A cell of emf `epsilon` 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.

Explanation

Terminal pot. Diff. is, `V=epsilon - Ir or V = - r I + epsilon` .
This equation is similar to a straight line equation y = mx +c. Hence the graph between V and I is straight line AB with a negative slope ( = - r) as shown in figure. For point A, I = 0, and `V=V_(A)` , so
`V_(A) =epsilon`= intercept on Y-axis.
For point B, V = 0, I = `I_(B)`
`0 = epsilon - I_(B) r or r= epsilon/I_(B)`