(b) charging and discharging of a capacitor or capacitor plates with…

The idea

A capacitor stores charge when a potential difference drives electrons around a circuit. During charging, electrons flow from the negative terminal of the supply onto one plate, giving it a net negative charge, while electrons leave the opposite plate towards the positive terminal, leaving it with an equal positive charge. The two plates therefore carry charges of equal magnitude and opposite sign. During discharging, the supply is removed and the plates reconnect through a component; electrons flow from the negative plate, through the external circuit, back to the positive plate until the potential difference across the capacitor falls to zero. The electron flow is opposite to conventional current in the external circuit.

What earns the marks

• Charging involves electron flow from the supply onto one plate and away from the other plate.
• One plate becomes negatively charged and the other becomes positively charged by an equal magnitude.
• The charge on the plates is equal in magnitude but opposite in sign.
• During discharging, electrons flow through the external circuit from the negative plate to the positive plate.
• The flow of electrons constitutes a current in the external circuit, opposite in direction to conventional current.
• Discharging continues until the potential difference across the capacitor is zero.

Common mistakes and how to put them right

• Thinking that positive charge physically moves onto a plate during charging.

  Put it right: In fact electrons move, and the positive charge on one plate arises because electrons have left it.

• Believing that the two plates carry the same sign of charge.

  Put it right: They carry equal magnitudes of opposite sign.

• Assuming that electrons flow through the dielectric between the plates.

  Put it right: In a capacitor, charge is stored on the plates and the external circuit carries the electron flow.

• Confusing the direction of electron flow with conventional current.

  Put it right: Electrons flow from negative to positive, opposite to conventional current.

Exam tips

• State clearly that it is electrons that move, not positive charge carriers.
• Use the phrase 'equal in magnitude and opposite in sign' when describing the charges on the two plates.
• When describing discharging, refer to the external circuit and the direction of electron flow from the negative plate to the positive plate.
• Link the end of discharging to the potential difference across the capacitor becoming zero.