Physics (AS & A Level) · May / June 2021 · Paper 11 · Question 38

A potentiometer circuit is used to determine the unknown electromotive force (e.m.f.) of a cell X. In the circuit shown, E is a cell with an e.m.f. that is known accurately. QR is the potentiometer wire, which has a movable contact S. Contact S is connected to a galvanometer and to cell X. What is not a necessary requirement to determine the e.m.f. of X from the circuit?

1. AThe e.m.f. of cell X must be lower than the e.m.f. of cell E.
2. BThe internal resistance of cell X must be known.
3. CThe lengths QS and QR must be determined accurately.
4. DThe resistance of the wire QR must be proportional to its length.

As printed in the paper