Uss module 4 chpt 4 Electrcity

Module 4:
Gadgets Work Wonders (II)
Chapter 4 Electricity
1© Copyright Star Publishing Pte Ltd

4.1 How can electric circuit symbols be
used to draw circuit diagrams?
4.2 How are current, voltage and
resistance measured?
4.3 What are the differences between
series and parallel circuits?
4.4 What are the applications of series and
parallel circuits in daily life?
4.5 How do we use electricity safely?
4.6 How can information on electric power
be used?

Objectives
 Draw simple series and parallel
circuits for daily applications (e.g.
table lamps use series circuits, ceiling
lamps use parallel circuits)
 Label and interpret circuit diagrams
which include cells, switches, resistors
(fixed and variable),
 voltmeters, ammeters, bells, bulbs,
lamps and fuses

What are circuit diagrams?
 Circuit diagrams show how electrical
components are connected together.
 In circuit diagrams, electrical
components are drawn using symbols.
© Copyright Star Publishing Pte Ltd 4
a circuit diagram

Circuit symbols

Circuits and their circuit diagrams

Circuits and their circuit diagrams
Activity Book Link
Activity 4.1
Using Circuit Symbols

be used?

Objectives
 Measure current, voltage and
resistance using a multimeter
 State that a voltage is required to
cause a current flow
 State that the effective resistance
increases when in series circuits
 State that the effective resistance
decreases when in parallel circuit

Current
 In the circuit, the bulb
lights up because
there is an electric
current flowing in the
closed circuit.
 Conventionally,
current is described
as flowing from the
positive terminal of
a battery to the
negative terminal.
current flows in a closed
circuit

What is current?
 Electric current is due to tiny electric
charges flowing in the circuit.
 When the electric charges are flowing
at a fast rate, the current will be big.
 Therefore, current is the rate of flow
of electric charge.

Symbol and SI unit of current
 The symbol for current is I.
 The SI unit of current is the ampere (A).
 A current of 2 amperes is written as I = 2 A.
 A smaller unit of current is the milliampere (mA)
where:

How do we measure current?
 Current can be measured using an
ammeter connected in series.
current can be measured using an ammeter
circuit diagram

Measuring current
 Besides the conventional type of ammeter,
another instrument called the multimeter can
be used as an ammeter to measure current.
a multimeter

How to use a multimeter as an ammeter
 To use a multimeter as an ammeter,
(a) insert the black lead into the 'com'
terminal,
(b) insert the red lead into the 'A'
terminal,
(c) turn the knob to 'A'.
 The value of the current will be
displayed on the LCD panel.
using a multimeter as an ammeter
circuit diagram
Activity Book Link
Activity 4.2
Measuring Electric
Current Using a
Multimeter

What is Voltage?
 Voltage is needed for current to flow. The
dry cell provides the voltage, therefore
there is a current.

What is Potential Difference?
 When we measure voltage across two
points, we measure the energy
needed to move one unit of electric
charge between the two points.
 The voltage across these two points
is also known as potential difference.

SI unit of voltage
 The symbol for voltage is V.
 The SI unit of voltage is the volt (V).
 A voltage of 3 volts is written as V = 3 V.
 A smaller unit of voltage is the millivolt (mV)
where

How do we measure voltage?
 Voltage can be measured using a
voltmeter connected in parallel.
measuring voltage
across a bulb with a voltmeter
circuit diagram

How to use a multimeter as a voltmeter
 To use a multimeter as an ammeter:
(a) insert the black lead into the 'com' terminal,
(b) insert the red lead into the 'V.Ω' terminal,
(c) turn the knob to 'V'.
 The value of the voltage will be displayed on the LCD
panel.
 When a multimeter is used as a voltmeter, its symbol will be
measuring voltage
using a multimeter
circuit diagram
Activity Book Link
Activity 4.3
Measuring
Voltage Using a
Multimeter

What is Resistance?
 Resistance opposes the flow of
current.
 The higher the resistance, the
smaller the current.

SI unit of resistance
 The symbol of resistance is R.
 The SI unit of resistance is the ohm (Ω).
 A resistance of 5 ohms is written as R =
5 Ω.
 All electrical components have
resistance.

What are resistors?
 Resistors are electrical components that
provide resistance.
 There are two types of resistors:
(a) Fixed resistor:
The resistance of a fixed resistor is fixed
and cannot be changed.
(b) Variable resistor:
The resistance of a variable resistor can be
adjusted, usually by turning a knob or moving
a slider.

Types of resistors

Resistors in series
 When resistors are connected in series, the
effective resistance becomes bigger.
 For example, when two10 Ω resistors are
connected in series, the effective
resistance becomes 20 Ω.

Resistors in parallel
 When resistors are connected in parallel,
the effective resistance becomes smaller.
 For example, when two 10 Ω resistors are
connected in parallel, the effective
resistance becomes 5 Ω.

How to use a multimeter as an ohmmeter
 To use a multimeter as an ohmmeter:
(a) insert the black lead into the 'com' terminal,
(b) insert the red lead into the 'V.Ω' terminal,
(c) turn the knob to 'Ω'.
 The value of the resistance will be displayed on the LCD
panel.
measuring resistance of a bulb
using a multimeter
Activity Book Link
Activity 4.4
Measuring
Resistance Using
a Multimeter

4.4 What are the applications of series
and parallel circuits in daily life?
be used?

Objectives
 Identify situations in which series and
parallel circuits are used in daily
electrical circuitry systems

Identifying Series and Parallel Circuits
 We use electrical gadgets ( e.g. torch lights and calculators )
at home every day.
 These electrical gadgets may be connected as series or
parallel circuits.
(a) The diagram below shows the connection of the
batteries, switch and light bulb in a torchlight.
We know that this is a series circuit because
there is only one path for current to flow.

Identifying Series and Parallel
Circuits
 (b) The figure shows a simplified diagram of the
circuitry used in homes for lighting. We
know that this is a parallel circuit because
there is more than one path for current to
flow.

be used?

Objectives
 Identify the potential dangers in the
use of electricity at home, such as (i)
damaged insulation (ii) overheating of
cables (iii) poor or loose connections
 State precautions to be taken for safe
electrical use at home, such as (i) use
of fuses (ii) earthing metal casings (iii)
double insulating wires (iv) use of
circuit breakers

Electrical Hazards
 Electricity is very useful but if it is not used
properly and correctly, electrical hazards
(dangers) can happen.
 Two common electrical hazards are:
(a) electric shock/electrocution
(b) electrical fire
warning of electric shock

What is an electric shock?
 An electric shock happens when an
electric current flows through the
body of a person.
 Electrocution is death as a result of
electric shock.
 A current of about 0.1 A may be
enough to kill a person.

Causes of Electrical Hazards
 Electric shock and electrical fire can
be caused by:
(a) damaged insulation
(b) overheating of cables
(c) damp conditions
(d) poor or loose connections

Damaged Insulation
 To prevent people from touching
bare wires, a layer of insulation
is used to cover the wires.
 When the insulation is
damaged, the wires will be
exposed.
 A person touching the exposed
wires may get an electric shock
or be electrocuted.
 For safety, damaged insulation
should be replaced immediately
by a qualified electrician.
damaged insulation

Overheating of Cables
 When many appliances are plugged into a
single mains socket, the current flowing
out of the socket will become large. This may
lead to overheating of the cables which may
in turn lead to electrical fire.
overloaded power socket

Overheating of Cables
 Overheating can also occur if a cable is used to
carry a current larger than its current rating. The
current rating of a cable is the maximum current
that the cable is meant to carry, e.g. a cable with a
current rating of 12 A is meant to carry a current of
up to 12 A.
fire caused by overheating

Damp Conditions
 Water is a conductor of electricity. When a
person uses an electrical appliance with wet
hands, the water may connect him to high
voltage. When this happens, the person will get
an electric shock.
 Wet skin reduces the resistance of the human
body. If a person accidentally touches a high
voltage when his skin is wet, a large current will
flow through his body, causing severe injury or© Copyright Star Publishing Pte Ltd 41
touching a switch with wet hands can
cause electric shock

Poor or Loose Connections
 Poor or loose connections can also cause
overheating, especially when exposed wires
touch each other (short circuit).
 Sparks may also be produced at the points of
loose connections. Both overheating and
sparks can lead to electrical fires.
badly wired plug
Activity Book Link
Activity 4.7
Home Circuitry &
Safety
Precautions

Safe Use of Electricity
 There are a number of safety features
and devices used to protect us against
electrical hazards.
 Examples of such safety features and
devices are:
(a) fuse
(b) circuit breaker
(c) earth wire
(d) double insulating wire
fuses melt and break the
circuit when current exceeds
their rating

Fuse
 A fuse is a safety device which prevents an
overly large current from flowing.
 It is usually made up of a thin piece of copper
wire housed in a case.
 The rating of a fuse is the largest current that
can flow through it without melting the fuse
wire.
 When the current becomes larger than the
fuse rating, the fuse wire will melt and the
circuit will be opened. For example, a fuse rated
5 A will melt when a 6 A current flows through it.

What is the correct fuse to
use?
 The correct fuse to use is one with a rating
just higher than the current that an
electrical appliance normally uses.
 For example, if a rice cooker uses a current
of 4 A, a suitable fuse will be one that is
rated 5 A and not one rated 10 A.
 In home circuitry, the fuse should be placed
along the live wire. In this way, the appliance
will be disconnected from high voltage
when the fuse is blown.

Circuit Breaker
 Another safety device found in
all houses in Singapore is the
circuit breaker.
 The circuit breakers prevent
an overly big current from
flowing. When the current
through a circuit breaker
becomes higher than its
rating, the circuit breaker will
‘trip’ and cut off the
electricity supply.
 When the circuit breaker ‘trips’,
electricity to some or all
parts of the house will be cut
off.
resetting a circuit
breaker that has tripped

Earth Wire
 Many electrical
appliances have metal
casings. If a fault occurs
causing the live wire to
touch the metal casing,
the metal casing will
have a high voltage.
 If a person touches the
metal casing, he or she
will get an electric
shock.
 To prevent this, a safety
feature called earth © Copyright Star Publishing Pte Ltd 47
earthing an electrical
appliance

The earth wire is connected to the metal
casing to protect users

Double Insulating Wire
 Double insulating
wires are wires with
two layers of
insulation. The extra
layer acts as a
safety feature in
case one layer of
insulation is
damaged.
double insulating wires as
safety feature

4.6 How can information on electric
power be used?

Objectives
 Relate power to energy transferred and time
taken, using appropriate example, and the
equation: Power =
 Use information on a label of electrical appliance
to determine its power consumption
 Give examples of ways to reduce electrical
energy wastage at home
 Use the equations Power, P = V x I, Energy, E =
P x t
 Calculate the cost of using electrical appliance
where the energy unit is kWh
 Understand and use information of electrical bills
Time
Energy

What is Electrical Power?
 Electrical power is the rate at which
electrical energy is converted to other
forms of energy.
 The SI unit of electrical power is the watt
(W). larger unit of power is the kilowatt
(kW).
 An electrical appliance with a power of 800
W will convert 800 J of electrical energy to
other forms of energy every second.

Electrical power
 Electrical power can be calculated
using the formula:
 where P = power (in W)
E = energy converted (in J)
t = time (in s)

Worked Example
 An electric hot plate uses 8000 J of
energy in 4 s. Calculate the power.
P =
t
E
= 2000 W
4s
8000J=

How do we calculate energy?
 To calculate energy, the formula can also be rewritten
as:
 A hairdryer with a power of 1 kW is used for 30 s.
Calculate the electrical energy needed
P = E × t
= 30 000 J
= 1000 × 30

Using Information on Labels of
Electrical Appliances
 Electrical appliances usually come with a label
that provides important information. The:
(a) voltage that the appliance should be used
with,
(b) power rating of the appliance.
the label on an rice cooker tells us that it should be used
with a voltage between 230 V and 240 V and that the power rating is 630 W

Using Information on Labels of
Electrical Appliances
 An appliance with high power rating can help us get
things done faster.
E.g. An electric kettle with a power rating of 2000W can
boil the same amount of water in a shorter time
compared to an electrical kettle with a power rating of
800 W.
 When used for the same period of time, an appliance
with high power rating will consume more electrical
energy and result in higher utility cost.
E.g. Using a 1000 W air-conditioner will consume 10
times more electrical energy than using a 100 W fan.

Energy efficiency labels
 From 2008, it became a
must for some electrical
appliances (air-
conditioners, refrigerators
and clothes dryers) to carry
an energy label.
 The number of ticks on
the label tells us the
energy efficiency rating
of the appliance.
 Choosing appliances with
high energy efficiency
helps to reduce energy
usage.
energy efficiency labels give
consumers clear information on
appliances that save energy

The number of ticks indicates how energy
efficient an applicance is.

Energy saving lamps
 The power rating of
the energy saving
lamp is 26 W but the
amount of light it can
give out is the same
as a 150 W
incandescent bulb.
 Therefore, another
way to reduce energy
usage is to use
energy saving
lamps instead of
incandescent bulb. © Copyright Star Publishing Pte Ltd 60
an energy saving lamp

Calculating Power
 Electrical power can be calculated
using the formula below:
 where P = power (in W),
V = voltage (in V),
I = current (in A).

Calculating Energy
 Combining E = P × t and P = V × I, the
following formula for energy is
obtained:
 where E = energy (in J)
V = voltage (in V)
I = current (in A)
t = time (in s)

Worked Example 1
 A light bulb is connected as shown.
 Calculate
(a) the power of the bulb,
(b) the electrical energy used by the bulb in 20
seconds.
Solution:
(a) Power = V × I
= 9 × 2
= 18 W
Solution:
(b) Electrical energy
= V × I × t
= 9 × 2 × 20
= 360 J

Worked Example 2
 The kettle is connected to the
240 V mains supply. Calculate
the energy used when the
kettle is switched on for 10
minutes.
Solution: E = P × t
= 800 × (10 × 60)
= 480 000 J

Calculating Electrical Consumption
 Although the SI unit of energy is the joule
(J), electrical energy is sold in a more
convenient unit called the kilowatt-
hour (kWh).
 Electrical energy in kWh can be
calculated by substituting the power in
kW and the time in h into the formula E
= P x t.
 If the cost of 1 kWh of electrical energy
is known, the cost of using an appliance
can be calculated.

Worked Example
 A 2 kW air-conditioner is switched on for 8 h.
(a) Calculate the energy used in kWh.
(b) If electricity is sold at 20 cents per kWh,
calculate the cost of using the air-conditioner
for 8 h.
Solution:
(a) E = P × t
= 2 kW x 8 h
= 16 kWh
Solution:
(b) Cost = 16 × $0.20
= $3.20

Example
 The figure below shows the utility bill of a
household. The household used 396 kWh of
electricity in the month. At a cost of $0.2628
per kWh, the total cost of electricity will be
396 x $0.2628 = $104.07.
Activity Book Link
Activity 4.8
Reading Energy
Labels and Utility
Bills

Uss module 4 chpt 4 Electrcity

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