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A cell of emf 4 V and internal resistance 0.5 Ω is connected to a 7.5 Ω external resistance. The terminal potential difference of the cell is

Asked in Re-NEET 2022 · Terminal voltage and internal resistance

(1) 0.375 V
(2) 3.75 V
(3) 4.25 V
(4) 4 V

Answer: (2) 3.75 V

Step-by-step solution

I=E/(R+r)=4/(7.5+0.5)=0.5 A.

V=E-Ir=4-0.5×0.5=3.75 V.

Check: IR=0.5×7.5=3.75 V.

Why the other options are wrong

(1) A factor of 10 low; the current is 0.5 A and 0.5×7.5=3.75 V.
(3) Adds the internal drop instead of subtracting it; that would be the terminal voltage while the cell is being charged.
(4) This is the emf; with current flowing, part of it is lost inside the cell.

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