5K40.70 - Motor - Back EMF

Code Number:
Demo Title:
Motor - Back EMF
Back EMF
Area of Study:
Electricity & Magnetism
DC Motor, 40 Watt light bulb, and DC power supply.

Hook the motor, power supply, and light bulb in series and turn to 80 Volts. Only when the motor is under load will the light bulb glow.  When the motor is running at constant RPM with no load, the light bulb is off as there is not enough current to heat the filament and thus light the bulb. When you apply load to the motor more current is drawn in the circuit and the light bulb will glow.

  • Lou Turner, "A Simple Demonstration of Back emf", TPT, Vol. 47, # 8, Nov. 2009, p. 513.
  • Robert J. Whitaker, "Motors and Bulbs in Series", TPT, Vol. 47, # 6, Sept. 2009, p. 353.
  • "Figuring Physics", TPT, Vol. 41, # 5, p. 303, May 2003. 
  • Steve Dail, "Overheating Motor Demo", TPT, Vol. 41, # 8, Nov. 2003, p. 444.
  • James N. Boyd, "Power and Efficiency", TPT, Vol. 29, # 7, July 1991, p. 457.
  • Jack A. Kaeck, "Power Transfer in Physical Systems", TPT, Vol. 28, # 3, March, 1990, p. 214.
  • Robert Lehrman, "The Back EMF of a Motor", TPT, Vol. 21, # 5, May 1983, p. 315.
  • B- 295,  "Back EMF in Motor", DICK and RAE Physics Demo Notebook.

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