PURPOSE: To demonstrate the timing of current in a switched inductor circuit by asking students a question.

DESCRIPTION: An inductor and light bulb are connected across a battery with a switch. In parallel with this inductor and light bulb is a second identical light bulb. Q: Predict what the light bulbs will do when the switch is closed. A: The separate light bulb will light immediately, but the light bulb in series with the inductor will be delayed in lighting until the current reaches a high enough level. The delay time is related to the L/R time constant of that leg of the circuit. Click your mouse on the photograph above to see a video of the action.

SUGGESTIONS: See Question of the Week #90 and Question of the Week #91 for information on using this demonstration to enhance class involvement.


EQUIPMENT: 7.5 volt battery, knife switch, two 12-volt light bulbs, large (approximately 0.92 henry) inductor.

SETUP TIME: 20 minutes.

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