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Flying circus of physics

The smallest electric train

Wednesday, April 01, 2015


The smallest electric train
Jearl Walker
April 2015   Small, strong magnets are snapped onto the two ends of a small battery and the assembly, said to be train, is then inserted into a copper coil.

Amazingly, the train races through the coil until it partially emerges at the far end. What drives this electric train? Well, the energy source is obviously the battery, but what is the force that propels the train?

First let me caution that this train is notoriously difficult to build. Each of the several YouTube videos about the trains have lengthy comment sections where people vent their frustrations on being unable to replicate a video. The magnets must be neodymium button magnets, which are small, light-weight, and provide strong magnetic fields. Also, they conduct electric current, which is essential for the train operation. The coil must also be conducting, should be wider than the train but not too much wider, and the spacing between adjacent turns must be not too wide or too narrow (that is, it has to be “just right”). Finally, the number of magnets on each end of the battery must also be “just right.” One video suggests 16 gauge, bare (uncoated) copper wire and a 1.5 volt battery. The coil’s turns must not be so closely spaced that the coil is effectively a cylinder but they cannot be too widely spaced either. Here is a video that visibly describes how well a train will move with variations in these parameters. long video but shows results of experiments

The force that drives the train is a magnetic force that acts on a current (due to the battery) in a magnetic field due to the magnet). When a charged particles such as electrons move through a magnetic field, a deflecting magnetic force acts on the electrons. When a steady stream of electrons forms a current in a magnetic field, a collective deflecting magnetic force acts on the current. Normally the current is in a wire and the collective force causes the wire to move in a direction that is perpendicular to both the direction of the magnetic field and the direction of the current. (In the special cases in which these two directions are the same or opposite, there is no magnetic force.)

In the train’s situation, the coil provides a conducting path from the positive terminal of the battery to the negative terminal.

However, to reach the coil, the current leaving the positive terminal must pass through the attached magnet and, more important, through the magnetic field lines inside the magnet. Those field lines flare near both the south (S) and north (N) poles of the magnet. Here is my rough sketch of the lines.

A far better picture can be seen at

To reach the picture, first type in the coordinates for a point of measurement, say 2 inches for x and 2 inches for y. Then click on “Calculate.” Then click on “See the Magnetic Field.”

The current from the positive terminal travels through this field to a contact point on the coil (wherever the contact happens to be at a given instant). The current that reaches that contact point has been continuously deflected by the field along its way there. I need a three-dimensional sketch to show the curved path but this sketch is the best I can do in two dimensions.

The result is that the current (and thus magnet) experiences a magnetic force that is tilted in the forward direction. That forward component of the force is the force that propels the train in the forward direction.

A similar arrangement occurs at the back of the train but there the current enters the attached magnet and curves toward the negative terminal (rear face of the battery).

If we replace the coil with a copper tube, the current would still leave the positive terminal and travel through the magnet, but now it would simply dive to the bottom of the magnet where the magnet contacts the tube. There would again be a magnetic force acting on the current but it would not have a forward component. Thus the train would not move.

More videos:


Are you looking for the stories from last five months? They are in the archives:

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