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First exams 2025

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Capacitor Discharge Graphs (CIE A Level Physics)

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Ann H

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Ann H

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Capacitor Discharge Graphs

  • Capacitors are discharged through a resistor
    • The electrons flow 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

Charging and discharging circuit, downloadable AS & A Level Physics revision notes

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
  •  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

Graphs of current, potential difference and charge against time

Discharging graphs, downloadable AS & A Level Physics revision notes

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

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Ann H

Author: Ann H

Expertise: Physics

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students no matter their schooling or background.