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
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.