MJ21 P21 Q5 Potentiometer Circuit Galvanometer | May/June 2021 | Cambridge A Level 9702 Physics

9702/21/M/J/21: (a) State Kirchhoff’s second law. (b) A battery has electromotive force (e.m.f.) 4.0 V and internal resistance 0.35 Ω. The battery is connected to a uniform resistance wire XY and a fixed resistor of resistance R, as shown in Fig. 5.1. Wire XY has resistance 0.90 Ω. The potential difference across wire XY is 1.8 V. Calculate:

(i) the current in wire XY
(ii) the number of free electrons that pass a point in the battery in a time of 45 s
(iii) resistance R.

(c) A cell of e.m.f. 1.2 V is connected to the circuit in (b), as shown in Fig. 5.2. The connection P is moved along the wire XY. The galvanometer reading is zero when distance XP is 0.30 m.

(i) Calculate the total length L of wire XY.
(ii) The fixed resistor is replaced by a different fixed resistor of resistance greater than R.
State and explain the change, if any, that must be made to the position of P on wire XY
so that the galvanometer reading is zero.
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[muhammadahmed]

The length can also be found, if we use the ratio of their resistances ( after finding it of 0.3m) ???