5. Electrostatic effects
Capacitance P.22
Photo of shoe on carpet - electrostatic charging
Picture 5.4. You can get charged by walking on a carpet.
When you walk on, for example, a nylon carpet, you can get charged up. This illustrates two things:

   1.  you can charge an insulator by rubbing it against another one;
   2.  you have the capacity to store charge.

You’ll also know that when you subsequently touch something like a metal rail, there will be a spark. This illustrates two more things:

   3.  that the Earth has a big capacity;
   4.  that air will break down and spark at a big enough voltage.

We have already seen how air breaks down (page 21) and will look at earthing on page 26. However, here we’ll take a look at the first 2 points.

What is your capacitance?

When you get an electrostatic shock, the voltage is high – it has to be to break down the air. However, because you have a small capacitance, there is only a small amount of charge (Q=C.V). So the current is small and short lived. Although it hurts, it shouldn’t do any lasting damage.

1. Charging by rubbing
If you walk across a nylon carpet in leather-soled shoes, you will become positively charged. This is because the leather has less desire (or affinity) to hold onto electrons than nylon. The nylon carpet steals electrons from the leather.

When two materials, particularly insulators, are rubbed together, they heat up on a microscopic level. This can free electrons from atoms close to the surface. If one of the materials has a greater electron affinity than the other one, then it will steal some of these freed electrons.

2. What is capacitance?
As you walk across the floor, you charge up – you have the capacity to store charge. As you get charged up, an electric potential, or voltage, builds up (see below).We find that the voltage is proportional to the amount of charge:

     Q µ V
or Q = C.V

The constant of proportionality is the capacitance. It is measured in farads and tells us how much charge a capacitor can store at a given voltage. A big capacitance can store lots of charge and a small one not so much.

Luckily, we are not very efficient charge storers (see box). However, we can build more efficient charge storers by putting two conducting plates close to each other, with an insulator in between.

Graphic of parallel plates capacitor
Picture 5.5 Pushing more charge onto a plate increases the voltage.
Voltage and charge

There are two ways of thinking about how the voltage and charge are related to each other on a capacitor.

Voltage as joules per coulomb. Voltage (or electrical potential) is the energy per unit charge (V=E/Q). When we push charge onto a capacitor, we have to do work (to push against the charge that is already there). As more charge builds up on the capacitor, so it becomes more difficult to add an extra bit of charge. Therefore, we have to do more work per unit charge to get the next bit of charge onto the capacitor. Therefore the voltage (or joules per coulomb) has gone up.

Voltage as a push. Although the volt is defined as the joules per coulomb, we can think of it as being like a push in an electric circuit. So, if we attach a capacitor to a battery, the voltage of the battery pushes charge onto the capacitor. The bigger the voltage, the more charge it will manage to push on.

Also, if there is more charge on a capacitor, it will tend to be forced off with a bigger push, i.e. the capacitor has a bigger voltage.

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Question 17
We often use a water analogy to think of ideas in electricity. A pretty good analogy for a capacitor is a water tower or reservoir. The charge is represented by the water that the reservoir holds.

a) Explain why the height (or extra height) of the reservoir would represent the voltage.

Click shift/return to get a line break in your answer

b) What would represent the electrical current?

c) What sort of reservoir would be analogous to a person who gets charged up on a carpet? Decribe it in terms of its depth, volume and any other relevant features.

Summary                                           Close
  • objects can get charged by rubbing
  • the amount of charge that something can store at a given voltage is its capacity or capacitance
  • Q = C.V