Electron Flow in Charging & Discharging (OCR A Level Physics)

Charging

• Capacitors are charged by a power supply (e.g., a battery)
• One plate of the capacitor is connected to the positive terminal of the power supply
  • The positive terminal 'pulls' electrons from this plate
  • Hence the plate nearest the positive terminal becomes positively charged

• These electrons travel around the circuit and are pushed from the negative terminal of the power supply
  • The negative terminal 'pushes' electrons onto the other plate
  • Hence the plate nearest the negative terminal becomes negatively charged

• As the negative charge builds up, fewer electrons are pushed onto the plate due to electrostatic repulsion from the electrons already on the plate
  • When no more electrons can be pushed onto the negative plate, the charging stops

Electron flow for a charging capacitor. Electrons are pulled from one plate and pushed on to the other

• At the start of charging, the current is large and gradually falls to zero as the electrons stop flowing through the circuit
  • The current decreases exponentially
  • This means the rate at which the current decreases is inversely proportional to the amount of charge on the plate

• Since an equal but opposite charge builds up on each plate, the potential difference across the plates slowly increases until it is the same as that of the power supply
• Similarly, the charge of the plates slowly increases until it is at its maximum charge defined by the capacitance of the capacitor

Discharging

• Capacitors are discharged through a resistor with no power supply present
• The electrons now flow back from the negative plate to the positive plate until there are equal numbers on each plate and no potential difference between them
• Charging and discharging is commonly achieved by moving a switch that connects the capacitor between a power supply and a resistor

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

• 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
  • 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 follow a pattern of exponential decay