5J10.20 • Inductance: Back EMF

Location: Cabinet 3

Photo of the Demonstration at the University of Texas at Austin.

Instructions / Notes:

• A large 3 Henry inductor is connected in parallel to a 37 volt DC light bulb circuit using the large knife switch which is at the center of this circuit board.
  • The circuit board is really two demos on board.   Only one side of the circuit board is the DC Back EMF demo.
• With the 3 Henry inductor connected to the circuit - the bulb glows dimly until the small knife switch is opened.
  • At this point, as soon as the small knife switch at bottom portion of the demo is opened - the magnetic field of the inductor collapses and bulb flashes brightly for a brief moment.
  • Also notice the arc/flash that occurs at the small switch at the moment that it is opened.
• If the large the knife switch in the center of the circuit board is opened - then all you are left with is a 37 volt DC light bulb circuit.
• The light bulb flash can be seen more brightly when the Small Variable Resistor is in its highest position - low resistance.

Photo below shows someone opening or closing the center knife switch.

• Once again - when the center knife switch is in the opened position - the coil is no longer connected to the circuit board.
• With the center knife switch in the open position - the light bulb will exhibit a normal on/off behavior when the smaller knife switch is opened or closed.
• In other words - no brilliant flash is observed, when opening the small knife switch, if the inductor is no longer part of the circuit.

• 5K10.40 the Iron Core in Mutual Inductance demonstration is a good companion demo to show another example of a back EMF produced by a collapsing magnetic field around an inductor.

Last updated on June 8, 2026