Demos: 6D-07 Eddy Currents

A permanent magnet is mounted such that the field in the pole gap is horizontal. A thin aluminum sheet is dropped between the poles of the magnet. It will not fall freely as long as any part of the sheet is in the B-field. Other materials made into sheets are also dropped through the field, producing falls of various rates.

Directions: Hold the sheet of material so that the lower portion is between the poles of the magnet. Release the sheet. Repeat, using other materials, including an aluminum sheet with slots.

Suggestions for Presentation: Drop the aluminum sheet through the field and point out how slowly it falls. Ask the students to speculate on why this happens. Suggest that competing fields may be at work here. What would be the source of the counter field? Currents produce B-fields. Where might there be currents in this situation? Drop a sheet of cardboard or other non-metallic material through the field. There is no effect on the fall rate. Let the discussion lead toward the idea that metals are necessary and that it might suggest currents are set up in the metals. This should clue them into the eddy current idea and Lenz’s Law.

Drop the slotted aluminum plate through the field. Note that when the slots are in the field, the fall rate quickly increases. Let this lead to the concept of disruption of the eddy currents.

Applications: Magnetic Braking

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