Demos: 6D-01 Induced EMF
A solenoid coil is connected to a projectable microammeter. A magnetized bar is slid in and out of the coil. The magnitude and sense of the meter reading is noted when (a) the bar is pushed in slowly, (b) the bar is pushed in quickly , and (c) the bar is pulled out both quickly and slowly. It is observed that the magnitude of the reading is associated with how fast the bar is inserted or removed and the sense of the reading is associated with whether the bar is being inserted or being removed.
Directions: With the ammeter projected onto the screen and focused, move the bar into the coil. As the bar enters there will be a movement of the needle on the ammeter. Hold the bar stationary within the coil and observe that the needle does not deflect. Then quickly insert and then quickly remove the bar and note that the needle moves in opposite directions.
Suggestions for Presentation: Emphasize that the induced emf in the coil arises from a changing magnetic field. If the bar is held steady, no matter where it is located, no induced emf results because the field is constant. Moving the bar causes an induced emf, but ask the students why moving the bar constitutes a changing field. (Because the magnetic field is not uniform around the bar, moving it closer to and farther away from the coil changes the field configuration at the coil.)