Electricity and Magnetism ** 5L10.17 ** AC Circuits

Impedance

AC circuit with only an R, only a C, and only an L

Equipment:

• PASCO PI-9587B fequency generator - shelf 8D
• Cornell-Dubilier decade resistance box - shelf 11A
• Aerovox decade capacitance box - shelf 11B
• Sprague decade inductance box - shelf 11D
• 1-ohm resistor - shelf 11D
• oscilloscope - in front of shelf 13
• 3 banana-to-banana cables
• 2 scope cables, banana to BNC
• video camera and holder - file cabinet between shelves 3 and 4

Theory: In an AC circuit with just a resistor, the current and voltage are in phase, and the resistance does not depend on frequency. With just a capacitor, however, the voltage lags the current by 90°, and the impedance decreases as the frequency, and capacitance, increases. With only an inductor, the voltage leads the current by 90° and the impedance increases as the frequency, and inducatnce, increases.

Description: If the following parameters are used, the capacitor or inductor can be sustituted for the resistor with needing to adjust the power supply or scope settings. Set the amplitude on the frequency generator to about halfway, and the frequency to 1500 Hz. R = 300 ohms; C = 0.04 microfarads; and L = 0.04 H. With any of the three components in place, the value of R, C, or L can be adjusted to see the effect on the current; the dependence of current on frequency can also be seen by changing the frequency.
In a small room, the students can view the scope directly. In a larger room, the video camera can show the scope screen on a TV monitor, or all 6 monitors in room 107.

Setup time: 20 minutes

References: (available in the demo room)