Magnetic Force
Electricity and Magnetism

# Magnetic train

Practical Activity for 14-16 16-19

Build a train with a cell, two magnets and a coil to test their understanding of electromagnetic forces and Lenz’s law.

## Equipment

• AA cell
• A small nut (that fits over the positive terminal of the cell)
• Two spherical neodymium magnets with a diameter of 15 mm or greater
• Bare copper wire
• 50 cm long pole with a diameter similar to magnets (eg broom handle or copper tube)
• Sticky or duct tape

## Preparation

Before the lesson wrap the copper wire around a pole to make a 40 to 50 cm long coil.

You can use standard cylindrical neodymium magnets to build your train, but they may get caught in the coil. For a more reliable demonstration, source spherical magnets.

## Procedure

1. Place the small nut over the positive terminal of the cell. Attach a magnet.
2. Attach a magnet to the negative terminal of the cell to complete your train. Like poles of the magnet should face each other.
3. Insert train into coil. If it moves backwards, turn the train around. If it doesn’t move at all, turn one of battery around. Explain that like poles need to be facing the cell for this demonstration to work.
4. Make a circular track shape out of the coil, joining the ends by slotting the end coils into each other. Secure your track to the bench with tape.
5. Separate the end coils, insert the train and re-join the coil again. Your train should go around in a circle.

## Discussion prompts

• What part of the coil does current flow through?
• Why does the train accelerate?
• Why does it reach a constant speed?

## Explanation

The train consists of two permanent magnets at either end of a cell. The magnets touch the bare copper wires of the coil, thereby completing the circuit so that there is a current in a section of the coil.

The current produces a magnetic field inside the coil which exerts forces on the two permanent magnets. It attracts the N pole of the left hand magnet and repels the N pole of the right hand magnet. These two forces act in the same direction on the train, and so it accelerates to the right.

If students ask about forces on the S poles of the magnets, explain that these will be in the opposite directions to the one on the N poles. These forces will be weaker than those on the N poles because the S poles are outside of the region where a current flows. The resultant force on each magnet will be to the right.

The train quickly reaches a constant speed around the track. This is because the moving magnets induce a magnetic field in the coils that acts to oppose the motion of the magnet (an example of Lenz’s law). The train reaches terminal velocity when the forces accelerating it forwards are balanced by the forces arising from electromagnetic induction.

## Learning outcome

Students explain how a simple magnetic train works.

This experiment was safety-checked in March 2020.