At the beginning, this current will be maximum and after certain time the current will become zero. The duration in which current changes in capacitor is known as transient period. The phenomenon of charging current or other electrical quantities like voltage, in capacitor is known as transient.
To understand transient behavior of capacitor let us draw a RC circuit as shown below, Now, if the switch S is suddenly closed, the current starts flowing through the circuit. Let us current at any instant is i(t).
Also consider the voltage developed at the capacitor at that instant is Vc(t). Hence, by applying Kirchhoff’s Voltage Law, in that circuit we get, Now, if transfer of charge during this period (t) is q coulomb, then i(t) can be written as Therefore, Putting this expression of i(t) in equation (i) we get, Now integrating both sides with respect to time we get, Where, K is a constant can be determined from initial condition.
Let us consider the time t = 0 at the instant of switching on the circuit putting t = 0 in above equation we get, There will be no voltage developed across capacitor at t = 0 as it was previously unchanged.
Therefore, Now if we put RC = t at above equation, we get This RC or product of resistance and capacitance of RC series circuit is known as time constant of the circuit. So, time constant of an RC circuit, is the time for which voltage developed or dropped across the capacitor is 63.2% of the supply voltage. This definition of time constant only holds good when the capacitor was initially unchanged.
Again, at the instant of switching on the circuit i.e. t = 0, there will be no voltage developed across the capacitor. This can also be proved from equation (ii). So initial current through the circuit is, V/R and let us consider it as I0.
Now at any instant, current through the circuit will be, Now when, t = Rc the circuit current. So at the instant when, current through the capacitor is 36.7% of the initial current, is also known as time constant of the RC circuit.
The time constant is normally denoted will τ (taw). Hence,