A cell of EMF E and internal resistance r is connected across a variable resistor R. Plot a graph of the terminal voltage V versus the current I. Using the plot, show how the EMF and the internal resistance of the cell can be determined.
Solution
From V=E−Ir: at I=0, V=E, so the intercept on the V-axis is the EMF. The slope of the line is −r, so the internal resistance is the magnitude of the slope. When V=0 (short circuit), Imax=E/r, giving r=E/Imax.
