Electromotive Force and Internal Resistance
If you measure the voltage across the terminals of a battery (below left), then connect the battery to a resistor and measure the voltage across the resistor (below right) it is never quite equal to the voltage you first measured across the terminals of the battery.
Some of the voltage battery is missing – the 'missing volts'. The reason is that the battery itself has some resistance, and some of the battery voltage – the emf or electromotive force, actually the energy per unit charge supplied by the battery – must be used to drive the current through the battery.
Alternatively we may say that the power made available to the external circuit – the resistor – is less than the power supplied by the chemical reaction inside the battery, with the difference being that power needed to drive the current against the internal resistance of the battery.
There is an equation to model the situation. If the emf of the battery is E and a current I flows through the resistance R, then the voltage across this resistance is IR and the lost volts isFor the circuit above right, the current through the batter isso if the internal resistance of the battery isthe lost volts isand we can write(1)
We can find the emfof a battery and it's internal resistanceusing the circuit below.
By varying the resistance, we can vary the voltage and current. Write (1) aswhere the voltage across the variable resistor. Thenwhich is of the form Values ofagainstcan be plotted. The gradient of the graph isand the intercept is