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Question:

In the circuit shown, a four-wire potentiometer is made of a 400 cm long wire, which extends between A and B. The resistance per unit length of the potentiometer wire is r = 0.01 Ω/cm. If an ideal voltmeter is connected as shown with jockey J at 50 cm from end A, the expected reading of the voltmeter will be:

0.20V

0.25V

0.50V

0.75V

Solution:

Correct option is B. 0.25V
Resistance of wire AB = 400 × 0.01 = 4 Ω
i = 3/6 = 0.5 A
Now voltmeter reading = i (Resistance of 50 cm length) = (0.5A)(0.01 × 50) = 0.25 volt