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

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Ashika

Expertise

## Electron Flow in Charging & Discharging

#### 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

#### Exam Tip

Describing the motion of electrons (electron flow) for a charging or discharging capacitor is a common exam question. Ensure you are able to specify from which plate electrons are 'pulled' (the plate connected to the positive terminal of the power supply) and to which plate electrons are 'pushed' (the plate connected to the negative terminal).

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### Author:Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.