A student was given the experiment to measure the emf of a unknown cell using potentiometer. He cheated result and wrote the observation arbitrarily and was caught. Which reading helped the teacher to arrive at the conclusion that he cheated?

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TestHub · Accepted Answer

Driver circuit: 20 V cell, 2 Ω series resistance and 100 cm wire of 8 Ω, with rheostat in series.

Potential gradient is maximum when rheostat = 0: current = 20/(2+8) = 2 A, so potential drop across the wire = 16 V over 100 cm.

Minimum possible balancing length for the 10 V cell: $ℓ_{min} = \frac{100}{16} \times 10 = 62.5 cm$

Any null point less than 62.5 cm is impossible, whatever the rheostat value.

Readings 1 (50 cm) and 2 (60 cm) are both less than 62.5 cm, so they are the fake readings.

Hence (A) and (B).

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