Experiment 12: An AC Circuit
PART ONE: AC Voltages.
Set up this circuit. The three boxes are a resistor, inductor,
and capacitor. Which one is which is not important. Use R
= 500 Ω, L = 5.0 mH and C = .01 μF. No black wire is
needed on EXT TRIG.
Set the frequency of the function generator at 60 Hz and
adjust its attenuation knobs for a 2.0 V amplitude as measured by the oscilloscope.
Connect a digital voltmeter set for AC volts to the same signal the oscilloscope is receiving.
Record the reading. In your conclusion, explain why this is not 2.0 V.
PART TWO: RLC Series Circuit.
In an AC circuit, we wish to see if calculated and observed voltages match, and if the phases of the
voltages are as predicted. A resistor, inductor and capacitor are connected in series across a
function generator. The amplitude of the voltage and the phase of the voltage across each of these is
observed using an oscilloscope.
Calculations. In the same circuit from part 1, the emf will be set for Vmax = 2.0 volt and f = 15,000
Hz. Find the impedance of the circuit, the current, and the potential difference across each of the
three circuit elements: the capacitor, resistor and inductor. (Solve this problem mathematically.)
Experimental Procedure. Now, check your results. Just remove the meter and otherwise it is the
circuit from part one. Adjust the function generator for the Vmax = 2.0 volt, f = 15,000 Hz output.
(Normally, V and I stand for RMS values. However, amplitude is easier to read from an
oscilloscope and behaves the same way. So, if you set the source for a 2.0 V amplitude, the
amplitudes of VL, VC, and VR should match what you calculated.)
Push the EXT TRIG button, located under TIMEBASE. The triggering circuits are what make the
spot start moving across the screen, drawing the trace. When triggering externally, using the signal
from the function generator, the spot will wait until the function generator reaches a certain voltage.
The scope will then start drawing the graph from whatever point the signal has reached at that
moment. Signals with different phases will be at different points in their cycles, so this lets you see
the phase differences between R, L and C.
The frequency dials on some of the function generators are a little off. Check that the period is 6 2/3
cm long at 10 s/cm. If not, ask if it's the function generator or scope which is miscalibrated.
When the function generator has been set to 2 V, move the scope's input wire so that it now displays
VL, VC, or VR, as shown below. If the start of the graph is beyond the left edge of the screen, adjust
x position so you can see it. If it doesn’t start right on a peak, a valley or the centerline, push the
AT/NORM button, then adjust the blue LEVEL knob so it does. (Then don't touch this again.)
- Record the peak voltage, being sure variable y amplitude (red knob) is fully
counterclockwise.
- Sketch the trace, paying attention to where in the cycle it begins at its left.
Next, switch the circuit element you just measured with one of the others by reconnecting them in
each other's places. Keep the scope’s black connected to the function generator’s black. Keep the
wires arranged the same way while switching the locations of R, L and C. Repeat the same
measurements. Repeat for the last of the three.
Get the instructor’s approval of your sketches before going on; there are often problems.
In your conclusion,
1. Comment on whether your measured and computed values for the three voltages agree with
each other if the measured values have a 10% uncertainty. (A typical tolerance when
components like these are manufactured.)
2. Comment on whether the voltages had the expected phase relationship:
a. Should VL lead or lag VR? Is this what you see?
b. Should VC lead or lag VR? Is this what you see?
PART THREE: Resonance
You will see if a circuit’s calculated resonant frequency matches the observed resonant frequency.
Keep the same circuit, except switch to a 47 Ω resistor. (Less resistance means more current around
the resonant frequency, making the peak stand out better on your graph.) Have the scope measuring
the voltage across the resistor. By I = V/R, the resistor's voltage is proportional to the current,
which is what we're really interested in.
Take data for a graph of resistor voltage as a function of frequency:
a) Find the frequency where the amplitude is largest. Record f from the dial on the function
generator. Record VR from the oscilloscope.
b) Record the same information for a frequency just 2 or 3 kHz above this, and again about 2 or
3 kHz below. We want this part of the graph to be clearly defined.
c) Record the same information at more widely spaced frequencies to fill out the range 100 Hz
to 100 000 Hz.
Plot the graph. Read the resonant frequency from it. Also, calculate the resonant frequency from L
and C. If your observed (graph) value has a 10% uncertainty, do these agree?
PHY 132
Report on Experiment 12: An AC Circuit
Part 1: Amplitude of graph = ____________
Part 2:
R=
Meter V = ____________
L=
Calculations:
C=
Measurements:
XL =
XC =
Z=
I=
VR =
VR =
VL =
VL =
VC =
VC =
Phase of voltage:
Part 3: f
I
_
a) ______________________
From graph, fR is _________________
b) ______________________
Calculate fR:
______________________
c) ______________________
______________________
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