5J10.20 Back-EMF: Light Bulb


Inductor back-EMF, Faraday's law, Lenz' law


When external current to an inductor is suddenly cut off, the inductor responds by temporarily producing a large EMF that can light a light bulb. In this way, a 6 V battery can momentarily illuminate a 120 V light bulb.



  • [1] Large inductor (~0.5 H)
  • [1] 40 W light bulb with socket
  • [1] 6 V battery
  • [1] SPST switch
  • [5] Banana cable
  • [4] Insulated alligator clip

Classroom Assembly

  1. Make sure switch is in the open position.
  2. Hook up banana cables.

Important Notes

  • The inductor is very heavy. Be careful when lifting it.
  • Do not short-circuit the battery for a long time.


  1. Close the switch for about 2 seconds.
  2. Open the switch.


Additional Resources


  • PIRA 5J10.20


  • Don't attempt this at home!

Last revised

  • 2019


Related AV

Related demos

  • Permanent magnet induction
  • Solenoid induction with a battery
  • AC induction

If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.