Demos: 6B-04 Field Inside a Solenoid

A loosely wound solenoid consists of a heavy copper wire threaded through a clear Lucite plate for overhead projection. When a large DC current is passed through the coil, iron filings sprinkled on the plate show that the field approximates that of a bar magnet and that the field inside the solenoid is uniform.

Directions: Place the apparatus on the overhead projector and adjust it so that the plate surface is in focus. Turn on the power supply. Begin sprinkling iron filings on the plate, watching carefully while the pattern develops. Tapping the plate periodically helps. As soon as you see a fully developed pattern emerge, stop sprinkling the filings because too many filings cause the pattern to be smeared and indistinct.

Suggestions for Presentation: For optimum effectiveness, we suggest that you show the single-loop pattern from 6B - 05 first. Then let the students think about what happens to the pattern as more loops are added. Next place the solenoid on the overhead and show the pattern. Accompanying this demonstration is an overhead transparency showing the field pattern for a single bar magnet. Point out the similarities. That the solenoid behaves like a bar magnet, but can be turned on and off, has many industrial and home applications. One is the solenoid in the starter motor of an automobile. A cut-away of a real starter motor is available to be shown to the class.

Applications: Relays of all sorts, giant electromagnets used in scrap metal yards, telephone speaker and earpieces.

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Last Updated: May 9, 2016 11:44 AM