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Ver. 0.3.4
Physics IA
Last updated 11.01.02
The Cathode Ray Oscilloscope
Syllabus reference
Assessment Criteria
Date delivered out
Date for handing in
Aim
4
Data Collection
Data Processing and Presentation
To gather knowledge, skills and understanding of the cathode ray tube
and to gain understanding of rms values and induced emf.
Equipment
Cathode Ray Oscilloscope, voltage source, signal generator, multimeter, solenoids, wires,
meterstick.
Theory
The cathode K is heated and emitts electrons, which are accellerated by the potential
difference U. The electrons pass through a tiny hole in the anode A. The electron beam
becomes deflected by the plates and hit the screen in point M. The reason why it is possible to
see this as a bright spot is fluorscens in the screen material.
A potential difference across the pair of plates P1 will move the electron beam vertically and
a potential difference across P2 will move the electrons horizontally. Channel 1 (CH1) and
Channel 2 (CH2) are both connected to the plates P1, and when you see the signal from both
channels it is the potential difference across the plates that alternates between the signals from
each channel. A potential difference across the plates P2 are set up by the sweepgenerator
inside the oscilloscope so that the beam will sweep over the screen during a time period
known as the sweep time. You may regulate the sweeptime by the SEC/DIV control.
Part One – How to Get Started
1. Switch on the oscilloscope by pushing the POWER ON button.
2. Set the 4 vertical lever switches to the top position.
3. Make sure the MAG control is pushed in (the control with ) and start with the
controls in midway positions (choose 1 if labelled).
4. Turn INTENSITY fully clockwise.
5. Push in the BEAM FINDER button and hold.
6. Centralise the display with the  control and the outermost control on the
VOLTS/DIV for both CH1 and CH2 (with vertical arrows beside).
7. Release the BEAM FINDER button.
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8. Adjust the INTENSITY and FOCUS controls to obtain a well-defined display of
suitable intensity.
9. Adjust the TRACE ROTATE (rear panel) control for a horizontal line.
10. Congratulations, the oscilloscope is now ready to be used, find out what happens as
you turn the SECS/DIV control counter clockwise:
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Prelab question
1. Study the usual voltage source. With which period T will you expect the voltage to
alternate (hint: use the frequency)? T = ____________________________.
PART TWO – about how to use the CRO to measure voltages and how the effective
and the maximum voltage of an alternator (=an alternating voltage source) relate.
1. Connect the alternating voltage source
to channel 2 of the oscilloscope. Start
with the lowest potential difference.
Choose the period found previously on
the SECS/DIV control. This should
give you a trace where the peaks seem
to be in relatively fixed positions.
Make a sketch of the screen.
Find out what happens when you
change the positions of the 2 vertical
lever switches on the left hand side
(controlling CH1 and CH2).
2. Measure the maximum voltage using the oscilloscope. Use the vertical arrows (the
outermost control on VOLTS/DIV to adjust the trace vertically and  to adjust it
horizontally. Adjust it so it becomes easier to read of the scale. Use peak to peak
measurement and divide by 2 if it is more convenient. One square on the screen has
sides that are one DIV long (div is a short for division).
3. Measure the effective voltage arross the unloaded source.
4. Repeat step 2 and 3 for the higher voltages.
5. Find the relationship between the effective and the maximum voltage. Explain your
method, and give your result.
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6. How does your value compare to the relationship between the rms value and the
peakvalue of the emf?
PART THREE – about how to use the CRO to study frequencies of different
waveforms.
1.
Connect the signal generator to CH 2 of the CRO. Try the sine formed waves first, and
measure different frequencies with the ocilloscope. Make a table of these together with
the the frequency on the signalgenerator.
2.
Do the same for the square wave.
3.
Enlarge the trace of the square signal as much as possible and make a drawing of it below.
Try to find the relationship between the
effective and the maximum voltage for
both the sine wave and the square
wave. Did you get what you expected?
Prelab questions
The magnetic field B inside solenoid 1 is given by
B1 = 4 pi 10-7 N1I1/L1
where I1 is the current through the solenoid, I1 = I0 sin(t). Here I0 is the peak value of the
current, and  = 2 f = 2  / T.
1. Find an expression for the flux linkage N2 in solenoid 2 with N2 turns, length L2 and area
A2 if placed inside solenoid 1 along its axis:
2. Use this to derive an expression for the induced emf 2 in solenoid 2 ,
when it is given that it is the negative derivative of the fluxlinkage with respect to time t :
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2 = - d(N2)/dt = - N2 d/dt
3. What will be the expression for the maximum (peak) value of the induced emf ?
4. If you measure the alternating current through solenoid 1, you’ ll find the effektive value or
the rms value. How can you find the peak value I0 from this?
PART FOUR – about how to use the CRO to study induced emf and phase difference.
Connect the solenoid with the biggest radius in parallel to the signalgenerator in such a way
that the current passes through all 240 turns. This will be solenoid 1. Use a multimeter in
series with the solenoid and adjust the effective current so that the peak value will be well
below the maximum value for the solenoid.
Find the peak value of the current from your measurement of the rms value
Place one half of the second solenoid (only the 120 turns) centred in the middle of the first
solenoid and connect it to CH2. Use the oscilloscope to find the maximum emf induced
Calculate the peak value of the induced emf if you use the peak value of the current found in
step1.
Find the % deviation of the measured value compared to the calculated value.
Try to connect the funny looking device connected to CH1 to solenoid 1 to be able to observe
how the voltage across it changes with time. You shold now see totally two traces on the
screen.
Make a sketch of the screen.
The phase difference between two points
one period T apart is 360o, so when the
phasedifference on the screen is measured
to t , the phase difference  (in degrees)
between the signals becomes
 = t * 360o / T
Use this to determine a value for the phase
difference between the original emf and
the induced emf.
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