The p.d. across the terminals of a cell can be measured using a high resistance voltmeter when the cell is in a circuit and current is flowing.The current is varied by adjusting a variable resistor and a fixed resistor is used to limit the maximum current that can pass through the cellAn ammeter is used to measure the current.
A graph of V against I is plotted
The terminal p.d. decreases as the current increases.The terminal p.d. is equal to the e.m.f. at zero current.The graph is a straight line with a negative gradient.Given ε = IR + Ir can be rearranged to:V = ε - Ir which is in the form y = mx + cThe gradient is -r and the y intercept is ε
A graph of V against I is plottedThe terminal p.d. decreases as the current increases.The terminal p.d. is equal to the e.m.f. at zero current.The graph is a straight line with a negative gradient.Given ε = IR + Ir can be rearranged to: