# Electric Capacitance And Capacitor - 5

In the above circuit (a), if at t=0, circuit is closed, and then the potential difference across the ends of capacitor increases with time, as shown in diagram (graph b). This is called charging of capacitor.

This can be represented by equation (1):

Similarly the discharging circuit is shown in figure (c). Closing the key at t=0, the discharging starts. The potential difference V_{C} which was equal to E at t=0, decreases exponentially with time t, as shown in graph (d).

The above process is mathematically represented as,

‘RC’ is called as the “time constant of circuit”, its dimension is M^{0}L^{0}T^{1}. Time constant is that time of circuit, at which the value of V_{C} during charging becomes 67% of maximum value, or during discharging becomes 33% of maximum value.

16) Different types of condensers and their capacitors:

1)

Capacitors: General parallel plate capacitor (plate area A).

Figure:

Capacity:

Remark: None.

2)

Capacitors: Parallel plate capacitor in which a dielectric medium is filled all through (constant e_{r}).

Figure:

Capacity:

Remark: Capacity is e_{r }times that given before in (a).

3)

Capacitors: Parallel plate capacitor in which a block of medium of dielectric constant e_{r} and thickness t is placed.

Figure:

Capacity:

Remark: None.

4)

Capacitors: Complicated parallel plate capacitor.

Figure:

Capacity:

Remark:

5)