Question

A cell supplies a current of $0.9A$ through a $2\Omega$ resistor and a current of $0.3A$ through a $7\Omega$ resistor. The internal resistance of the cell is :-

• A. $1.0\Omega$
• B. $0.5\Omega$
• C. $2.0\Omega$
• D. $1.2\Omega$

Answer 1

The correct option is B $0.5\Omega$

Let $E$ be the emf of cell and r be the internal resistance.
here, $E=0.9(2+r)...\left(1\right)$ and $E=0.3(7+r)...\left(2\right)$
From (1) and (2), $0.9(2+r)=0.3(7+r)$
or $1.8+0.9r=2.1+0.3r$
or $0.6r=0.3$
or $r=0.5\Omega$