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In an experiment, current flowing through a cell and potential difference across its terminals are measured and the following observation table is prepared. r is the internal resistance of the cell.

S.No.	V (V)	I (A)
1	1.0	0.08
2	0.5	0.18
3	0.8	0.12

What is the emf of the cell used in the experiment?

Asked in GUJCET 2013 · Terminal voltage and internal resistance

• (1) 2.5 V
• (2) 1.4 V
• (3) 2 V
• (4) 1.5 V

Step-by-step solution

Why the other options are wrong

• (1) No single slip gives 2.5 V. Sanity check: ε = V + Ir with r = 5 Ω gives 1.4 V from every row of the table.
• (3) 2 V comes from taking V/I = (1.0)/(0.08) = 12.5 Ω from row 1 as the internal resistance: 1.0 + 0.08×12.5 = 2 V. But V/I is the external resistance; r comes from how V changes as I changes.
• (4) 1.5 V is the familiar emf of a dry cell, not something these readings give. From the table, ε = V + Ir = 1.4 V for every row.

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