A cell of emf `' epsi '` 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 'l'. Using the plot, show how the emf of the cell and its internal resistance can be determined.

Explanation

We know that terminal potential difference V across a cell of emf `epsi` and internal resistance varies with current I drawn from it as per relation :
`V = epsi - Ir`
Thus, V-I graph is a straight line graph with a negative slope as shown in Fig.
If I = 0, then `V = epsi` = emf of the cell. Hence, emf of a cell can be determined by the intercept of V-I graph on the V-axis.
Again when `I = I_(max)` then V = 0, hence internal resistance of cell `r = (epsi)/(I_(max))`
where `I_(max)` is the intercept of V-I graph on the I-axis