Download 6_2 Using an Osc - Animated Science

Topic6.2
6.2
Using an oscilloscope
The oscilloscope: impossible or really,
really useful?
Aims
In this activity you will think about the controls on an oscilloscope, describe the functions of
the main controls and use them to measure voltage and frequency.
You will apply your knowledge to two square wave oscillations as well as a sinusoidal
oscillation and discover that the shape of a wave affects its root mean square value.
Using an oscilloscope
Figure 1 shows the front face of an oscilloscope.
Figure 1
Note: Oscilloscopes vary widely in appearance. For example, some may use a grid divided
into centimetre squares, as in the textbook, while others may simply use divisions, as above.
The control labelled Y-INPUT is sometimes known as the Y-gain control. The position
control near the Y-INPUT is sometimes known as the Y-shift and the position control near
the TIMEBASE control is sometimes known as the X-shift.
1 Describe what happens to the sinusoidal trace on the screen when you alter the controls in
the following ways:
(a) The Y-gain is changed from 5 V/div to 10 V/div.
(b) The Y-shift is changed in either direction.
(c) The TIMEBASE is changed from 2 ms/div to 1 ms/div.
(d) The X-shift is changed in either direction.
AQA Physics A AS Level Extension Activity © Nelson Thornes Ltd 2008
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Topic6.2
6.2
Using an oscilloscope
2 Suppose you turn on an oscilloscope to look at a trace. You find the trace at the top of the
screen. The trace is very small peak to peak and also has far too many oscillations on the
screen for you to be able to make any useful measurements.
What adjustments to the controls move the trace to the middle of the screen, and increase
or decrease the height and width of the trace? In your answer, make clear which way you
turn a control to have more or less volts/div and time/div.
Not all oscillations are sinusoidal!
Figure 2
The three diagrams in Figure 2 show the trace on an oscilloscope screen of a sinusoidal
oscillation and two types of square wave oscillation.
In each case the Y-gain is 1 V/div and the timebase is 10 ms/div.
The dotted line shows the trace obtained when no input is connected to the Y-input, i.e. it
shows zero volts.
1 (a) How can you tell that diagrams 1 and 3 have the same peak voltage?
(b) Find the value of this peak voltage.
2 (a) How do you know that the two square waves have different frequencies?
(b) Find the frequency of trace 2.
3 (a) On each of the three traces above, draw a graph to show how voltage squared varies
with time. (Hint: look at Figure 3 in Topic 6.1 of your textbook, which shows how
the sinusoidal value of current squared varies with time.)
(b) What is the mean value of the voltage squared over a full cycle in each of the graphs
you have drawn?
(c) What is the value of Vrms in each of the three diagrams?
(d) Explain how trace 3 produces a heating effect in a resistor that is constant and does
not vary with time.
AQA Physics A AS Level Extension Activity © Nelson Thornes Ltd 2008
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Topic6.2
6.2
Using an oscilloscope
Teaching Notes
Aims

To think about the controls on an oscilloscope.

To apply knowledge to square wave oscillations as well as a sinusoidal oscillation and
also determine peak voltage and frequency.

To sketch a graph of voltage squared against time and determine the mean value of the
voltage squared.

To understand that a square wave symmetrical about the origin has a root mean square
value equal to the peak value.
Students should write at length describing the functions of the main controls of an
oscilloscope and how to adjust them to display a reasonable wave.
Students can practise using square roots in surd form but may use a calculator.
Teaching notes
Students should have investigated practically the use of an oscilloscope, preferably with an
actual oscilloscope. Alternatively, use the simulation of a simple oscilloscope which is part of
the Nelson Thornes online resource for Physics.
Students may need help in taking a plotted graph and drawing the variation of the square
value of that graph and finding its average. Other examples could be shown to students
before they actually undertake this exercise.
Answers
Using an oscilloscope
4 (a) The trace becomes half the height.
(e) The trace moves up or down but keeps exactly the same shape.
(f) The trace becomes more spread out horizontally. For example, a trace that previously
covered 3.4 divisions along the x-axis will now cover 6.8 divisions.
(g) The whole trace moves to the left or right but keeps exactly the same shape.
5 Use Y-shift to move the trace to the middle of the screen.
Rotate Y-input anticlockwise so making the trace of larger height on the screen.
Rotate timebase anticlockwise so making the trace of larger width on the screen.
Not all oscillations are sinusoidal!
1 (a) They are both of the same height.
(h) 2 V
2 (a) The time for one wave as measured by the distance along the x-axis is different for
the two square waves.
AQA Physics A AS Level Extension Activity © Nelson Thornes Ltd 2008
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Topic6.2
6.2
Using an oscilloscope
(i) One complete wave takes 20 ms, so frequency =
1
1
=
= 50 Hz
0.02
time for one wave
(This is mains frequency.)
3 (a) In trace 1 the variation of voltage squared varies at twice the frequency of the input
signal.
For mathematicians, if the input signal is V = V0 sin (2ft)
(1  cos (4ft))
then V 2 = V02 sin2 (2ft) = V02
2
(j) Mean value of voltage squared over a full cycle:
trace 1
halfway up the V 2 graph (by symmetry)
trace 2
halfway up the V 2 graph (by symmetry)
trace 3
constant value of V 2
(k) For trace 1: Vrms =
(V0 )
=
= 2V
= 2V
= 4V
2
= 2V
2
2
or the square root of average value of V 2 = 2 V
For trace 2: square root of average value of V 2 = 2 V
For trace 3: square root of average value of V 2 = 2 V
The value of V 2 is constant and thus the heating effect
V2
is constant.
R
Alternatively, in trace 3 the voltage is either +4 V or −4 V. The current in a heater
will be either one way or the other but will be constant in value. The direction of the
current does not affect the amount of heat produced.
AQA Physics A AS Level Extension Activity © Nelson Thornes Ltd 2008
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