Demos: 6C-07 Energy Stored in an Inductive Circuit

An electromagnet is connected to a DC source through a resistance that keeps the voltage across the terminals of the electromagnet below the source voltage when the battery is engaged. A light bulb is connected in parallel with the electromagnet. When the switch is closed, the bulb lights and energy is stored in the inductor. When the switch is opened, the battery is out of the circuit and the bulb and inductor are in series. The self-induced emf in the coil causes a greater potential difference across the coil than was there previously and the light bulb flashes brighter before going out.

Directions: Press the momentary-on switch and hold it in the closed position for a moment or two. Then release the switch quickly. A large self-induced emf will occur across the coils.

Suggestions for Presentation: First, disconnect one wire from the coil so that only the bulb is in the circuit. Close the switch and make note of the brightness of the bulb. Ask what will change, if anything, if the coil is placed in the circuit, parallel with the bulb. Then close the switch and hold, showing that the brightness does not change. But when you release the switch, the bulb flashes brightly before going out. Discuss.

Applications: Large currents develop in some switch circuits when the switch is opened, causing an arc.

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