Exp 11: Procedure

Procedure:

1. Set up the circuit shown in Fig. 3. The signal generator is to be used for the applied emf and the oscilloscope will be used to measure and I_o. The single pole double throw (SPDT) switch allows the oscilloscope to be connected either across the output of the generator (position A) or across the resistance R (position B). Note the ground of the generator and the ground of the oscilloscope should be connected together. The vertical input of the oscilloscope is connected to the pole of the switch. Use a standard 10 ohm resistor for the resistance R. It should be approximately the same as R_L .

2. Check the calibration of the oscilloscope (see pg. 61-sec. II).
3. Set the frequency of the signal generator to =100 Hz and throw the switch to position A with the oscilloscope input scale (3) set on the 1 scale. Adjust the amplitude of the signal generator to get as long a line as possible. Measure the amplitude of the applied signal. The output of the signal generator changes with frequency, so this adjustment has to be made for every frequency used. Now throw the switch to position B and read the p-p voltage V_R across the resistance R. Note the amplitude of the voltage across R will be V_R/2. It may be necessary to reset the VOLTS/CM scale to a smaller range to be able to read this voltage. The amplitude of the current will then be I_o = (V_R)/(2R) . Repeat these measurements for several (10) frequencies between 100 and 3500 Hz. Tabulate your data.

4. Calculate and plot as a function of . Determine the slope of this line and calculate L (see equation (2)).

5. To the circuit of Fig. 3 add a capacitor of about 10^-7 F in series with L. Make sure the oscilloscope is still calibrated. The circuit in Fig. 4 will now be used to find L from the resonant frequency. For this set of measurements it is important to check the output of the generator at each point.
6. For a coil with a large number of turns, start at a frequency of about 500 Hz. If a coil with a small number of turns is used, start at about 2 x 10³ Hz. Adjust the emf so that it is about 3V as before and measure V_R . Change the frequency by intervals of 500 Hz over the frequency range from 500 to 1 x 10⁴Hz (large coil) or from 3 x 10³ to 1.4 x 10⁴ Hz (small coil), determining V_R for each frequency. Take points closer together when V_R is near its maximum value. Tabulate your results.
7. Plot the value of and note the maximum. This is . Repeat the procedure with R = 30 ohms Note that remains the same. Using this value for and given value of C, calculate the value of L and compare with that found in the L-R circuit.