Electromagnetic braking in a copper pipe
Practical Activity for 14-16
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
- Data logger plus computer
- Coil to act as sensor
Health & Safety and Technical Notes
Do not allow pupils to stand on the bench where they may fall over the tube.
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).
- 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.
- Drop non-ferrous metal from the top of the pipe as a control.
- Drop a magnet down the pipe and wait for
wows. Repeat if required using a stopwatch to time the magnet.
- 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
opposesthe 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.