Electrical Engineering ⇒ Topic : Discharging of a Capacitor
Discharging of a Capacitor
Consider a capacitor of C farad charged to a p.d. of V volts and connected in series with a resistance R through a switch S as shown in Fig. (a) (i). When the switch is open, the voltage across the capacitor is V volts. When the switch is closed, the voltage across capacitor starts decreasing.The discharge current rises instantaneously to a value of V/R (= IM) and then decays gradually to zero
Let at any time t during discharging,
v = p.d. across the capacitor
i = discharging current
q = charge on capacitor
By Kirchhoff's voltage law, we have,
Note that negative sign is attached to IM. This is because the discharging current flows in the opposite direction to that in which the charging current flows.
Fig. (b) as well as adjoining table shows the percentage of initial voltage (V) after each time constant interval during discharging of capacitor. A fully charged capacitor discharges to about 37% of its initial fully charged value in first time constant. The capacitor is fully discharged in a 5 time constant interval.
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