When a charged capacitor is disconnected from a battery, its energy remains in the field in the space between its plates. To gain insight into how this energy may be expressed (in terms of Q and V), consider a charged, empty, parallel-plate capacitor; that is, a capacitor without a dielectric but with a vacuum between its plates.
The same ideas also apply to charging the capacitor. During charging electrons flow from the negative terminal of the power supply to one plate of the capacitor and from the other plate to the positive terminal of the power supply.
This process will be continued until the potential difference across the capacitor is equal to the potential difference across the battery. Because the current changes throughout charging, the rate of flow of charge will not be linear. At the start, the current will be at its highest but will gradually decrease to zero.
As charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls and the rate of decrease of potential difference also falls. Eventually the charge on the plates is zero and the current and potential difference are also zero - the capacitor is fully discharged.
DC charging is one of the most common methods of charging capacitors. In this method, a direct current (DC) power source is connected to the capacitor, allowing current to flow from the source into the capacitor. During DC charging, the voltage across the capacitor gradually increases as charge accumulates on its plates.
When a voltage is placed across the capacitor the potential cannot rise to the applied value instantaneously. As the charge on the terminals builds up to its final value it tends to repel the addition of further charge. (b) the resistance of the circuit through which it is being charged or is discharging.