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Lesson: Chapter 13 — Electromagnetism and electromagnetic induction

The force on a current in a magnetic field 1 of 4

A wire that moves itself

You have learned that a current in a conductor sets up a magnetic field around it. Now, with activity 13.3, find out whether a force acts on a conductor carrying a current when it is placed in a magnetic field.

Activity 13.3

You will need: a U-shaped (horseshoe) magnet, a piece of conductor, two brass or other conducting rods, two cells

  1. Put the horseshoe magnet on a table. Hold the two brass rods as in figure 13.7, supported by a piece of thick cardboard with two holes in it. Connect dry cells E and switch S across the ends A and D of the rods.
  2. Lay the conducting wire BC across the two rods, between the north and south poles of the magnet.
  3. Press switch S to supply the current. It flows from the cell along one rod in the direction AB, through the wire BC, and back to the cell along the other rod in the direction CD.

Figure to be added

Figure 13.7 — two brass rails held in a block of cardboard run into the gap of a U magnet; a short wire BC lies across them between the poles; a cell E and switch S are joined to the ends A and D.

The observations

  • With the current on, the wire BC rolls along the rails away from the magnet (to the right).
  • Swap the battery terminals to reverse the current: BC moves into the magnet (to the left).
  • Turn the magnet over so its poles swap top and bottom: BC now moves opposite to both the directions above.

When a current is passed through a conductor placed in a magnetic field, the conductor moves because a force acts on it. The direction the conductor moves shows the direction of the force. In the activity the field and the current were arranged at right angles to each other, and the movement happened at right angles to both.

Why there is a push

From the last pages, the current in BC makes its own field circling the wire. That field and the magnet's field add together on one side of the wire and partly cancel on the other. The wire is pushed from the stronger side towards the weaker, like being squeezed out — which is why the effect is sometimes called the catapult effect. Reverse either field and the stronger side swaps, so the push reverses. Reverse both and the push stays the same.

A current in a field feels a force

A wire carrying a current across a magnetic field is pushed at right angles to both. Reversing the current or the field reverses the push.