Topic 6.2
Using an oscilloscope
Using an oscilloscope
Aims
In this experiment you will use an oscilloscope to measure and compare the emf of cells,
batteries and a power supply. This will provide you with experience of operating an
oscilloscope and interpreting the display. It will also enable you to understand how dc and ac
voltages can be displayed.
Safety
 Check all plugs and electrical wiring for fraying, loose leads, etc.
 Check cells/batteries for leakages.
 The outputs of the cells/batteries and the 12 V power supply are low enough not to be a
hazard. However, the mains voltage is 220–240 V which is a dangerous level that you
should be aware of.
Equipment, materials and method
1 For this experiment you will need an oscilloscope with the necessary connecting lead, a
number of different cells and batteries and a 12 V power supply.
2 Turn on the oscilloscope. Switch the timebase off and turn down or off the non-calibrated
adjustment if there is one. Adjust the x-shift and y-shift so that the dot is laterally in the
centre of the screen and vertically towards the bottom. Align the dot so that it is exactly
level with one of the horizontal lines. This will make it easier to measure its movement.
3 Connect the terminals of a standard cell of known emf to the input lead of the
oscilloscope. If the dot jumps down instead of up, reverse the connections. Adjust the
V/cm (it may be labelled in volts per division) so that the dot moves vertically as far as
possible without leaving the screen and measure the number of cm (or divisions) the dot
has moved. How does increasing the distance the dot jumps improve your measurement?
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Topic 6.2
Using an oscilloscope
4 Use the number of cm (or divisions) the dot has moved and the V/cm setting to calculate
the emf of the cell. If necessary, calibrate the volts per division or V/cm using the known
value of the emf of the standard cell.
5 Repeat the experiment for a number of different cells. Use the calibrated value of V/cm to
calculate the emf of each cell.
6 Finally, measure the ac voltage supplied by a 12 V power supply. Adjust the timebase
(ms/cm or ms/div) to get a wave. Use the time base setting of ms/cm (or ms/div) to
measure the period of the ac supply. How can you increase the precision of your
measurement?
Questions
1 Calculate the frequency and the root mean square value of measured ac voltage where:
1
Vrms =
V0
2
2 In what way is the oscilloscope better than a voltmeter for measuring voltage?
Results
Record your results by sketching and labelling graphs of each of the outputs.
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Topic 6.2
Using an oscilloscope
Technicians Notes
Equipment and materials required
Each student or pair of students will require the following equipment:
 an oscilloscope with the necessary connecting lead
 a standard cell of known emf
 a number of different cells and batteries
 a 12 V ac power supply
 extra connecting leads for the power supply, depending on the connections
 graph paper
Safety
 All plugs and electrical wiring should be checked for fraying, loose leads, etc.
 Cells/batteries should be checked for leakages.
 The outputs of the cells/batteries and the 12 V power supply are low enough not to be a
hazard. The mains voltage is 220–240 V and is a dangerous level. A distinction should be
made between these.
 CLEAPSS ‘Use of oscilloscopes’ Laboratory Handbook/CD-ROM section 12.4.
AQA Physics A AS Level © Nelson Thornes Ltd 2008
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Topic 6.2
Using an oscilloscope
Teachers Notes
Aims
 To observe and calculate the output voltages of a variety of different power supplies,
including root mean square values, using an oscilloscope.
 To be able to operate the controls of an oscilloscope and interpret the display.
 To display different dc and ac voltage outputs graphically.
Safety
 All plugs and electrical wiring should be checked for fraying, loose leads, etc.
 Cells/batteries should be checked for leakages.
 The outputs of the cells/batteries and the 12 V power supply are low enough not to be a
hazard. The mains voltage is 220–240 V and is a dangerous level. A distinction should be
made between these.
Teaching notes
3 It may be advisable to combine this practical with another task, if the availability of
oscilloscopes is an issue.
4 The practical section of this experiment is likely to take 25–30 minutes.
5 It is advisable to enlist extra support from a lab technician, if available, to adjust the
settings in order to find the trace on the screen etc.
6 Ensure that the volts/division setting is set as large as possible for each reading in order to
minimise percentage uncertainty.
7 Use the standard cell to check the calibration of the V/cm or volts per division setting of
the oscilloscope.
Answers to questions
1
, where T is the time period.
T
Put the maximum values of V into the equation to calculate the root mean square value.
2 The oscilloscope shows how voltage varies with time.
1 Calculate the frequency f using f =
Practical assessment opportunities
3 ISA preparation:
written task Section A and B – how to increase the precision of a measurement
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