Demonstration Description: A coil with many turns (number of turns unknown) is connected in series to a small light bulb. When the coil is passed between the poles of a magnet the light bulb glows. It is possible to burn out the light bulb.
Demonstration Instructions: Equipment:
Small magnetron magnet and coiled wire attached to a light bulb.
Move the coil in and out of the magnetic field and observe the brightness of the bulb. If done in rapid succession this may burn out the bulb.
The area through which the magnetic field "flows" increases as the coil is placed in the field. Thus, the magnetic flux changes over time, inducing an emf, which in turn induces a current. The current always flows in a direction that "cancels out" the change in magnetic flux. When the loop is fully in the magnetic field there is no current. When it is pulled back out of the field the induced current is in the opposite direction, this time opposing the decreasing magnetic flux.
Of course, the light bulb doesn't care about the direction of current, just the magnitude of the induced emf. The bulb will burn brighter if the coil is inserted with rapid succession because this will create a greater change in magnetic flux.
See chapter 29, starting page 993, University Physics, for information on electromagnetic induction.
Young, Hugh D., Roger A. Freedman, and A. Lewis Ford. Sears and Zemansky's University Physics: With Modern Physics, 12th ed. San Francisco: Pearson Addison-Wesley, 2008.