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Practicals

Lenz's Law
AKA: Electromagnetic Braking

Summary

A magnet is dropped through a copper pipe, whereupon the rate at which it falls slows down dramatically.

Operation

This Practical is usually conducted as a demonstration using a Lenz's Law kit or can be made into a class practical using standard copper gas pipe and magnets. As such some preparation time is required if a class set of the equipment is required.

Lenz's Law kits are available through some lab suppliers and usually consist of a length of copper pipe (sometimes with a section cut out so that you may see the magnet in motion), a magnet whose diameter is just slightly smaller than that of the tube so that it may be dropped down the tube easily and a ‘dummy' slug which is usually the same size and shape as the magnet but made of metal. Kits are simple to make, the only difficulty is obtaining the correct size and shaped magnets but many suppliers now stock suitable types.

Equipment typically required (per set):

  • Length of copper pipe, approx 30-50cm in length or longer depending upon level of practical
  • Cylindrical magnet which fits down the tube without difficulty
  • A dummy slug or metal cylinder (non-ferrous)
  • Tray of sand to cushion the fall of the magnet
  • Stopclock (optional)
Firstly the dummy slug is dropped through the pipe. It should fall through the tube at an expected rate (fairly quickly). The magnet is then dropped through. Students should be able to see the difference in that the magnet will brake as it falls through the pipe.

This electromagnetic braking is caused by eddy currents being induced in the copper pipe by the falling magnet. The pipe effectively acts as a one-turn coil. The resultant induced current creates a magnetic field which opposes the changes that causes it. This theory is known as Lenz's Law.

This equipment can be used as an end of term fun demonstration, for higher ability groups investigating electromagnet induction or with post-16 groups, learning about Lenz's Law itself.

If students intend to investigate the rate at which the magnet slows, they may ask for datalogging equipment including light sensors so that the duration of the drop can be accurately measured. Stopclocks provide an alternative where accuracy is not so important. Also the length of pipe may be varied to see how it affects the rate of electromagnetic braking.

Safety

 CautionThe contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.