Topic 5.1
Circuit rules
Conservation of energy in a circuit
Aims
In this experiment you will investigate a circuit using two laws known as Kirchhoff’s laws.
The first law states:
At any junction in a circuit, the total current entering the junction equals the total
current leaving the junction.
In other words, charge is conserved.
The second law states:
Around any closed loop in a circuit, the sum of the emfs is equal to the sum of the
products of current and resistance in that loop.
In other words, energy is conserved.
In this investigation you will set up a circuit using various resistors and cells in order to
investigate a circuit using these laws. Recall that, according to Ohm’s law, the product of the
current and resistance across a device corresponds to the potential difference.
Safety
Check electrical equipment for loose wires. The resistors may heat up during the experiment.
Equipment, materials and method
1 For this experiment you will need a multimeter, 5 labelled resistors of varying resistance
from 10  to 22  in suitable holders, 2 × 1.5 V cells and connecting leads.
2 Measure the exact resistance of each of the five resistors using the multimeter set on the
ohmmeter setting.
3 Connect the circuit shown below. Check that the cells are arranged with the correct
R1
R3
R2
R4
R5
polarity.
4 Measure the pd across each cell and across each resistor using the multimeter as a
voltmeter. Record your readings. As soon as you have recorded the readings, disconnect
the cells.
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Topic 5.1
Circuit rules
5 Disconnect the wire between R1 and R2; connect the multimeter as an ammeter in the gap
and measure the current through the branch containing R1 and R2. Remove the multimeter
and replace the disconnected wire. Similarly, measure the current through the branch
containing R3, R4 and R5. Record your readings.
Results
1 Use your measurements to calculate:
(a) the pd across the two cells
(b) the current through the cells.
2 (a) Calculate the pd across each of the two branches using the resistance and current
measurements for each resistor.
(b) Compare the result of your calculations in 2(a) of the pd across each resistor branch.
Discussion
1 What effect might the multimeter have on the circuit when measuring the current in step
5?
2 What are the sources of experimental error in this experiment?
3 Estimate the uncertainty in your result for 1(a) to discuss whether your pd values for 1(a)
and 2(a) can be considered the same?
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Topic 5.1
Circuit rules
Technical Notes
Equipment and materials required
Each student or pair of students will require the following equipment:
 a multimeter capable of reading milliamps. (Many multimeters have protective fuses on
the milliamp setting which will need to be checked before the experiment is carried out.)
 5 labelled resistors of varying resistance from 10  to 22  in suitable holders
 2 × 1.5 V cells (new if possible)
 9 connecting leads. Alternatively, a Locktronics-type system would be useful.
Students will connect the following circuit. A circuit board could be used if available.
R1
R3
R2
R4
R5
It is important that students do not leave the cells connected because this will cause them to
run down and will affect their results.
Safety
All electrical equipment should be checked for loose wires and connections.
The resistors may get hot.
AQA Physics A AS Level © Nelson Thornes Ltd 2008
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Topic 5.1
Circuit rules
Teachers Notes
Aims
 To use Kirchhoff’s laws to investigate the current and the voltage in a circuit.
 To use a multimeter appropriately to measure resistance, voltage and current in a circuit.
 To compare theoretical values with measured values critically, taking errors into account.
Safety
All electrical equipment should be checked for loose wires and connections.
The resistors may get hot.
CLEAPSS information on cells: Laboratory Handbook/CD-ROM section 9.5.
Teaching notes
4 This experiment may take about 30 minutes.
5 Some students may need help setting up the circuits. It is particularly important that the
polarity of the cells is correct.
6 If the cells are not disconnected they may run down significantly and affect the results.
7 An important source of error is the tolerance of the resistor values. Students could use
their multimeters to check the values of the resistors beforehand.
8 In order to measure the current through each resistor branch, the multimeter as an
ammeter must be connected in series with the resistors in each branch.
Answers to discussion
1 The multimeter, when used as an ammeter, will slightly increase the resistance and
therefore reduce the current flowing in the loop.
2 Sources of error include the pd drop in the wires and at the connections (particularly if any
connections are loose), the cell running down during the experiment, the pd drop across
the ammeter and any current that flows through the multimeter when used as an ohmmeter
or a voltmeter. This could lead on to discussion about the limitations of meters when used
in circuits.
3 The uncertainty in 1(a) is twice the uncertainty in the voltmeter readings. Taking account
of this uncertainty, the pd values for 1(a) and 2(a) should be the same because the resistor
branches are in parallel with the cell. If they are not, the uncertainty in the resistor pd
values need to be considered. If the pd values for 1(a) and 2(a) still differ, the resistance of
the wires and the current passing through them needs to be considered.
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