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A potentiometer circuit has been set up for finding the internal resistance of a given cell. The main battery, used across the potentiometer wire, has an emf of 2.0 V and a negligible internal resistance. The potentiometer wire itself is 4 m long. When the resistance R, connected across the given cell, has values of
(i) infinity
(ii) 9.5 Ω
the balancing lengths on the potentiometer wire are found to be 3 m and 2.85 m, respectively. The value of internal resistance of the cell is

Asked in CBSE AIPMT 2014 · Potentiometer

• (1) 0.25 Ω
• (2) 0.95 Ω
• (3) 0.5 Ω
• (4) 0.75 Ω

Step-by-step solution

Why the other options are wrong

• (1) Check: r=0.25 Ω would give l₂=3×(9.5)/(9.75)=2.92 m, not 2.85 m.
• (2) Check: r=0.95 Ω would give l₂=3×(9.5)/(10.45)=2.73 m, not 2.85 m.
• (4) Check: r=0.75 Ω would give l₂=3×(9.5)/(10.25)=2.78 m, not 2.85 m.

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