AQA A Level Physics

Revision Notes

7.7.1 Charge & Discharge Graphs

Capacitor Discharge Graphs

  • 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
  • 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
  • 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

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