Capacitor Discharge Graphs (CIE A Level Physics)

• 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 the discharge, the current is large (but in the opposite direction to when it was charging) and gradually falls to zero

Capacitor Charging and Discharging Circuit

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
  • This 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
  • The initial value starts on the y-axis and then decreases exponentially

Current, Potential Difference and Charge Against Time Graphs

Exponential decay graphs of the variation of current, p.d. and charge with time for a capacitor discharging through a resistor

•  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 the charge will flow from the capacitor plates quickly, meaning the capacitor will discharge faster