. of 6V will give how much energy to each coulomb of charge passing through the battery? 6 J
  • 67. Voltage or p.d. Voltage (or p.d.) is measured in volts and is given the symbol V
  • 68. Summary of Units Quantity Symbol Units Symbol charge Q coulombs C time t seconds s current I amperes A voltage V volts V
  • 69. How can we measure voltage? Voltage (or p.d.) can be measured using a voltmeter. V An ammeter is connected in the circuit but a voltmeter must be connected across the component.
  • 70. You can’t measure voltage… in a circuit through a circuit through a component flowing
  • 71. Build a series circuit Build a series circuit which contains a 6V battery, two 6V lamps, and a meter used for measuring potential difference across each lamp. What is the meter called? Where is it positioned in the circuit?
  • 72. Drawing a circuit diagram Now draw a circuit diagram of the series circuit which you built. Remember to use a ruler and pencil, draw components carefully, draw wires as straight lines (with corners as right angles!), and make sure all components are correctly draw and joined in the circuit.
  • 73. Series Circuits and Voltage We are measuring the potential difference (V) in a series circuit. What have we observed? We find that the How can this be written mathematically?
  • 74. Parallel Circuit Now use the same components to construct a parallel circuit. This is a circuit with different branches.
  • 75. Parallel Circuits and Voltage We are measuring the potential differences in a parallel circuit. What have we observed? How can this be written mathematically?
  • 76. Tasks & Homework Yellow Practice Questions: 2.10, 2.11 Numerical Questions: p33-36 qu 5-14 Complete for homework for Tuesday 27th November
  • 77. What have you learned today?
  • 78. Quick Quiz What have we learned?
  • 79. What have you learned today?
  • 80. Key words: electrical resistance, voltage, current, Ohm’s law, ohms, resistor, variable power supply By the end of this lesson you will be able to: State that V/I for a resistor remains approximately constant for different currents. State that an increase in resistance of a circuit leads to a decrease in the current in that circuit. draw the symbol or a variable power supply and resistor.
  • 81. Key words: electrical resistance, voltage, current, Ohm’s law, ohms, resistor, variable power supply By the end of this lesson you will have practised: building a series circuit using an ammeter and a voltmeter to find current and voltage. graphing results
  • 82. Resistors The symbol for a resistor is
  • 83. Resistors Resistors oppose (or resist) the flow of electric current. They have a property called resistance (R) which is measured in ohms (Ω).
  • 84. What is the relationship between current and voltage in a resistor? Current is measured using an ammeter. Voltage is measured using a voltmeter. Investigation: relationship between current and voltage in a resistor.
  • 85. Relationship between current and voltage in a resistor I / Amps p.d. / Volts Straight line through the origin tells us that current is directly proportional to voltage The ratio V/I is constant and is equal to resistance in the circuit.
  • 86. Relationship between current and voltage in a resistor is approximately constant The constant is resistance R R V I V = I
  • 87. Relationship between current and voltage in a resistor R V =I Ohm’s Law V = IR
  • 88. Resistors A What do you expect to happen to the current if you increase the value of the resistor in the circuit shown? lamp cell resistor Demonstration
  • 89. Calculate For a voltage of 12V, calculate the current for a resistant of (i) 1 Ω (ii) 2 Ω (iii)4 Ω (iv)24 Ω (v)1 k Ω
  • 90. What can you say about current and resistance for a fixed voltage? Complete the sentences. As resistance increases, the current As resistance decreases, the current
  • 91. Varying Resistance The opposition to current or resistance of a material (measured in Ω) depends on several things. Think and discuss what some of these might be.
  • 92. Varying Resistance The opposition to current or resistance of a material (measured in Ω) depends on - type of material (the better the conductor, the lower the resistance) - length of material (the longer the material, the higher the resistance) - thickness of material (the thinner the material, the higher the resistance) - temperature of material (the higher the temperature, the higher the resistance)
  • 93. Varying Resistance The relationship between length of the material and resistance allows us to make a variable resistor (or rheostat).
  • 94. Variable Resistor Incoming current Outgoing current A B Demonstration
  • 95. Variable Resistors In the above diagram, if the slider is moved in the direction A→B the resistance will increase because the length of wire through which the current passes increases.
  • 96. Uses of Variable Resistors? Variable resistors can be used • as volume or brightness controls on televisions • volume control on MP3 players • light dimmer switches.
  • 97. Key words: resistance, series, parallel, ohms, ohmmeter By the end of this lesson you will be able to: State the relationships between total resistance and individual resistances in series and parallel circuits Carry out calculations involving the relationships between resistors in series and in parallel
  • 98. Key words: resistance, series, parallel, ohms, ohmmeter By the end of this lesson you will have practised: building a series circuit building a parallel circuit drawing circuit diagrams using an ohmmeter to measure resistance in a circuit
  • 99. Variation of Resistance and Current for a Lamp Filament Look at the circuit diagram below: Handout
  • 100. Name each of the components Is this a series or parallel circuit? As the voltage across the lamp increases, what do you expect to happen to the current? Sketch a graph of your prediction of the relationship between current and voltage.
  • 101. In the resistor, current and voltage are directly proportional. But in a filament lamp, heat is generated. We know that resistance increases as temperature increases. So we see that as voltage increases, temperature increases, resistance increases and current increases – but more slowly than we might predict.
  • 102. Measuring Resistance We can find the resistance of a component by measuring voltage across the component using a voltmeter current through the component using an ammeter
  • 103. Measuring Resistance or we can measure it directly using an ohmmeter Ω Demonstration & experiment
  • 104. Series and Parallel Circuits Voltage, Current and Resistance Vs I3 V1 V2 V3 I2 I1 R3 - + R2 R1 What type of circuit is this?
  • 105. Vs I3 V1 V2 V3 I2 I1 R3 - + R2 R1 One electrical path from negative to positive therefore series.
  • 106. Vs I3 V1 V2 V3 I2 I1 R3 - + R2 R1 What is the relationship between the three currents? The current is the same at each point. 1 2 3 I = I = I
  • 107. Vs I3 V1 V2 V3 I2 I1 R3 - + R2 R1 What is the relationship between the four voltages? They add to equal the supply voltage. 1 2 3 V V V V s = + +
  • 108. Disadvantages of Series Circuits? When one component fails the whole circuit fails. The current is the same at all points and the voltage is divided between the bulbs. The more bulbs added the dimmer each one is.
  • 109. Vs I3 V1 V2 V3 I2 I1 R3 - + R2 R1 How do you find total resistance in series? Add each resistance together. 1 2 3 R R R R total = + +
  • 110. Vs - + IT IT I1 I2 I3 V1 R1 V2 R2 V3 R3 What typ e of circuit is this?
  • 111. Vs - + IT IT I1 I2 I3 V1 R1 V2 R2 V3 R3 More than one electrical path – components connected on different branches therefore parallel.
  • 112. What is the relationship between the four currents? The four currents add to give the total current. Vs - + IT IT I1 I2 I3 V1 R1 V2 R2 V3 R3 1 2 3 I I I I T = + +
  • 113. What is the relationship between the four voltages? Each voltage is equal to the supply voltage. Vs - + IT IT I1 I2 I3 V1 R1 V2 R2 V3 R3 1 2 3 V V V V S = = =
  • 114. - + IT IT I1 I2 V1 R1 V2 R2 V3 1 1 1 1 = + + R R R RT 1 2 3 Vs I3 R3 The resistance in parallel?
  • 115. If more resistors are connected in parallel the total resistance will always decrease This is because there are more branches through which the electricity can flow.
  • 116. Advantages of the Parallel Circuit? When one bulb fails the rest of the circuit continues to work. The more components, the lower the resistance. The total current drawn increases. Voltage in each branch is the same as the supply voltage therefore bulbs in parallel will each be as bright as a single bulb.What have you learned today?
  • 117. Handout 3 Key words: resistor, resistance, series, potential, potential divider By the end of this lesson you will be able to: State that a potential divider circuit consists of a number of resistors, or a variable resistor, connected across a power supply. Carry out calculations involving potential differences and resistance in a potential divider.
  • 118. Name each component. What type of circuit is this? V V
  • 119. The supply voltage is 6V. What is voltage V1? V2? 10Ω 10Ω V1 V2
  • 120. The supply voltage is 10V. What is voltage V1? V2? 10Ω 10Ω V1 V2
  • 121. The supply voltage is 5V. What is voltage V1? V2? 10Ω 10Ω V1 V2
  • 122. The supply voltage is 6V. What is voltage V1? V2? 5Ω 10Ω V1 V2
  • 123. A series circuit with two resistor and a power supply is known as a potential divider. V1 V2 Why is it called a potential divider?
  • 124. The potential difference of the supply is divided between the two resistors. When the two resistors are identical (i.e. have the same value of resistance), the potential difference is split equally.
  • 126. Potential Divider Circuits A voltage divider consists of two devices, usually resistors, connected in series. V1 V2 R1=100 Ω 6V R2=100 Ω V1 V2 R1=4.5 kΩ 6V R2=9 kΩ
  • 127. The current in each resistor is calculated using Ohm’s Law: I = V R
  • 128. What can we say about the current in a series circuit? It stays the same throughout the circuit. I2 = V 2 R2 I1 = V = 1 R1
  • 129. In a voltage divider circuit I2 = V 2 R2 I1 = V = 1 R1
  • 130. This can also be written V2 R2 V1 R1 =
  • 131. If the resistance of one resistor is increased, the voltage across this resistor will This means the other voltage must
  • 132. Potential Dividers 1 2 S V R V R V R V R = V R T S T S S V R V R R V R R 2 2 1 1 1 2 2 1 2 1 = + = + = What do the symbols mean? V1 is the voltage across resistor R1 V2 is the voltage across resistor R2 VS is the supply voltage RT is the total resistance
  • 133. Potential Dividers = 1 Look again at the worksheet. 2 S V R V R V R V R = V R T S T S S V R V R R V R R 2 2 1 1 1 2 2 1 2 1 = + = + Use the formula to calculate V1 and V2 for each circuit. The answers found using the formula match the values measured using the voltmeter.
  • 134. Potentiometer The potentiometer is a special type of voltage divider. It is a variable resistor with a sliding contact.
  • 135. What range of output is it possible to obtain from a potentiometer? Range of output voltages 0V to supply voltage.
  • 136. Key words: electrical energy, power, voltage, current, resistance By the end of this lesson you will be able to: State that when there is an electrical current in a component there is an energy transformation and give some examples. State the relationship between energy and power. Carry out calculations using E = Pt State that in a lamp electrical energy is transformed into heat and light. State that the energy transformation in an electrical heater occurs in the resistance wire.
  • 137. What is electricity? What is a voltage? What is a volt?
  • 138. What is “potential difference” ? What is voltage? It is the same thing! The potential difference (p.d.), or voltage, of a battery is a measure of the electrical energy given to one coulomb of charge passing through the battery.
  • 139. What is the energy change which takes place in a battery? Chemical to Electrical
  • 140. When a battery is in a circuit… The electrical energy is carried by the electrons that move round the circuit. It is converted into others forms of energy.
  • 141. If there is a bulb in the circuit, it is converted from to Virtual Int 2 Physics -> Electricity ->Electrical Energy & Power ->Energy Transformation in a Lamp
  • 142. Filament lamps Filament of tungsten wire Glass How does it work?
  • 143. Filament Lamp Tungsten (metal) filament becomes so hot it glows. Why isn’t oxygen used inside the bulb?
  • 144. Filament lamps Electric current passes through the resistance wire which is made of tungsten. Electrical energy is changed into heat energy and the wire glows white hot. Filament lamps produce both heat and light.
  • 145. In an electric fire, energy is converted from to
  • 146. Resistance in a wire We have learned that when a voltage is applied across a lamp, the resistance increases. What happens to the temperature?
  • 147. Resistance in a wire As current passes through a resistance wire, the wire gets hot. This is how electric fires and filament lights work. The filament becomes hot enough to glow and emit light. The bar of the electric fire is a length of wire which also glows when hot.
  • 148. Electrical appliances change electrical energy into other forms. What are the energy changes taking place in these appliances?
  • 149. Power and Energy Electrical energy has the symbol and is measured in
  • 150. Power The power rating of an appliance or a component is defined as the amount of energy used by the component / appliance in one second
  • 151. Power The power rating tells us the rate at which energy is transformed, that is the energy transformed each second.
  • 152. Power For example, an appliance with a power rating of 250 W converts 250 Joules of electrical energy into another form each second.
  • 153. Power How can this be written as a formula? Power in Watts (W) time in seconds (s) t E Energy in Joules (J) P = Demonstration / experiment
  • 154. Investigating Energy and Power Connect the joule meter to the voltage supply and a ray box bulb to the joule meter. Set the supply voltage at 6V and switch on. You’ll see the counter on the joule meter increasing (note each time the counter increases by 1, this is 100J of energy). Record the number of joules used in 50s and 100s. Calculate the number of joules used per second. Power is energy used per second, in watts. Write the formula: If the supply voltage was increased to 12V, what would you expect to happen? Increase supply voltage to 12V and repeat the experiment. Worksheet / experiment
  • 155. Power and Energy Ray box bulb, 6V supply Ray box bulb, 12V supply Number of joules used in 50 s? Number of joules used in 50 s? Number of joules used in 100 s? Number of joules used in 100 s? Number of joules used each second? Number of joules used each second? Power (W) Power (W) Were your results as expected? 1 watt is equivalent to the transfer of 1 joule per second.
  • 156. Power & Energy Example If an electric fire uses 1.8 MJ of energy in a time of 10 minutes, calculate the power output of the fire.
  • 157. Power & Energy Example P = ? E = 1.8 MJ = 1.8x106 J t=10 minutes = 600 s
  • 158. Formula? t E P =
  • 159. Power Ratings of Appliances Different appliances have different power ratings. What is meant by power?
  • 160. Watt’s my power rating? 500 W, 150 W, 1200 W, 100 W, 3000 W, 300 W, 800 W, 1500 W, 30 W, 60 W, 11 W
  • 161. Watt’s my power rating? 60 W, 11 W 1200 W 3000 W 30 W 150 W 800 W 1500 W 500 W 100 W 300 W
  • 162. What have you learned today?
  • 163. Key words: electrical energy, power, voltage, current, resistance By the end of this lesson you will be able to: State that the electrical energy transformed each second = VI Carry out calculations using P=IV and E=Pt Explain the equivalence between VI, I2R and V2/R. Carry out calculations involving the relationships between power, current, voltage and resistance.
  • 164. Watt’s my power rating? 500 W, 150 W, 1200 W, 100 W, 3000 W, 300 W, 800 W, 1500 W, 30 W, 60 W, 11 W
  • 165. Watt’s my power rating? 60 W, 11 W 1200 W 3000 W 30 W 150 W 800 W 1500 W 500 W 100 W 300 W
  • 166. Current through Appliances Different appliances have different power ratings. P = IV For appliances which use the mains supply V =
  • 167. Current through Appliances As power increases for a fixed voltage, what happens to the current? As power increases the current increases
  • 168. Red flag indicates 9V. Live Neutral
  • 169. Even with the switch open and zero current the lamp is still at 9V. Live Neutral
  • 170. This time, when the switch is open, the lamp is at 0V and is safe to touch. Live Neutral
  • 171. The red flags indicate that voltage at these points is 9V.
  • 172. Closing the third switch results in a current greater than 1A, blowing the fuse.
  • 173. Inserting a voltmeter across a bulb shows that the bulbs are at zero volts. If you touch them, you won’t receive an electric shock as they are isolated from the voltage supply.
  • 174. The red flags indicate that voltage at these points is 9V. The fuse is now in the neutral wire.
  • 175. Closing the third switch results in a current greater than 1A, blowing the fuse. The red flags show that at these points the voltage is still at 9V. If you touch this now, you’ll complete the circuit and receive an electric shock – you become the “neutral wire” and allow electricity to flow through you.
  • 176. Why can a bird sit safely on this high voltage power line? What will happen if the bird spreads its wings and touches the pylon?
  • 178. Which fuse to use? How would you calculate which fuse is required for an appliance? An appliance operating from the mains supply has a supply voltage of 230V. The rating plate gives you information on the power of the appliance.
  • 179. The formula which links voltage, power and current: P = VI
  • 180. The general rule for fuses The fuse value needs to be just above the normal operating current If the appliance has a power rating of: Fuse value should be: Less than 700W 3A More than 700W 13A
  • 181. Example What is the appropriate choice of fuse for a mains appliance with a power rating of 330 W? V = V 230 P = W = ? I 330 P IV = I P V =
  • 182. I 330 230 I = 1 . 44 A = Example What is the appropriate choice of fuse for a mains appliance with a power rating of 330 W? V = V 230 P = W = ? I 330
  • 183. Power Ratings of Appliances Which type of appliances tend to have the highest power ratings? Generally, appliances which produce heat.
  • 184. Power Ratings of Appliances Which type of appliances draw the highest current? Generally, appliances which produce heat.
  • 185. Power Ratings of Appliances Which type of appliances need the largest value of fuse? Generally, appliances which produce heat.
  • 187. What have you learned today?
  • 188. Key words: electrical energy, power, voltage, current, resistance By the end of this lesson you will be able to: State that the electrical energy transformed each second = VI Carry out calculations using P=IV and E=Pt Explain the equivalence between VI, I2R and V2/R. Carry out calculations involving the relationships between power, current, voltage and resistance.
  • 189. Investigating… power, voltage, current and resistance. What do you notice about IV I R V 2 R , 2 , Worksheet / experiment
  • 190. Power can be calculated from the voltage across the appliance and the current flowing through it. Written as an equation: P = IV
  • 191. Relationship between power, current, voltage and resistance Our experiments showed that IV I R V 2 R = 2 =
  • 192. Relationship between power, current, voltage and resistance P IV I R V 2 R = = 2 =
  • 193. Equations for Power P = VI and V = IR Substituting P IxRxI 2 = P = I R
  • 194. Equations for Power P = VI V = IR I = V Substituting P = VxV P V R R R 2 and =
  • 195. What have you learned today?
  • 196. Key words: alternating current, direct current, mains supply, frequency By the end of this lesson you will be able to: Explain in terms of current the terms a.c. and d.c. State that the frequency of the mains supply is 50Hz. State that the quoted value of an alternating voltage is less than its peak value. State that a d.c. supply and an a.c. supply of the same quoted value will supply the same power to a given resistor.
  • 197. Direct Current (d.c.) • The voltage drives a steady or direct current. • The electrons move in one direction. • The current (or voltage) does not change with time.
  • 199. Alternating Current (AC) •An alternating current is continually changing direction •The alternating voltage and current has a distinctive waveform
  • 200. Alternating Current Using the oscilloscope, we can measure the peak voltage of the a.c. supply. The declared, quoted or “effective”, voltage is always less than the peak voltage.
  • 201. Calculating Declared Voltage The declared (or effective) voltage can be calculated from the peak voltage. The quoted voltage is ~ 0.7 x peak voltage. The declared voltage is the value of a.c. voltage which gives the same heating or lighting effect as d.c. voltage.
  • 202. Mains Supply What is the frequency of the mains supply? 50 Hz
  • 203. Mains Supply What is meant by the frequency of the supply? Alternating current flows one way then the other. It is continually changing direction. The rate of the changing direction is called the frequency and it is measured in Hertz (Hz) which is the number of forward-backward cycles in one second.
  • 204. Mains Supply Why does the current change direction? Voltage pushes the current. The voltage changes polarity causing the current to change direction.
  • 205. Mains Supply What is the declared value of the mains supply voltage? 230V What is meant by the voltage of the supply? The voltage of a power supply or battery is a measure of how much “push” it can provide and how much energy it can give to the electrical charge.
  • 206. Measuring effective voltage / current in an a.c. circuit The effective voltage or current in an a.c. circuit can be measured using a.c. voltmeter or ammeter.
  • 207. Measuring peak a.c. voltage using an oscilloscope 1. Adjust the position so the trace is central on the screen. 2. Adjust the volts/div so the trace fills the screen. 3. Count the number of boxes from the axis to the peak. 4. Multiply the number of boxes by the volts / div.
  • 208. What have you learned today?
  • 209. Key words: electromagnetism, induced voltage, field strength, turns. By the end of this lesson you will be able to: State that a magnetic field exists around a current carrying wire. Identify circumstances in which a voltage will be induced in a conductor. State the factors which affect the size of the induced voltage i.e. field strength, number of turns on a coil, relative movement.
  • 210. Permanent Magnets A magnetic field is the region around a magnet in which a magnetic force can be detected.
  • 212. Magnetic Field Around a Current Carry Wire What happens when the direction of the current is reversed? The direction of the magnetic field is reversed.
  • 213. Electromagnets When an electric current passes through a wire which is coiled around an iron core, the core becomes magnetised and an electromagnet is produced. When an a.c. current is used, the current changed direction and so the magnetic field changes direction. e-m demo
  • 214. Electromagnets Strength of electromagnet with/without iron core? Effect of increasing current through the coil? Effect of increasing number of turns in the coil (while keeping current constant)?
  • 215. How is an electromagnet constructed? A current through a wire can be used to create an electromagnet. http://micro.magnet.fsu.edu/electromag/java/compass/index.html
  • 216. How is an electromagnet constructed? A conducting wire is wound round an iron core. When a current passes through the conductor there is a magnetic field around the conductor. By wrapping it round a soft iron core, the magnetic field is concentrated.
  • 218. How can the strength of an electromagnet be increased? By increasing the current through the coil. By increasing the number of turns on the coil of wire.
  • 219. What are the advantages of an electromagnet over a permanent magnet? The electromagnet can be switched off. The magnetic field strength can be varied (how?) The electromagnet provides a much stronger magnet field for the same size than a permanent magnet.
  • 220. Electromagnetic Induction What happens when a wire is moved in a magnetic field? A voltage is created – or induced. For this reason we call this electromagnetic induction.
  • 221. Electromagnetic Induction http://micro.magnet.fsu.edu/electromag/java/faraday2/ What happens when a permanent magnet is moved towards or away from a coil of wire?
  • 223. What do we know so far? When a current passes through a coil of wire, there is a magnetic field around the wire. Changing direction of the current changes the direction of the magnetic field.
  • 224. What do we know so far? When we move a wire in a magnetic field, voltage is induced. When we move a magnet in a coil of wire, a voltage is induced. What do we have in common? Changing magnetic field leading to electricity!
  • 227. FARADAY’S EXPERIMENT 1832 I A current in B is only present when the current in A is changing. A B
  • 228. I HAVE DISCOVERED ELECTROMAGNETIC INDUCTION
  • 229. Now I understand! VOLTAGE IS ONLY INDUCED WHEN THERE IS RELATIVE MOTION BETWEEN A CONDUCTOR AND A MAGNETIC FIELD
  • 230. S N
  • 231. S N
  • 236. THE INDUCED VOLTAGE IS DIRECTLY PROPORTIONAL TO THE RATE OF CHANGE OF MAGNETIC FIELD
  • 237. What is observed when… the magnet is stationary next to the coil? Nothing! No voltage is induced. The magnet is moved in the opposite direction (towards the coil instead of away from it)? The voltage produced has opposite polarity.
  • 238. What is observed when… the magnet is moved backwards and forwards? Voltage induced which has a changing polarity. What does this mean for the current? The current will change direction – it is a.c.!
  • 239. Generating Electricity A voltage can be induced in a coil of wire if a magnet is moved towards (or away from the coil). This effect is known as induction. What does the induced voltage depend on?
  • 240. Generating Electricity Induced voltage depends on: strength of the magnetic field (the stronger the greater the induced voltage) speed of movement (the faster the greater the induced voltage). number of turns in the coil (the more turns of wire on the coil the greater the induced voltage). Virtual Int 2 Physics – Electricity & Electronics – em induction
  • 241. Generating Electricity To summarise: A voltage is induced across the ends of a wire coil is the coil experiences a changing magnetic field.
  • 243. I HAVE DISCOVERED ELECTROMAGNETIC INDUCTION
  • 244. Now I understand! VOLTAGE IS ONLY INDUCED WHEN THERE IS RELATIVE MOTION BETWEEN A CONDUCTOR AND A MAGNETIC FIELD
  • 245. THE INDUCED VOLTAGE IS DIRECTLY PROPORTIONAL TO THE RATE OF CHANGE OF MAGNETIC FIELD
  • 246. Generating Electricity How do we “create” electricity?
  • 247. A Simple Generator A current can be passed through a wire to result in movement (a motor!). Electrical energy was changed to kinetic energy.
  • 248. A Simple Generator The motor can work “in reverse”. Kinetic energy can be used to create electricity in a dynamo or simple generator.
  • 249. Transformers What is a transformer? Demonstration.
  • 250. Transformers A transformer consists of two separate coils of wire wound on the same iron core. The first coil, the primary, is connected to an a.c. voltage supply. There is therefore a changing magnetic field around the core. This changing field induces a voltage across the other coil, the secondary. A current flows as a result of the induced voltage.
  • 251. Transformer Terms We talk about Primary coil (the first one – connected to a.c. voltage) Secondary coil (the second one – voltage is induced) Number of turns – number of “loops” of wire in coil
  • 252. Transformer Terms We talk about Np – the number of turns on the primary coil Ns – the number of turns on the secondary coil Vp – the voltage applied to the primary coil Vs – the voltage induced across secondary coil Ip – the current in the primary coil Is – the current in the secondary coil
  • 253. PRIMARY Np Turns AC input VP Volts THE TRANSFORMER Laminated soft iron core SECONDARY NS Turns AC output VS Volts S P V S = P N N V
  • 254. Equipment 2 coils 1 x a.c. voltmeter Four wires A variable power supply. Set your power supply to 2V. YOU MUST NOT EXCEED 2V as the primary voltage.
  • 255. Measure the output voltage for a 2V input for each of the combinations of number of turns in the primary and secondary. Record your results in your table.
  • 256. V Calculate and S V p N S N p and record your results in your table.
  • 257. Investigating Transformers V s V P N s N P V p 2V 125 125 2V 125 500 2V 125 625 2V 500 125 2V 500 500 2V 500 625 s V s N p N
  • 258. Transformers V V N V s N s P P s V p N s N p 2 V 125 2 V 125 1 1 2 V 125 8 V 500 4 4 2 V 125 10 V 625 5 5 2 V 500 0.5 V 125 0.25 0.25 2 V 500 2 V 500 1 1 2 V 500 2.5 V 625 1.25 1.25
  • 259. Transformers A step-up transformer is one in which the secondary voltage is greater than the primary. A step-up transformer has more turns on the secondary coil than the primary coil. Which of the transformers are step-up?
  • 260. Transformers A step-down transformer is one in which the secondary voltage is less than the primary. A step-down transformer has fewer turns on the secondary coil than the primary coil. Which are step-down transformers?
  • 261. What would happen if a d.c. supply was connected to a transformer? At the moment of switching on, there is a changing magnetic field which would induce a voltage in the secondary coil. The same at the moment of switching off. Once switched on, no changing magnetic field (since steady current) and therefore no induced voltage.
  • 262. Step-Up Transformer What is a step up transformer? What can you say about the relationship between the number of turns in the secondary and primary? > NP s N and the voltage in the secondary and primary? P > V s V
  • 263. Step-Down Transformer What is a step down transformer? What can you say about the relationship between the number of turns in the secondary and primary? < NP s N and the voltage in the secondary and primary? P < V s V
  • 264. Energy Losses in Transformers For calculations, we often assume that the transformer is 100% efficient. however in reality they are about 95% efficient. What causes the energy losses?
  • 265. Energy Losses in Transformers - Heating effect of current in coils (coils are long length of wire with resistance hence electrical energy changed to heat) - Iron core being magnetised and demagnetised - Transformer vibrating -> sound - Magnetic field “leakage”
  • 266. Voltage and Current in Transformers Assuming an ideal transformer with no energy losses total energy input must equal total energy output. Since rate of energy input is power: power input = power output
  • 267. Voltage and Current in Transformers Power is given as P = V I so p p S S V I =V I which can be rearranged as P S V s = P I I V
  • 268. Voltage and Current in Transformers P S V s = P I I V In a step-up transformer, the voltage in the secondary is greater than the primary. What happens to the current?
  • 269. Voltage and Current in Transformers P S V s = P I I V The current in the coils is in the reverse ratio to the voltage therefore as voltage increases, current decreases.
  • 270. Voltage and Current in Transformers P S V s = P I I V In a step-down transformer, the voltage in the secondary is less than the primary. What happens to the current?
  • 271. Voltage and Current in Transformers P S V s = P I I V The current in the coils is in the reverse ratio to the voltage therefore as voltage decreases, current increases.
  • 272. Transformers I s p n V = p = s p s I V n np = number of turns on primary coil ns = number of turns on secondary coil Vp = voltage across primary coil Vs = voltage across secondary coil Ip = current in primary coil Is = current in secondary coil
  • 273. Type of transformer Turns ratio? Effect on VOLTAGE? Effect on CURRENT? Step-up Step-down
  • 274. What have you learned today?
  • 275. Key words: electromagnetism, induced voltage, field strength, turns. By the end of this lesson you will be able to: State that high voltages are used in the transmission of electricity to reduce power loss. Carry out calculations involving power loss in transmission lines.
  • 276. Transmitting Electrical Energy Transformers are used by the National Grid system through which electrical energy is transmitted. Demonstration
  • 277. Electricity Transmission Electrical energy is transferred from the power station to the consumer via the National Grid. • Electricity is sent for many kilometres along transmission lines on pylons.
  • 278. Transformers in Electrical Transmission What happens as current flows through the wires? The length of the wires means large resistance and hence heating in the wires.
  • 279. Transformers in Electrical Transmission Energy is changed from electrical to heat resulting in large power losses in the wires. Relationship between power, current and resistance? P = I 2R
  • 280. Transformers in Electrical Transmission At the power station, a step-up transformer is used to increase the voltage. Why?
  • 281. Transformers in Electrical Transmission P S V s = P I I V As voltage stepped up, current stepped down by the same factor. And since P = I 2R by reducing current the power losses due to heating are reduced.
  • 282. Transformers in Electrical Transmission This stepping up of the voltage and hence stepping down of the current makes the transfer much more efficient. The losses due to heating are reduced.
  • 283. Transformers in Electrical Transmission At the consumer end, a step-down transformer reduces the voltage to 230V, increasing the current.

Editor's Notes

  1. Print for lab books
  2. 21//11 and 22/11.
  3. Wednesday 28th / Thursday 29th
  4. Thursday 29th / Friday 30th
  5. Worksheet – review values for next year. This year completed using Croc Physics rather than practical – this seemed to work better than with practical values.
  6. Thursday 29th / Friday 30th
  7. Notes p22
  8. As a group – use tablet to write on power ratings – score off as go. What do you notice about those with the highest power rating?
  9. As a group – use tablet to write on power ratings – score off as go. What do you notice about those with the highest power rating?
  10. Thursday 29th / Friday 30th
  11. As a group – use tablet to write on power ratings – score off as go. What do you notice about those with the highest power rating?
  12. As a group – use tablet to write on power ratings – score off as go. What do you notice about those with the highest power rating?
  13. Thursday 29th / Friday 30th
  14. Thursday 29th / Friday 30th
  15. 18th December / 8th / 9th January
  16. Plotting magnetic field. Solenoid (on OHP)
  17. Coil without core (10V); coil with core (10V) (pick up paper clips) Magnetic knife Coils with different number of turns (balance to find mass of paperclips collected).
  18. Got to this point 9th Jan – continue on 10th
  19. 10TH January – pupils having difficulty with using this formula. Left with problem to solve overnight – pick up tomorrow 11th.
  20. 18th December / 8th / 9th January