Electricity and Magnetism > AC Circuits DCS# 5L40.20

POWER IN AC CIRCUITS

ac power

APPARATUS
function generator/amplifier 202-17-A2
radiometer 202-21-B
6 V flashlight bulb 202-19-E3
oscilloscope 202

DESCRIPTION
The idea is to use a sinusoidal current to drive a lightbulb for frequencies varying from 10 kHz down to zero.  At very low frequencies we see heating due to a direct current, with the bulb glowing brightest twice per cycle, obviously not caring which direction the current is flowing.  As f increases, the bulb flickers, eventually so fast that it appears to be constantly on, but obviously not as brightly as it was during the peaks of the slow cycles.  This illustrates the importance of the rms, or time average, power.
-JHS

Use the scope to display the voltage across the bulb and power as measured by the radiometer.  The two will be out of phase at higher frequencies because of the inductance of the bulb filament.

If using the digital scope, use Horizontal Scan Mode:

The oscilloscope displays waveform updates from the left to the right of the screen and erases old points as it displays new points.  A moving, one-division-wide blank section of the screen separates the new waveform points from the old.  The oscilloscope changes to the Scan acquisition mode when you turn the SEC/DIV knob to 100ms/div or slower, and select the Auto Mode option in the TRIGGER Menu.  To disable Scan mode, push the TRIG MENU button and set the mode option to Normal.

   
Connect a second identical bulb to a DC source set to the same rms voltage as the first, to show that their average brightnesses are the same.