Question

A cell of e.m.f. $\epsilon$ and internal resistance $r$ is used to send current to an external resistance R. The current drawn from the cell is :

• A. $\frac{\epsilon }{(R+r)}$
• B. $\frac{\epsilon Rr}{R+r}$
• C. $\frac{\epsilon R}{R+r}$
• D. $\frac{\epsilon r}{R+r}$

Answer 1

Answer: (A)

$\frac{\epsilon }{(R+r)}$

Batteries and cells have an internal resistance (r) which is measured in ohms. When electricity flows around a circuit, the internal resistance of the cell itself resists the flow of current and so thermal (heat) energy is wasted in the cell itself.

$\epsilon =I(R+r)$

where,

$\epsilon$ = electromotive force in volts, V

I = current in amperes, A

R = resistance of the load in the circuit in ohms,

r = internal resistance of the cell in ohms.

The above equation can be written as $I=\frac{\epsilon }{R+r}$.

Hence, the expression for the current drawn from the cell is $I=\frac{\epsilon }{R+r}$.