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. | | | | | |

| 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. | | | | | | |