cell supplies a current of 0.9 A through a 2 ohm resistor and current of 0.3 A

Discussion Forum : Current Electricity

Question -

cell supplies a current of 0.9 A through a 2 ohm resistor and current of 0.3 A through a 7 ohm resistor. What is the internal resistance of the cell?

Options:

A . 0.5 ohm

B . 1 ohm

C . 1.2 ohm

D . 2 ohm

Answer: Option A
:
A

If E the emf of the cell and r its internal resistance then
$\frac{E}{2 + r} = 0.9$ and
$\frac{E}{7 + r} = 0.3$
dividing the two equations we get
$\frac{7 + r}{2 + r} = \frac{9}{3}$
r=0.5 ohm

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