Question

A cell having emf of 1.5V, when connected across a resistance of 14 Ω, produces a voltage of only 1.4V across the resistance. The internal resistance of the cell must be
(a) 1Ω (b) 14Ω (c) 15Ω (d )21 Ω​

Answer 1

Given:

A cell having emf of 1.5V, when connected across a resistance of 14 Ω, produces a voltage of only 1.4V across the resistance.

To find:

Internal resistance of battery?

Calculation:

• Let internal resistance be r and external resistance be R .

Under discharging conditions, we can say:

$E = V - ir$

$\implies 1.4 = 1.5 - \bigg(\dfrac{1.5}{14 + r} \times r \bigg)$

$\implies \dfrac{1.5}{14 + r} \times r = 0.1$

$\implies 1.5r = 1.4 + 0.1r$

$\implies 1.4r = 1.4$

$\implies r = 1 \: ohm$

So, internal resistance is 1 ohm.