## The effect of a magnetic field on an electric charge

The fact that electric currents create magnetic fields establishes a link between electricity and magnetism. It could perhaps be expected that magnetic fields in turn should exert a force on electric charges. But the experimental evidence does not support this idea:

A stationary electric charge is not affected by a magnetic field.

This is by no means the whole story. Such immunity of charge to magnetism is not complete, and applies only if the charge is 'sitting still' in the magnetic field!*

Ampere's discovery that parallel currents repel one another and anti-parallel currents attract one another provided the first clue that something new happens when electric charges move. The mechanism for the force between currents can be formulated as follows:

The moving charges in one conductor create a magnetic field represented by circular magnetic lines of force around that conductor. The charges in the second conductor are moving in that magnetic field, which in this case is perpendicular to their direction of motion, and causes them to be subjected to a force. Similarly, the moving charges in the first conductor are affected by the magnetic field from the current in the second conductor. In each case the charges experience a force which is at right angles to their direction of motion, as illustrated in Figure 10.14.

We can now formulate a law: To experience a force from a magnetic field a charge must be moving in a direction at an angle to the magnetic lines of force. This force is perpendicular

* We will not try to define the meaning of 'stationary' at this point.

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