Download 127-10_Resonance

PHYSICS 127 EXPERIMENT NO. 10
RESONANCE
In this experiment, we observe the response of a series RLC circuit when it
is driven by a sinusoidal voltage whose frequency can be adjusted.
A. Equipment
1 Oscilloscope, 1 Function Generator, 1 Inductance Coil, 1 Decade
Resistance Box, 1 Decade Capacitance Box, 1 Frequency Counter.
B. Method
We use a dual-trace oscilloscope to monitor the input voltage applied to the
series RLC circuit (channel 1) and simultaneously observe the voltage across
parts of the circuit on channel 2. By triggering the scope from the channel 1
input, we can determine the relative phase relationships of the voltages
appearing across each circuit element.
to Oscilloscope
Channel 1
Oscillator
5 kHz
Sine
Waves
Frequency
Counter

C
L
R
12345
to Oscilloscope
Channel 2
Figure 1. Circuit Configuration.
C. Procedure
Connect the circuit shown in Figure 1. Adjust the resistance box R to 50
and the capacitance box C to 0.1 F. Set the oscilloscope for dual trace
operation and trigger from channel 1. Be sure all vertical sensitivity and
time base controls are in the "calibrated" position. Set the oscillator for
maximum amplitude 5 kHz sine waves.
Now adjust the frequency of the oscillator until the voltage across R is
maximum. This is the resonance frequency.
Q1. At what frequency should the maximum occur? Use the value obtained last
week for L to answer this question.
Keeping the frequency and everything else the same, interchange the resistor
and capacitor, so that channel 2 now shows the voltage across the capacitor.
Q2. What is the phase of the capacitor voltage relative to the input voltage at
resonance?
Q3. What is the amplitude of the capacitor voltage at resonance? Is it larger than the
input voltage?
Still keeping the frequency the same, interchange the capacitor and the
inductor so that channel 2 of the oscilloscope now shows the voltage across
the inductor.
Q4. What is the phase of the inductor voltage relative to the input voltage at
resonance?
Q5. What is the phase of the inductor voltage relative to the capacitor voltage at
resonance?
Q6. What is the amplitude of the inductor voltage at resonance? Is it larger than the
input voltage?
Q7. Explain how it is possible for the inductor and capacitor voltages to be larger
than the input voltage.
To Channel 1
R
L
To Channel 2
To Os cillator
C
To Scope Ground
Figure 2. Measurement of
Voltage across L-C combination.
Reconnect the channel 2 lead so that the scope measures the voltage across
the L-C series combination as shown in Figure 2.
Q8. At resonance, what is the voltage and phase across the L-C combination relative
to the input voltage? What connection does this have to question 7?
Now connect the circuit as shown in figure 3.
To Channel 1
L
To Os cillator
To Channel 2
C
R
To Scope Ground
Figure 3. Measurement of Resistor Voltage.
Measure and record the voltage across the resistor for at least 8 frequencies
above and below resonance. Use R=50 and C=0.1F.
Q9. Plot the voltage across the resistor as a function of frequency. This is called a
resonance curve.
Repeat the above procedure with R=10 and C=0.1F.
Q10. Make a similar plot as in Q9 and comment on any difference you may observe.