Showing the current decay of a capacitor
Fig. 45.29 shows a voltage divider chain which includes a capacitor and a resistor. When the switch is in position A the battery is connected across the combination of the capacitor C and resistor R1• To plot the current against time, move this switch,
then note the value of the current on the centre-zero IlA meter every 10 seconds until the value becomes too small to read.
To plot the discharge graph for the capacitor, move the switch to position Band
again read the current value every 10 s. The capacitor is discharging through R2• Fig. 45.29 (b) shows typical charge and discharge graphs for the suggested capacitor/ resistor combinations. The time constant of the combination is roughly the time taken for the current to fall to 1/3 of its maximum (or to reach 2/3 ifit is increasing). These times are shown on the graph. Notice that the discharge time constant here is greater. This is because R2 is a larger value resistor than R1.