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, q0−qC = R dqdt
or, dqq0−q = 1RCdt .......... (ii)
Integrating equation (ii), we get
or, ∫q0dqq0−q = ∫t01RCdt
or, [-ln(q0−q)]q0 = 1RC[t]t0
or, - ln(q0−q) + lnq0 = tRC
or, ln(q0−q)−lnq0 = −tRC
or, lnq0−qq0 = −tRC
or, q0−qq0 = e−tRC
or, q0−q = q0e−tRC
or, q = q0(1 - e−tRC) .......... (iii)
A graph between charge (q) and time (t) during charging of a capacitor as shown in figure.
If t=RC, then from equation (iii) we get,
or, q = q0(1− e−RCRC) = q0(1− e−1)
or, q = q0(1−0.37)
or, q = q0∗0.63
or, q = 63% of q0 .......... (iv)
This is the equation for growth of charge.
∴ The charging time constant (or R−C 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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