4.Main Content

experiments

Electromagnetic braking in a copper pipe

Demonstration

This demonstration always amazes 14-16 year olds, with whom I show some 'electromagnetic magic' and demonstrate electromagnetic forces. Post 16 you can use it to illustrate em 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

Technical notes

Clamp pipe vertically - usually two stands are needed, one on the floor and one on a bench or stable stool.

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

Safety

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

Read our standard health & safety guidance

Procedure

a 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.
 
b Drop non-ferrous metal from the top of the pipe as a control.
 
c Drop a magnet down the pipe and wait for 'wows'. Repeat if required using a stopwatch to time the magnet.

Teaching notes

1 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.
 
2 Does the magnet reach a terminal velocity? This is a question to investigate.
 
3 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.