Lenz's Law
Electricity and Magnetism

Electromagnetic braking in a copper pipe

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

This demonstration always amazes 14-16 year olds, with whom it can be used to show some electromagnetic magic and demonstrate electromagnetic forces. Post-16 it can be used to illustrate electromagnetic induction and Lenz's law.

Apparatus and Materials

  • Copper pipe, 2 m, 16 mm diameter with smaller magnets
  • Copper pipe, 2 m, 22 mm diameter with larger magnets
  • Magnet, cylindrical rare earth (e.g. diameter 1 cm, 7 mm long)
  • Non-ferrous metal, pieces, of similar shape and size
  • Bucket or container of sand to cushion impact of magnet at floor level
  • Stopwatch
  • Data logger plus computer
  • Coil to act as sensor
  • Leads

Health & Safety and Technical Notes

Do not allow pupils to stand on the bench where they may fall over the tube.

Read our standard health & safety guidance


The photo comes from the work of a year 13 girl, Bethan James, who has been investigating the phenomenon.

Note that students should not work with bags on the bench (as shown in the photograph).

Procedure

  1. Clamp copper pipe vertically with sand bucket (or similar) underneath, so that the bottom of the pipe is about 20–30 cm above the sand.
  2. Drop non-ferrous metal from the top of the pipe as a control.
  3. Drop a magnet down the pipe and wait for wows. Repeat if required using a stopwatch to time the magnet.

Teaching Notes

  • The falling magnet induces eddy currents in the copper pipe (which acts effectively as a single one-turn coil). The magnetic field created by induced current opposes the change that caused it - this is Lenz's Law.
  • Does the magnet reach a terminal velocity? This is a question to investigate.
  • With the larger tube and cubical magnets, watch the magnet tumble as it falls down the pipe - lots of scope for student investigation.

This experiment was submitted by David Grace who teaches at Ysgol Y Preseli, Crymych, Pembrokeshire.

Lenz's Law
is expressed by the relation V=-N(dΦ/dt)

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