Physics 42 Lab AC Circuits
Introduction: The Oscilloscope
The oscilloscope is a versatile instrument from which quantitative measurements of voltage
and time can be made. Advantage is taken of this capability in order to measure the peak-topeak voltage and frequency for alternating current (AC) signals.
The vertical distance from a crest to a trough for a
sinusoidal signal represents the peak-to-peak value of
the voltage. Count the number of vertical divisions on
the CRT and multiply this value by the value on the
VOLTS/DIV setting. For example, if the number of
division is 3.7 and the VOLTS/DIV setting is 0.2, then
the peak-to-peak voltage is (3.7 div) x (0.2 volts/div) =
0.74 volts.
The frequency of the sinusoidal signal is found by first
determining the period of the oscillation, the
calculating the reciprocal (f = 1/T). The period is the
time for one complete oscillation and is represented on
the oscilloscope by the horizontal distance from a crest
to an adjacent crest. The value of the period is found by
counting the number of horizontal divisions from one
crest to the next adjacent crest, then multiplying this
number by the TIME/DIV setting. For, example, if the
number of division is 5 and the TIME/DIV setting is 0.5
ms/div, then the period is (5 div) x (0.5 ms/div) = 2.5
ms = 2.5 x 10-3 sec. The corresponding frequency is 2.5 x
10-3 = 400 Hz.
Another use of an oscilloscope is to find the
phase difference between two signals.
In this case, the phase difference between
two sinusoidal signals of the same
frequencies (#1 on the left and #2 on the
right) can be calculated from :
In this equation, f is the phase difference, t is the time
difference between two similar points on each curve
(e.g. two peaks) and T is the period.
Another way of observing the phase delay between two signals is to input each signal
into the two separate channels and plot one
channel against the other channel. If the
two signals are completely in phase, you will get a
45 degree line. For an RC and RLC circuit, if the
frequency of the AC applied voltage is equal to the
natural frequency of the circuit, the current and
applied voltage are in phase. The natural
frequency of an LC and RLC circuit is given by:
0 
1
LC
Where L is the inductance and C is the capacitance.
Equipment
Oscilloscope, signal generator, 100mH inductor with 100 ohm resister (wired together),
variable capacitor, 500 and 1000 turn transformer, cables.
Experimental Procedure: Make data tables in excel for your data for each part.
Part 1: Basic O-scope Measurement
1. Hook up channel A to the frequency generator. Set the frequency to 55kHz and the
amplitude to 11am.
2. Set the vertical volts/Div is set to 1 volts/div and the horizontal time/div set to 5
µs/div.
3. Make sure that the calibration dials are set all the way in the direction of the arrow. This
is VERY important!!!
4. Use the horizontal and vertical direction knobs to move the wave pattern so that it lines
up with the squares.
5. Find the period of the wave and then calculate the frequency and angular frequency.
6. Compare the calculated frequency with the frequency of the generator.
7. Find the peak-peak voltage and max voltage.
Part 2: LRC Resonant Frequency to Find Capacitance
1. Leave Ch A hooked up to the frequency
function generator with 55kHz as in Part 1.
2. Set up the LRC circuit as shown. Connect Ch
B across the resistor at G and B.
3. Set the vertical knob on Channel B to 1
volt/div. Display both A and B together by
pushing both blue buttons together. You
should see two sine waves.
4. Set the capacitor distance to 5mm. Adjust the
distance and watch the waves change.
5. Push the blue SWAP button to show the phase. You should see an ellipse that changes
as the phase angle changes.
6. Set the plate separation to 2 mm, then slowly change the frequency of the generator to
get a line (start at 30KHz and tune upward). Use the voltage knobs to zoom in and
readjust the frequency to maximize accuracy. Record the frequency from the generator.
This is the natural frequency of the LRC circuit. Use this to calculate the capacitance of
the capacitor.
7. Determine the theoretical value for the capacitance from the geometry of the plates.
Compare your results found from the oscilloscope.
Part 3: Transformers
1. Hook up channel A to the frequency generator. Set the frequency to 60Hz and the
amplitude so that the peak to peak is six divisions.
2. Set the vertical volts/Div to 1 volts/div and the horizontal time/div set to 2ms/div.
3. Make sure that the calibration dials are set all the way in the direction of the arrow. This
is VERY important!!!
4. Use the horizontal and vertical direction knobs to move the wave pattern so that it lines
up with the squares.
5. Find the period of the wave and then calculate the frequency and angular frequency.
6. Compare the calculated frequency with the frequency of the generator.
7. Find the peak-peak voltage and max voltage.
8. Connect alligator clips from the frequency generator to the 500 turn coil on the
transformer.
9. Connect Channel B of the Oscope to the 1000 turn coil of the transformer. Set the
Volts/Div to 2V.
10. Display both A and B waves by pushing the A and B vertical display buttons
simultaneously. Use the vertical position to adjust waves so that their peaks are aligned.
11. Determine the voltage of the 1000 turn coil. Is it double the primary voltage? Explain.
12. Repeat for a different frequency and voltage, adjusting the horizontal and vertical time
and voltage settings so you can see both waves. Repeat steps 5-11.