Capacitor Discharge Graphs (CIE A Level Physics)

• 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