Attraction and repulsion
3.1
The sticky balloon
If you rub a rubber balloon on a woollen jumper, they may both
become charged. You can use the charged balloon to attract small
pieces of paper, or your hair.
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Rubber, wool, paper and hair are all electrical insulators. Two
insulators may become charged when they are rubbed together. The
electric charge on an insulator is called its electrostatic charge.
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Any two charged objects exert a force on one another. The
electrostatic force can either pull the objects together or push them
apart. For example, the charged balloon and jumper attract each other.
But two charged balloons repel each other.
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Rubbing charges on and off
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Here is an explanation of the observations described above.
There are two types of electrostatic charge, called positive charge and
negative charge. After it has been rubbed, the jumper has a positive
charge. The balloon has a negative charge. They will attract each other
because opposite charges attract. On the other hand, two charged
balloons repel each other. Each has a negative charge, and charges that
are the same repel one another.
First object
Second object
negatively charged balloon
negatively charged balloon
negatively charged balloon
positively charged nylon thread
positively charged piece of wool
positively charged nylon thread
Attract/repel/
no effect?
Questions
a Copy and complete the table, to show how the pairs of objects will interact.
b If you comb your hair, your hair is attracted to the comb. After combing,
it is light and fluffy – your hairs are repelling each other. What do these
observations tell you about the electrostatic charges on hair and comb?
Here is a deeper explanation of the observations. You have to think
about the atoms of which matter is made. An atom has a positively
charged nucleus with negatively charged electrons around it.
When you rub one object against another, charged particles are
transferred from one to the other. Because the electrons are on the
outside of the atoms, it is these particles which are transferred.
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194
An uncharged material has equal amounts of positive and
negative charge.
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A material which gains electrons becomes negatively charged.
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A material which loses electrons becomes positively charged.
P2
 The Van de Graaff generator in the
photograph can be charged up to two and
a half million volts. Electric charges build
up on the metal domes; when the charge is
great enough, sparks jump through the air.
In the picture, the balloon has gained a negative charge. The jumper
has an equal amount of positive charge. If we put the jumper and
balloon back together, the two charges would cancel each other out.
Questions
c Look at the picture of the jumper and balloon. Which has gained electrons, and
which has lost them?
d Use the idea of electrons to explain why, when two objects are rubbed together,
their charges are equal in size (although one is positive and the other negative).
e Explain why your explanation of electrostatic charge in Question d is ‘deeper’
than your explanation in Question b.
Dangerous sparks
The more charged up an object becomes, the greater is the potential
difference (p.d.) between the object and the earth. This is the same
as saying that there is a large voltage between the object and the earth.
If the p.d. is great enough, a spark may jump from the object to earth,
or to any nearby conductor which is earthed (connected to earth).
This is why lightning occurs – a cloud becomes highly charged, so that
the p.d. between it and the ground is many millions of volts. This p.d.
is big enough to make a current flow through the air, and we see a
flash of lightning.
The tip of a lightning conductor is high up, closer to the cloud, and it is
connected to earth. This gives the lightning a shorter distance to travel
to reach the earth, and so it strikes the conductor and the electrostatic
charge flows through the conductor and is discharged safely to earth.
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 If the charge on the big sphere is large
enough, a spark will jump across to the
small sphere. The charge then flows down
the conducting wire to earth.
Static electricity can be dangerous. Sparks can give you a shock, or
ignite a fire.
This petrol tanker is delivering fuel to an aeroplane. The fuel may
become charged by friction as it flows along the pipe, like a balloon
rubbed on a jumper. A spark could then jump through the air, and
this might ignite the petrol vapour. To avoid this danger, the pipe is
connected to the earth using a metal wire.
Question
f How does connecting the pipe to the earth with a wire avoid the danger
of sparks?
Key points
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When insulating materials are rubbed against each other, electrons are
rubbed off one material onto the other and both become charged.
Opposite charges attract one another; like charges repel one another.
If an object has a high enough p.d. compared to the earth, a spark can
jump from the object to the earth.
Electric charge
195
Using static electricity
3.2
Friend or foe?
We have seen that static electricity can be
dangerous. But it also has many uses. For example,
liquid crystal displays are found on many computer
monitors, mobile phones, calculators and TV sets.
Each dot on the screen can be changed from black
to white by applying a static voltage. (Coloured
filters give the different colours.)
The picture shows another use of static electricity
– in painting metal components. The spray gun is
connected to one terminal of a high-voltage supply,
so that the droplets of paint are charged. The metal
component is connected to the other terminal. The
paint is attracted to both sides of the door panel.
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 Static electricity helps in spray-painting this car door panel.
Question
a Explain why the paint is attracted to the car door panel.
Smoke precipitator
Many power station chimneys send large amounts
of dust up into the atmosphere. This is a major
source of pollution but, in the UK, it has now been
greatly reduced, thanks to static electricity.
In a power station, coal is burned, producing hot,
dirty exhaust gases. These are passed through a
precipitator where they are ‘scrubbed’ to remove
as much of the dust as possible.
The precipitator chamber has several wires running
down the middle, connected to the negative terminal
of a high voltage supply (approximately 40 kV).
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 The large blue structure at the centre
of this photo is a pair of dust precipitators
fitted to a power station. Waste gases
travel through these before passing up the
chimney stacks at the back.
As the exhaust gases pass through this high voltage
region, the dust particles gain electrons.
The particles are then repelled by the negative
wires so that they move towards the walls of the
chamber. The walls become covered in dust, which
is periodically scraped off and disposed of.
Questions
b What force causes the dust particles to move towards
the walls of the chamber?
c Why might a small-scale smoke precipitator be useful
for an asthma sufferer? Where else might smoke
precipitators be useful?
196
P2
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 A smoke precipitator uses static electricity to remove particles of
dust which would otherwise pollute the atmosphere.
What’s the idea?
Photocopier
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You put down the lid and press the ‘Copy’ button. A
bright light scans the page and a copy emerges – all
thanks to static electricity.
White paper reflects light, black ink absorbs it. Inside the
photocopier, the light reflected from the white areas of
the page is focused onto a charged drum which turns as
the light scans the page. When the light hits the charged
drum the electric charge gains energy and can flow to
earth, leaving these parts of the drum uncharged.
The only parts of the drum that stay charged are the parts
that have not been hit by light. These correspond to the
dark areas of the page. They pick up a special dry ink
called toner, which is then transferred onto the paper.
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 Inside a photocopier.
Finally, the paper with toner is heated. This melts the
toner and sticks it to the paper.
Question
d The drum, toner and paper are all charged. For each, say
what charge it must have, positive or negative. Explain your
answers.
It is easier to understand the stages in making a copy if
we picture the drum as a flat metal plate with a special
coating. It is also easier to think in terms of positive
charges moving, even though it is really electrons
which move in the opposite direction.
1. Positive charge is sprayed onto the plate (drum).
2. Where light falls, it gives the charges enough energy
to flow to earth. This happens wherever the original
page was white.
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4. A positively charged sheet of paper picks up the
toner powder, which is then melted on.
e After the image has been focused on the base plate, what
charge do the black areas have? What charge do the white
areas have?
f Why is the drum not made of metal?
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3. The rest of the plate remains positive. These areas
attract negatively charged toner powder.
Questions
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Key points
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Static charges can be useful.
They are used in devices such as the smoke
precipitator and the photocopier.
Electric charge
197