4.4 Charging of a Capacitor through a resistor:


Let us consider a capacitor of capacitance (C) and a resistor of resistance (R) are connected in a series with a potential (V), as shown in the figure.
Figure 1: Circuit diagram for a charging Capacitor
Initially, there is no charge in the capacitor. i.e. at time t=0, the charge in the capacitor C is also zero.
After a time (t) sec, the charge on a capacitor is q. Let the current in the circuit is I, with a potential difference across the capacitor is VC, and the potential difference across the resistor is VR. Then,
VC = qc  and VR = IR
If q0 is the maximum charge stored in the capacitor, then
q0 = CV
From Figure, V = VC + VR .......... (i)
or, q0C = qC + IR
or, q0qC = R dqdt
or, dqq0q = 1RCdt .......... (ii)
Integrating equation (ii), we get
or, q0dqq0q = t01RCdt
or, [-ln(q0q)]q0 = 1RC[t]t0
or, - ln(q0q) + lnq0 = tRC
or, ln(q0q)lnq0 = tRC
or, lnq0qq0 = tRC
or, q0qq0 = etRC
or, q0q = q0etRC
or, q = q0(1 - etRC) .......... (iii)

A graph between charge (q) and time (t) during charging of a capacitor as shown in figure.
Figure 2: Variation of charge with time in a charging capacitor 
If t=RC, then from equation (iii) we get,
or, q  =  q0(1 eRCRC)  =  q0(1 e1)
or, q = q0(10.37)
or, q = q00.63
or, q = 63% of q0 .......... (iv)
This is the equation for growth of charge.
∴ The charging time constant (or RC time constant) of a capacitor is defined as the time interval in which the capacitor charges by about 63% of its maximum charge.

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