Topic 5.3
Emf and internal resistance
Measurement of the emf and the internal
resistance of a battery
Aims
In this experiment you will measure the pd across the terminals of a battery for different
currents through the battery. The measurements will enable you to determine the emf and the
internal resistance of the battery.
Safety
The voltmeter must be connected directly across the battery terminals. A wire-wound resistor
is to be included in series with the battery to limit the current when the resistance of the
variable resistance in the circuit is reduced, otherwise the battery could be damaged.
Equipment, materials and method
1 In this experiment, you will need a 3 V battery, variable resistor, ammeter, voltmeter,
switch and a wire-wound resistor (10 ).
2 When a battery delivers energy, chemical energy is converted to electrical energy inside
the battery. Some of the electrical energy is wasted inside the battery due to internal
resistance r. The pd across the battery terminals V is related to the current I by
V =  − Ir
where  is the battery emf and I is the current.
3 Use the circuit shown below to measure the pd across the terminals of the battery under
test when a measured current passes through the battery. The wire-wound resistor R is
included in the circuit to limit the current. Use the variable resistor to alter the current and
repeat the procedure for different measured currents.
4 Plot a graph of the pd across the battery terminals (on the vertical axis) against current. In
accordance with the above equation, the graph should be a straight line with a negative
gradient equal to −r and a y-intercept equal to .
AQA Physics A AS Level © Nelson Thornes Ltd 2008
1
Topic 5.3
Emf and internal resistance
Results
Record your results in a table that allows you to display repeated readings.
Questions
1 What does your graph tell you about the variation of the battery pd with current?
2 Use your graph to find:
(a) the emf of the battery
(b) the internal resistance of the battery.
Technicians Notes
Equipment and materials required
For this experiment, each student or pair of students will require the following equipment:
 a 3 V battery
 a variable resistor
 an ammeter
 a voltmeter
 a switch
 a 10  wire-wound resistor
 graph paper
Safety
The wire-wound resistor is included to limit the current when the resistance of the variable
resistance is reduced, otherwise the battery could be damaged.
The voltmeter must be connected directly across the battery terminals.
AQA Physics A AS Level © Nelson Thornes Ltd 2008
2
Topic 5.3
Emf and internal resistance
Teachers Notes
Aims
 To measure the terminal pd of a battery for different battery currents.
 To tabulate measurements and use the measurements to plot a graph.
 To relate the graph to the equation for internal resistance and determine the internal
resistance.
Safety
The voltmeter must be connected directly across the battery terminals. A wire-wound resistor
is to be included in series with the battery to limit the current when the resistance of the
variable resistance in the circuit is reduced, otherwise the battery could be damaged.
CLEAPSS information on cells: Laboratory Handbook/CD-ROM section 9.5.
Teaching notes
3 This experiment should take about 30 minutes.
4 Take particular care with the setting up of the circuit and the position of the voltmeter.
5 Students should switch the circuit off when not taking a measurement.
Answers to questions
1 The pd drops as the current is increased. The graph is a straight line which means that the
drop of pd is the same for equal increases of the current.
2 (a) The y-intercept (the intercept on the pd scale) gives the battery’s emf.
(c) The drop of pd per unit current gives the internal resistance. This is the negative of
the gradient.
AQA Physics A AS Level © Nelson Thornes Ltd 2008
3