Electric Charge & The Atom
An object is charged if it has gained or lost
Like Charges Repel & Opposites
The removed images show that two positive
charges attract, two negative charges attract,
while a positive and a negative charge will
attract each other.
There is no need to load the image or read the
pages linked below.
An electric field is a
region of space in
Electric charges (eg
create electric fields
The removed image
showed a woman standing
on top of a mountain with
her hair standing up. This
was because there was a
strong electric field between
the ground and the clouds.
Moments after her and her
brother left the spot it was
struck by lightning.
So NEVER stay around if
your hair stands up – go
Lightning occurs during a
thunderstorm when cloud
movement causes a negative
charge to build up in the cloud,
which 'induces' a positive charge
beneath it on the ground.
If these charges become large
enough, the electrons will 'jump'
to the Earth, causing lightning.
Thunder is the sound of
lightning, and occurs at the
same time, but is heard
separately since sound travels
so much slower than light.
Most lightning occurs within or
A lightning rod provides an 'easy' way for the
electrons to pass through a building or other
structure, instead of it passing through the
material it's made of, which often results in
Many famous structures, including Osaka
castle (twice in the 1600s) have been destroyed
by lightning. The Eiffel tower lost its top in 1902.
One man has survived seven lightning strikes.
Never shelter under a tree during a
Creating a Current
Instead of passing through the atmosphere,
electricity can run through a conductor.
Electrons are very small, so physicists measure
them in Coulombs.
How do we Measure Charge
Charge is measured in Coulombs.
One Coulomb (C) = 6 * 1018
electrons (but not
1. How many electrons in half a Coulomb?
2. What is the elementary charge (on one
The unit of the Coulomb was used before the
elementary charge was known.
Image: Charles-Augustin de Coulomb
Current is a measure of how much charge are
flowing past a given point in a conductor per
Its units are Amperes.
One Amp = 1 C/s.
1. If five Coulombs flow past a point in a wire in ten
a) what is the current in the wire?
b) how many electrons flow past in two seconds?
Image: Charles-Augustin de Coulomb
1) 12 * 1018
electrons pass a point in one second. What is the current
2) 3 * 1018
electrons pass a point in one second. What is the current in
3) How many Coulombs pass a given point in three seconds if the
current is five Amps? Harder: how many electrons is this?
4) Draw a diagram to explain an analogy between electric current
a) a bakery delivering bread to a grocery store
b) cars on a freeway
c) water flowing down a river
5) An average lightning bolt carries 30 000 Amps and transfers 15
a) How many electrons is this?
b) How long does the lightning bolt last?
6) A new Macbook Air contains a 7150mAh battery. If it takes five
hours to charge the battery, calculate the average charging
Optional extension: Do some research and explain why we
calculated the 'average' current. Sketch a charging curve for a
(lithium ion) battery.
One Coulomb is 6*1018
electrons. I = Q/t
a) How many Coulombs in 18 *1018
b) How many Coulombs in 3 *1018
c) If Three Coulombs pass a point in 1.5 seconds,
what is the current in the wire?
d) Why do we use Coulombs instead of electrons?
Give another example of a unit representing a
number of objects.
Conductors and Insulators
Make the bulb light up.
Materials: 1 wire, 1 bulb and 1 battery.
Make a circuit to test whether or not something
A) Draw the circuit in your book (without using
symbols). It should be possible for another
student to reconstruct it the same way.
B) Test the following for conductivity:
glass, iron, plastic, aluminium, copper, wood,
Complete everything on the worksheet.
Electrons carry electrical energy. Work is done to
move electrons through a conductor, so the
electrons lose potential energy.
Potential difference is the difference in energy
between one point and another. Potential
difference is measured in Volts.
1 Volt = 1 Joule per Coulomb (J/C or V)
e.m.f = electromotive force is the voltage generated
by a power source (eg ______). It drives the
charge around a circuit.
e.m.f and P.D. Measure the same quantity (voltage)
so of course have the same units.
Voltmeters measure the potential difference
between two points in a circuit.
It is difficult for electricity to get through a
voltmeter (we say its __________ is high – next
class) so that it has minimal disruption on the
This image is worth loading to show how
voltmeters are connected.
Energy is required to push electrons through
Resistance measures how difficult it is for
electrons to get through.
Resistance is measured in Ohms (Ω).
1. Calculate the resistance of a light bulb which
uses three Amps from a 9V battery.
2. Calculate how many Amps will flow through
a heater with an effective resistance of 10Ω
when connected to the Japanese 100V
Ohm'sLaw :resistance= voltage
Image: Georg Simon Ohm
More Resistance Problems
1. Tammy takes a flashlight to Phuket. It runs on two
1.5Volt AA batteries (total 3V) and draws a current
of 0.2A. Calculate the resistance of the LED (light
2. A kettle connected to the Japanese mains voltage
(100V) draws five Amps. Calculate its resistance.
3. How much current would the kettle use if it were
connected to the mains power in Europe (240V).
Why might this be a problem?
4. A small solar-powered motor with an effective
resistance of 100Ω uses 0.5 Amps. What is the
voltage of its power source?
Length and Thickness
The resistance of a resistor (or anything else) is
greater if the resistor is thinner, since there is
less area for the electrons to pass.
The resistance is greater if the resistor is longer,
because the electrons need to travel further
through the resistor.
A length of metal has a resistance of 10Ω. It is cut
in half, widthwise, and the two ends are placed
lengthwise. What is its new resistance?
Why are pylons made of aluminium, when copper
is a better conductor?
Photo of pylons under Mt Fuji unnecessary.
Series and Parallel Circuits.
Current that leaves the battery must all come
back. It can not disappear. If 10 Amps leaves,
10 Amps must come back.
Voltage is 'used up'.
If a Coulomb leaves a battery with 12V, it must
'use it all up' in the circuit.
ie. If the EMF of the power source is 12V, the
sum of the PD of all components in one circuit
must be 12V.
In a series circuit, the electricity can only flow one way.
The electricity goes through each bulb.
The current is the same everywhere in the circuit.
The P.D. Of all the components equals the EMF of the
In a parallel circuit, each bulb
has its own path to the power
The electricity goes through
each bulb only once.
The current from the power
source is shared between all
the different branches.
The PD of each component is
Current and Voltage
Current and Voltage
All bulbs are
Current and Voltage
All bulbs are
Resistors in Series
Two resistors in series are just like one long
Calculate the current
in this circuit.
Resistors in Parallel
With two resistors in parallel, there are two paths for the
electricity to travel through, so it is easier, so the resistance
Calculate the resistance of a 10Ω, 1Ω and 5Ω resistor, all in
Don't forget the final reciprocal at the end!
Note for IGCSE students:IGCSE only requires two in parallel.
Five 10Ω resistors are connected in parallel to a 10V
A) Calculate the current through each resistor.
B) Calculate the current in the whole circuit.
C) If all the resistors were to be replaced with one
resistor (to save space) what would its resistance
D) Can you find a connection between the total
resistance and the additional resistances? What if
they were different resistances?
How is Power Different in Different
Photos of different power sockets in different countries unnecessary.
Five Amps flow through a 12 Volt car bulb. How
many Joules flow through the light bulb each
This is called the power. Power measures how
much energy something uses or produces per
A Joule is a measure of energy.
One Joule is about the amount of
energy required to lift a calculator
out of a bag onto a desk.
Image: James Joule
Energy and Power
Power (Watts) = Voltage (Volts) * Current (Amps)
Mr Duffield bought a small heater in Japan. It was labelled “600W”.
Japan's mains voltage is 100V.
1) Calculate the current through the heater element in Japan.
2) Calculate the resistance of the heater element in Japan.
He then took it to Taiwan, where the voltage is 110V.
3) Assuming the resistance stays the same in Taiwan (?),
calculate the new current through the heater.
4) Calculate the new power of the heater in Taiwan.
5) How many extra Joules of energy must be dissipated by the
fan in the heater each second in Taiwan (instead of Japan)? Is
this likely to be dangerous?
6) How many Joules of heat does the heater produce in Taiwan
in ten seconds?
7) Repeat calculations for New Zealand (240V). What will
happen if the heater is used there?
Iphones and Fridges
Does an iphone use more power than a fridge?
Some studies have claimed this, but the results
have been disputed. Research this and explain
One Kilowatt hour (kWh) is a measure of energy.
It is the amount of energy something which has a
power of 1kW uses in one hour.
It is a useful unit for the general public since
electricity is sold in kWh. In Japan 1 kWh costs
approximately 20 Yen.
How many Joules in 1 kWh?
If a vending machine uses 3 000 kWh per year,
calculate its power in Watts (assuming it is on all
ANS: ~ 340 Watts.
Why Different Voltages?
Electric current interferes with the body's nerve system, and
as a resistor the body produces heat (thus shocks burn).
Assume a person has a resistance of 1000 Ohms (it varies a
lot person to person, and depends on the skin in particular).
1. Calculate how much current they will have pass through
them in Japan (100V).
2. Calculate how much current the same person would have
pass through them if they were electrocuted in Australia
3. On a cold winter's day a particular house uses 2000 Watts.
Calculate the current through the wires coming into the
house in Japan and Europe. Why might a higher current be
4. Draw a table to show the advantages and disadvantages
of high and low voltage mains electricity.