5L30.25 - DC vs. AC Voltages and the Meaning of Root Mean Square

Code Number:
Demo Title:
DC vs. AC Voltages and the Meaning of Root Mean Square
Root Mean Square
Area of Study:
Electricity & Magnetism
12 volt bulb, 0 - 20 v, 8 amp. power supply, 24 volt transformer, 6 Volt Battery, Variable Pot., Oscilloscope, Variac, DC Power Supply, Light Bulbs, Isolation Transformer, Multimeters (Keithley) (2).

Arrange the apparatus so the the oscilloscope is monitoring the AC or DC current.  Attach the light bulb to the AC current and observe the trace on the oscilloscope.  Then unhook the bulb from the AC and hook onto the DC leads and again observe the trace.  The AC trace will show the usual sine wave as the voltage is increased while the DC will show a straight horizontal line which moves upward on the oscilloscope as the voltage is increased.

NOTE: Make sure you use the center tap on the 24 volt transformer so you don't accidentally burn out the 12 volt bulb.

Set the demo up as shown.  Turn the voltage of the AC and DC power supplies to the same voltage as observed by the light bulbs and as read on the meters.  Measure the voltages with the oscilloscope and see that the AC voltage is actually larger when measured peak to peak than the observed RMS measured voltage.

The demonstration in picture 3 is used to show that DC voltage gives a straight line and not a sine wave.  Turn the variable pot and observe how the trace moves on the scope.

  • Patrik Vogt, Stefan Kuchemann, Jochen Kuhn, "The Flashing Light Bulb: A Quantitative Introduction to the Theory of Alternating Current", TPT, Vol. 59, #2, Feb. 2021, p. 138.
  • Donald S. Ainslie, "Showing  the Ratio of Ac Peak", TPT, Vol. 19, # 8, Nov. 1981, p. 551.
  • Yaakov Kraftmakher, "1.14 Electric Power in AC Circuits", Experiments and Demonstrations in Physics, ISBN 981-256-602-3, p. 54.
  • Curt Suplee, "Electricity in motion", Everyday Science Explained, National Geographic, p. 90.
  • W. Bolton, "Alternating Current and Meters", Book 4 - Electricity, Physics Experiments and Projects, 1968, p. 55-56.

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