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Al Tobias (wat4y) - Office: Gibson S123 & Physics 218, (434) 924-0538

Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography


Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.

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 Electric Fields And Potential
 Current and Resistance
 Electromotive Force And Current
 DC Circuits
 Magnetic Materials
  video  - Back EMF in an Inductor
  video  - LR Circuit w/ Function Generator
 LR Circuit w/ Battery
  video  - Ringing LC Circuit w/ Function Generator
 Ringing LC Circuit w/ Battery
 Complex Circuits
 Semiconductors And Tubes
 Transmission Lines and Antennas

 video  - Back EMF in an Inductor


To show that a very large back EMF is produced in an inductor when the current changes rapidly.


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  DEMO VIDEO DOWNLOAD w/ sound / 854x480 / 5.69Mb/s / 75MB size  

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  DEMO VIDEO DOWNLOAD w/ sound / 1920x1080 / 24.19Mb/s / 260MB size  

  DEMO VIDEO DOWNLOAD w/ sound / 854x480 / 5.69Mb/s / 64MB size  

After the electromagnet has been energized, the knife switch can be opened and a large arc can be observed. Be very cautious of the high voltage and heat of the spark. Main spark is produced at break rather than make.


The electromagnet coil pair has a measured inductance of roughly 3 Henries and wire resistance of 15.7 ohms.

For more information on high voltage sparking when a current carrying inductor is suddenly cut off from its power source, see Giancoli, Physics for Scientists & Engineers, 4th Ed., Chapter 30, Problem 27.


  • 1 110 volt DC source
  • 1 12 V Car Battery
  • In-Line Knife Switch
  • Induction Coil w/ Solenoid Interrupter
  • Large DC electromagnet
  • Solid State Induction Coil