 # Charging and Discharging

## Charging and Discharging

#### Charging Capacitors

What happens to current as time passes? As explained above, current falls away as it becomes less attractive for electrons to move to the plate from the cell.

Note: The area under the current-time graph is equal to the amount of charge stored on the plates.

What happens to the charge on the plate? Charge builds up - quickly at first (a lot of electrons arriving each second) and then more slowly. We have already said that potential difference is proportional to charge, so the p.d.-time graph is exactly the same shape as the charge-time graph.

When the capacitor is fully charged, the pd across the plates will equal the emf of the cell charging it. Look at the diagram. The cell is trying to push electrons clockwise (with its 'push' of 2V) and the capacitor is trying to push electrons anticlockwise (with its push of 2V). Neither wins so no charge flows.

#### Discharging Capacitors Initially there is a large current due to the large potential difference across the plates. The current drops as pd drops. Notice that the electrons are now moving the opposite way round the circuit so the graph shows the current as negative to show this. Charge drops quickly at first (due to the large current - which is of course, a large flow of charge). As the charge and therefore the pd across the plates drops, so the charge drops more slowly. As the potential difference across the plates is directly proportional to the charge on the plates, the p.d.-time graph is the same shape as the charge-time graph as before.