In this topic, you study Working of Capacitor.
Consider a simple capacitor C (Fig. 3.3), be connected to a battery through a galvanometer G and a key K. When the key is closed, the galvanometer gives momentary deflection. This shows that even though the meta ic circuit is open between the plates, current flows for a short time. This is because the positive terminal of the battery attracts some of the free electrons from plate A of the capacitor, the battery e.m.f. then pumps them from the positive terminal to the negative terminal and the negative terminal ultimately repels them to plate B Of the capacitor. So, plate A is positively charged and plate B is negatively charged. As a result, potential difference arises across the plates. This potential difference which is established across the plates acts as a counter e.m.f. and starts opposing the movement of the electrons. Its magnitude is obviously proportional to the charge that accumulates on the plates. The movement of electrons, therefore, continues till the potential difference between the plates A and B becomes equal to the e.m.f. Of the battery. If the battery is then disconnected, the capacitor remains in the charged condition for a considerable period of time. However, if a conducting wire is connected across this charged capacitor and galvanometer as shown by the dotted line in the figure, the galvanometer again gives a momentary deflection but this time in the opposite direction. This is because there is again a flow Of current due to rushing Of the electrons from plate B to A. Thus, the energy stored in the capacitor is released and ultimately dissipated as heat in the resistance of the wire. This is known as discharging of the capacitor. It should be remembered that during charging and discharging, the conventional current is always in the opposite direction to that of flow of electrons.If the voltage of the battery is increased, the galvanometer deflection on charge and on discharge also increase proportionately, indicating that charge on the capacitor is proportional to the voltage applied across its terminals.