• So far, only capacitors charged by a battery have been considered
  • This is when the electrons flow from the positive to negative plate
  • At the start, when the capacitor is charging, the current is large and then gradually falls to zero
• Capacitors are?discharged?through a resistor
  • The electrons now flow back from the negative plate to the positive plate until there are equal numbers on each plate
• At the start of discharge, the current is large (but in the opposite direction to when it was charging) and gradually falls to zero

The capacitor charges when connected to terminal P and discharges when connected to terminal Q

• As a capacitor discharges, the current, p.d and charge all decrease?exponentially
• The means the rate at which the current, p.d or charge decreases is proportional to the amount of current, p.d or charge it has left
• The graphs of the variation with time of current, p.d and charge are all identical and represent an?exponential decay

Graphs of variation of current, p.d and charge with time for a capacitor discharging through a resistor

• The key features of the discharge graphs are:
  • The shape of the current, p.d. and charge against time graphs are identical
  • Each graph shows exponential decay curves with decreasing gradient
  • The initial value starts on the y axis and decreases exponentially
• The rate at which a capacitor discharges depends on the?resistance?of the circuit
  • If the resistance is?high, the current will decrease and charge will flow from the capacitor plates more slowly, meaning the capacitor will take longer to discharge
  • If the resistance is?low, the current will increase and charge will flow from the capacitor plates quickly, meaning the capacitor will discharge faster