Question

A cell of emf $E$ and internal resistance $r$ is connected across a variable resistor $R$. Plot a graph showing variation of terminal voltage $V$ of the cell with the current $I$. Using the plot, show how the emf of the cell and its internal resistance can be determined.

Answer 1

The terminal voltage 'V' of the cell is given by

$V=-Ir+E$

where $E$ is the emf of the cell,

$r$ is the internal resistance of the cell, and

$I$ is the current through the circuit.

Comparing with the equation of a straight line $y=mx+c$, we get

$y=V;x=I;m=-r,c=E$

Variation of terminal voltage $V$ of the cell with the current $I$ is shown in the graph below.


Emf of the cell = Intercept on $V$ axis.

Internal resistance = slope of the line.