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The resistance of the four arms P, Q, R and S in a Wheatstone’s bridge are 10 ohm, 30 ohm, 30 ohm and 90 ohm, respectively. The e.m.f. and internal resistance of the cell are 7 volt and 5 ohm respectively. If the galvanometer resistance is 50 ohm, the current drawn from the cell will be:

1. 0.2 A
2. 0.1 A
3. 2.0 A
4. 1.0 A

Correct answer: 0.1 A

Solution

The Wheatstone bridge is not balanced as P/Q ≠ R/S. The equivalent resistance of the circuit is calculated by combining the resistances in series and parallel. The total resistance is approximately 70 ohms, and using Ohm's law (I = V/R), the current drawn from the cell is 7/70 = 0.1 A.

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