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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 versus the current 'I'. Using the plot, show how the emf of the cell and its internal resistance can be determined.
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Solution
Terminal voltage 'V' of the cell is V = E − Ir
E is the emf of the cell, r is the internal resistance of the cell and I is the current through the circuit.
So, V = −Ir+E
Comparing with the equation of a straight line y = mx+c, we get:
y = V; x = I; m = −r; c = E
Graph showing variation of terminal voltage 'V' of the cell versus the current 'I' is
Emf of the cell = Intercept on V axis
Internal resistance = slop of line.
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