Objectives * Equipment * Activity
 


Electromagnetic Oscillations
With proper referencing, educators are welcome to use this for instructive purposes. Any other permission of publication (written or electronic) is denied without express prior consent from Dr. Philip Casabella.

OBJECTIVES :

  • To examine voltage oscillations of a simple LC circuit.

EQUIPMENT:
Function Generator
Diode
Capacitor Box
Oscilloscope
Inductor
Jumper Wires
 
ACTIVITY:

In this experiment you will examine voltage oscillations of a simple LC circuit. A function generator will provide a square wave which will start the oscillations, and an oscilloscope will be used to observe the oscillations. One inductor, of inductance L = 30 mH (30 ×10-3 H), and two capacitors, each of capacitance C = 0.10 mF (0.10 ×10-6 F), are provided.  The two 0.10 mF capacitors are labeled A and B on the box.

Connect the function generator to the oscilloscope. Take the output from the signal generator through the white cap, and connect it to the input of the oscilloscope. Connect the ground of the signal generator to the ground or black connector on the oscilloscope.  The white cap on the signal generator is attached to a diode. A diode is a device that passes current in one direction only.  It is inserted in the circuit to isolate the function generator, so it will not affect the oscillations of the LC circuit once the oscillations are established.  Make sure that you connect the ground of the function generator to the ground of the oscilloscope and the output of the signal generator to the input of the oscilloscope.  Set the generator to provide a square wave of about 50 Hz (it’s ok to be approximate here), and get a display on the oscilloscope.  The square wave will be used to start the oscillations of the LC circuit.  Make sure you can display a square wave before continuing.  The AutoSet button on the oscilloscope may prove handy.  Once you can see a square wave, adjust the Time/Div knob (and the trigger if necessary) until you can see the top portion of only one cycle.

Taking Measurements

1.   Connect the inductor and two capacitors as shown in the diagram.  Use the red and black connections on the inductor.

a)   Calculate the frequency at which the circuit should oscillate.

b)   Now you will use the oscilloscope to measure the frequency at which the circuit oscillates. When you apply the 50 Hz square wave, the circuit will oscillate.

What is your time/div setting? How many divisions are there in only one oscillation? What is the period of the oscillation? What is the frequency at which the circuit oscillates?

The frequency should be close to the calculated frequency. If it is not, find out what is wrong.

2.   Connect the inductor and two capacitors as shown in the diagram.

a)   Calculate the frequency at which the circuit should oscillate.

b)   Now, measure the frequency at which the circuit oscillates, as you did before.

What is your time/div setting? How many divisions are there in only one oscillation?What is the period of the oscillation? What is the frequency at which the circuit oscillates? (The frequency should be close to the calculated frequency. If it is not, find out what is wrong.)
3. Ratios:

a) What is the ratio of the equivalent capacitance of the circuit used in part 1, to that used in part 2? Express your answer as C1/C2.

b)   What is the ratio of the frequency of oscillation of the circuit used in part 1, to that used in part 2? Express your answer as f1/f2. Explain why this answer is not the same as the answer to part a) above (or is it?).

4.   Change the frequency of the signal generator.  Does the frequency at which the circuit oscillates change?  If so, how?

5. Does this LC circuit exhibit pure simple harmonic motion?  Justify your answer.
6.   All inductors possess an inherent capacitance, called a “parasitic” capacitance. The inductor acts as if it has a capacitance connected in parallel. Disconnect the capacitors and measure the frequency at which the inductor oscillates. Calculate the value of the parasitic capacitance.