Eddy Current – Tank | Te Taika Konukura

Introduction

Abstract
A strong magnet rolls down a copper plate, turning toward the centre of the copper plate as eddy current drag slows the inner face of the magnet more than the outer face.

Principles Illustrated
A tank or bulldozer turns by slowing or even reversing one of its tracks. To turn left, the left track is slowed. A magnet rolling down an inclined conducting slab at an angle toward the edge will generate eddy currents in the slab. But the eddy current drag on the inner face of the rolling magnet will be larger than that on the outer face, and the magnet will turn inward.
This resource is an extension of the Eddy Current Drag resource.

Content

Video

Instructions

Students like this demonstration because it seems like magic and yet can be understood qualitatively even by students in junior science. The interaction force between the eddy currents and the magnet is large enough to be obvious only with very strong magnets. These are rare earth magnets model N42 from MacMill Magnets. It is essential to use thick slabs of copper to maximize the effects as only those field lines cutting through the copper will contribute. Aluminium works poorly as it is not a good enough conductor. Unfortunately these copper slabs are rather pricey.
You do not need magnets as large as the ones in this video. It works well enough, but is less dramatic, with smaller rare earth magnets. A 30 mm diameter, 10 mm thick rare earth magnet is adequate.

Other Information

Safety

You really can get a serious injury from large rare Earth magnets. They will fly toward another magnet or even a steel object and will be moving fast enough to shatter when they hit. The chunks that fly out of these collisions tend to have sharp edges. You don’t want to get your fingers between the magnet and some steel! See the video in the Eddy Current Drag resource for an example collision. People with pacemakers or other medical electronics should never approach these very strong magnets! Swallowing a rare Earth magnet is particularly dangerous and they need to be kept away from small children.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

Related Resources

We have lots of eddy current resources. See in particular Eddy Current Drag.

Notes, Applications, and Further Reading

Eddy current drag is used to make brakes in certain applications. See for example the following exchange on physlink: eddy currents brakes. Also see a discussion of eddy currents in Wikipedia.

Credits

This teaching resource was developed by the Te Reo Māori Physics Project with support from

• Te Puni Kōkiri
• The MacDiarmid Institute
• Faculty of Science, Victoria University of Wellington
• School of Chemical and Physical Sciences, Victoria University of Wellington

• The New Zealand map shown on the poster frame above is used with permission from www.nz.com.
• The strong magnets were supplied by the Magnetic Resonance Physics research lab founded by the late Prof. Paul Callaghan. His lab is still operating and is located on the Victoria University of Wellington campus.
• This teaching resource was developed in collaboration with Mabel Stewart, a New Zealand Science, Mathematics and Technology Teacher Fellow, 2008, hosted by Victoria University School of Chemical and Physical Sciences. Mabel teaches at Bishop Viard College in Porirua, New Zealand.