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For a cell terminal potential difference is 2.2 V when circuit is open and reduces to 1.8 V when cell is connected to a resistance of R=5 Ω. Determine internal resistance of cell (r).

Asked in CBSE AIPMT 2002 · Terminal voltage and internal resistance

• (1) (10)/9 Ω
• (2) 9/(10) Ω
• (3) (11)/9 Ω
• (4) 5/9 Ω

Step-by-step solution

Why the other options are wrong

• (2) Inverts the last step: current over lost volts, (0.36)/(0.4), instead of lost volts over current.
• (3) (11)/9=(2.2)/(1.8) is only the ratio E/V; r=R(E/V-1) still has to be taken.
• (4) No single slip gives 5/9 Ω; it would need lost volts of 0.2 V, but 2.2-1.8=0.4 V.

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