Building charge on the plates
A capacitor can be charged by connecting it to a power supply through a resistor.
Electrons are removed from the plate connected to the positive terminal of the supply and transferred around the circuit onto the plate connected to the negative terminal. The plates therefore develop equal and opposite charges.
At the start, very little charge has accumulated and the current is large. As electrons build up on the negative plate, their electrostatic repulsion makes it progressively more difficult for further electrons to arrive.
The charging current consequently decreases with time and eventually approaches zero.
As the opposite charges on the two plates increase, the potential difference across the capacitor also rises. Charging ceases when the capacitor potential difference has reached the value imposed by the supply and the current has fallen to zero.
Exam Tip: When describing charging, distinguish electron motion from conventional current. Electrons move in the opposite direction to conventional current.