This problem6 illustrates how a digital voltmeter affects the voltage across a capacitor in an RC circuit. A digital voltmeter of internal resistance r is used to measure the voltage across a capacitor after the switch in Figure P28.76 is closed. Because the meter has finite resistance, part of the current supplied by the battery passes through the meter.
(a) Apply Kirchhoff€™s rules to this circuit, and use the fact that iC = dq/dt to show that this leads to the differential equation where Req = rR/(r % R).
(b) Show that the solution to this differential equation is After Joseph Priest, "Meter Resistance: Don't Forget It!" The Physics Teacher, January 2003, p. 40. and that the voltage across the capacitor as a function of time is
(c) What If? If the capacitor is fully charged, and the switch is then opened, how does the voltage across the capacitor behave in this case?

r+ R + =- Voltmeter