What happens to the voltage across a capacitor when it is fully charged in a circuit?

When a capacitor is fully charged in a circuit, the voltage across it equals the source voltage. This occurs because the capacitor functions as a storage device for electrical energy and, once charged, it builds an electric field that balances the voltage supplied by the source. As it charges, the capacitor's voltage gradually rises and, upon reaching its maximum charge, it stabilizes at the same level as the supplied voltage. At this point, there is no potential difference to drive current through the capacitor, leading to an open-circuit condition.

The other options do not accurately reflect the behavior of a fully charged capacitor. For instance, it doesn't drop to zero since that would imply that it hasn't been charged. Similarly, a fluctuation in voltage suggests that the capacitor is still charging or discharging, which is not the case once the capacitor is fully charged. Lastly, exceeding the source voltage would contradict the fundamental operation of capacitors in ideal conditions, as a charged capacitor cannot maintain a voltage higher than the source voltage unless specific conditions or components (like inductors or other energy sources) are involved in the circuit.