Saturday, April 8, 2017

Action of a Capacitor

      Consider a capacitor formed by two flat metal plates X and Y, facing each other and separated by an air gap or other insulating material used as a dielectric medium. There is no electrical contact or connection between them. Such a capacitor is called parallel plate capacitor.
      Consider a circuit in which such a capacitor across a battery with the help of a switch 'S' and a galvanometer 'G' in series. The arrangement is shown in the Fig 1.

      Let us see what happens when the switch 'S' is closed. As soon as the switch 'S' is closed, the positive terminal of the battery attracts some of the free electrons from the plate 'X' of the capacitor. The electrons are then pumped from positive terminal of the battery to the negative terminal of the battery due to e.m.f. of the battery. Now, negative terminal and electrons are repelled by the negative terminal to the plate 'Y' of the capacitor.
      The action is shown in Fig 2. 
      So, plate 'X' become positively charged while plate 'Y' becomes negatively charged. The flow of electrons constitutes a current, in the direction opposite to the flow of electrons. This is the conventional current called charging current of the capacitor as shown in the Fig. this can be experienced from the momentary deflection of the galvanometer 'G'. Because of this, there builds a potential difference across 'X' and 'Y'. There builds an electric field between the two fields.
      But this potential difference across the plates, acts as a counter e.m.f. and starts opposing the movement of the electrons. The magnitude of this potential difference is proportional to the charge that accumulates on the plates. When this potential difference becomes equal to the battery e.m.f., the flow of electrons ceases.
      If under such condition, the battery is disconnected then the capacitor remains in the charged condition, for a long time. It stores an electrical energy and can be regarded as a reservoir of electricity. Now, if a conducting wire is connected across the two plates of capacitor, with the galvanometer in series, then galvanometer shows a momentary deflection again but in the opposite direction.
      This is due to the fact that electrons rush back to plate X from plate Y through the wire. So, there is a rush of current through the wire. This is called discharging current of a capacitor. Thus, the energy stored in the capacitor is released and is dissipated in the form of the heat energy in the resistance of the wire connected.
      The direction of the conventional current is always opposite to the flow of electrons. If the voltage of the battery is increased, the deflection of the galvanometer also increases the time of charging and discharging.
Note: so, charge on the capacitor is proportional to the voltage applied to it.

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