Question

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

Answer 1

Terminal potential difference is, 

V = ε − Ir 

Or V = -rI + ε 

This equation is similar to a straight-line equation,

y = mx + c 

Hence,

The graph between V and I is a straight line AB with a negative slope (= -r) as shown in figure.

For point A, 

I = 0 and 

V= V_A, 

So, 

V_A = ε = intercept on y-axis. 

For point B, 

V = 0, 

I = I_B

O = ε − I_Br