Question

A cell of e.m.f. $\epsilon$ and internal resistance r is used to send current to an external resistance R. The total resistance of circuit,

• A. $\frac{Rr}{(R+r)}$
• B. $\frac{R+r}{2}$
• C. $Rr$
• D. $R+r$

Answer 1

The correct option is D $R+r$

Let us consider the circuit as given in the diagram below.
Batteries and cells have an internal resistance (r) which is measures in ohms. When electricity flows round 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 $\epsilon =IR+Ir$.
Hence, the total resistance in the circuit is given as R+r.