 BCIT Physics Department

# Demonstration Manual

## Electricity and Magnetism

### DEM 52B - AC resonance damping in L, R and C circuits. (Simple Harmonic Motion Damping).

#### Objective:

1. Use an oscilloscope to show the effects of varying the values of L, R and C have on the damping of an AC signal (square wave).

2. To measure a value for the damping constant and the period T of the damped resonant circuit, from the scope screen, for fixed values of L, R and C.

3. To show how one can obtain a value of the Inductor L (or the Capacitor C), by measuring the period of the damped resonant circuit.

#### Apparatus:

• Wavetech Function Generator Model 182A
• Solenoid (from lab EM 11 - Hall Effect Probe equipment set) and 1.5H Inductor
• STARK Resistance Substitution Box RS1
• STARK Capacitor Substitution Box CS1
• Tektronix 2205 (or similar) oscilloscope #### Method:

Set up the circuit as shown: • Set the function generator to 200 hz square wave signal (Output: Hi, 20V p.p., 0.2A)
• Set Resistance Hi/Low switch to low and select about 2 to 200 ohms.
• Select Capacitor value of 0.015 microfarads
• On the scope, set the amplitude scale to 2.0 V/div (between the "1x probe" markers) and set the time scale to 50 microseconds/div on the connected channel.

#### Note:

On the scope screen, you should see a damped oscillation of the LRC circuit during the time of one pulse of the square wave signal provided by the AC generator.
(You can temporarily increase the time scale to demonstrate that this damping happens with each half cycle of the square wave signal - or 400 times a second).
You may vary the values of R, L and C to show how these factors effect the damping and period of the resonance. #### The following values can be calculated from measurements taken from the scope screen:

1. Using the solenoid and the settings above to show how to measure the height of the first peak , the height of the fifth peak , and the time difference between these two peaks . Then show how to calculate the damping constant:

(exponential decay of amplitude with time.)

2. See Lab VW4 for Electronics, Part C

3. Show how to measure and calculate the period of the damped oscillation from the oscilloscope screen. .  (Note: There are 4 periods between and . peaks).

4. Show how to calculate the Inductance L of the solenoid with the selected capacitor value and the period measured off the oscilloscope screen, in step 2. (or one may calculate the C value for a given L value). 