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MIT Physics Demo -- Back "emf" in a Large Solenoid

File: Esidman-MITPhysicsDemoBackEmfInALargeSolenoid984.flv

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A large solenoid is connected to DC power through a knife switch.  When the switch is closed, the current builds up to approximately 30 amps.  When the switch is opened, the solenoid attempts to keep the current flowing, creating a huge emf which ionizes the air in a huge spark.

Read more about inductors and emf. 

Comments:

The phrase "solenoid attempts to keep the current flowing" is not very descriptive and somewhat incorrect. Once the current in the inductor reaches a steady 30 amps the magnetic field formed around the solenoid is stable. When the power source is removed the field collapses at very high rate of speed. This very fast moving magnetic field causes a reverse emf in the coil that creates the spark. -RH

— Anonymous User posted about 162 days ago
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