Question

A cell of e.m.f. 1.5 V, records a p.d. of 1.35 V, when connected to an external resistance R, such that current flowing through the circuit is 0.75 A. Calculate the value of R and internal resistance of the cell.

Answer 1

E = 1.5 V; V = 1.35 V; I = 0.75 A; R = ?

By Ohm’s law, \[I = \frac{V}{R}\]

\[\therefore R = \frac{V}{I}\]

\[= \frac{1.35}{0.75}\]

= 1.8 Ω

Also, \[I = \frac{E}{R + r}\]

\[\Rightarrow 0.75 = \frac{1.5}{1.8 + r}\]

\[\Rightarrow 1.35 + 0.75r = 1.5\]

\[\Rightarrow 0.75r = 0.15\]

\[r = \frac{0.15}{0.75}\]

\[\therefore r = 0.2 \Omega\]