A designer needs a circuit that will cause a delay in switching off the interior light in a

car after the door is shut.

She uses a circuit with a resistor and a capacitor. She knows that the time constant T is

given by

T = RC

where R is the resistance in ohms and C is the capacitance in farads.

With the switch in position X the capacitor is charged to 12 V. When the switch is

moved to position Y the capacitor discharges through the resistor and the potential

difference (p.d.) across the resistor falls steadily from 12 V.