Question

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

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

Answer 1

Answer: (D)

$0.5\,\Omega$

Answer 2

E = $I_1 \, (R_1 + r)$ and E = $I_2 (R_2 + r)$
$i.e.$ E = 0.9 (2 + r) and E = 0.3 (7 + r)
$i.e.$ 1.8 + 0.9 r = 2.1 + 0.3$r$
$i.e.$ $x = \frac{0.3}{0.6} = 0.5 \, \Omega$