Resistance of four arms P, Q, R and S in a Wheatstone bridge are 10 Ω, 30 Ω, 30 Ω and 90 Ω. Cell emf is 7 V with internal resistance 5 Ω and galvanometer resistance 50 Ω. Current drawn from cell is:
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Question

Resistance of four arms P, Q, R and S in a Wheatstone bridge are 10 Ω, 30 Ω, 30 Ω and 90 Ω. Cell emf is 7 V with internal resistance 5 Ω and galvanometer resistance 50 Ω. Current drawn from cell is:

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G1 Physics · Accepted Answer

Step 1: Bridge is balanced because 10/30 = 30/90 = 1/3, so galvanometer carries no current. Step 2: Series arms: (10+30)=40 Ω and (30+90)=120 Ω, in parallel. R(eq) = (40×120)/(40+120) = 30 Ω. Step 3: Add cell internal resistance: 30+5=35 Ω. Step 4: I = 7/35 = 0.2 A.