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A cell of emf $\varepsilon$ and internal resistance r drives a current I through an external resistor. The terminal potential difference V across the cell is
A$V = \varepsilon + Ir$
B$V = \varepsilon - Ir$
C$V = \varepsilon r - I$
D$V = \varepsilon / (Ir)$
Answer & Solution
Correct answer: B. $V = \varepsilon - Ir$
1. The emf is the total energy per charge supplied by the cell.
2. Some of it is lost across the internal resistance as a drop $Ir$.
3. The remaining terminal voltage on discharge is $V = \varepsilon - Ir$.
4. The $+Ir$ form (A) applies only during charging; the others are dimensionally wrong.
_Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.13_
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