Page 334 - Schaum's Outline of Theory and Problems of Applied Physics
P. 334

CHAPTER 27







                                                         Magnetism














        NATURE OF MAGNETISM

        Two electric charges at rest exert electric forces on each other according to Coulomb’s law. When the charges
        are in motion, however, the forces are not the same. The differences are customarily said to arise from
        magnetic forces that occur between moving charges in addition to the electric forces between them. As it
        happens, the total force on a charge Q at a certain time and place can always be separated into two parts. One is
        an electric force that depends only on the value of Q, and the other is a magnetic force that depends on both Q
        and the velocity v of the charge.
            Nevertheless there is only a single fundamental interaction between charges, the electromagnetic interaction.
        The theory of relativity provides the link between electric and magnetic forces by showing how the electric force
        between two charges appears altered to an observer when the charges are moving with respect to the observer.
        Thus magnetism is not distinct from electricity in the way that, for instance, gravitation is. Because the electric
        effects the electromagnetic interaction is responsible for are distinct from the magnetic effects, electric and
        magnetic forces can be considered separately and we can think in terms of separate electric and magnetic
        fields.
            Of course, the word magnetic brings to mind ordinary magnets and their ability to attract nearby iron objects
        and, depending on their arrangement, to attract or repel each other. How this behavior is related to magnetic
        forces between moving charges will be considered later.






        MAGNETIC FIELD

        A magnetic field B is present wherever a magnetic force acts on a moving charge. The direction of B at a certain
        place is that along which a charge can move without experiencing a magnetic force; along any other direction
        the charge would be acted on by such a force. The magnitude of B is equal numerically to the force on a charge
        of1Cmoving at 1 m/s perpendicular to B.
            The unit of magnetic field is the tesla (T), where
                                                 newton        weber
                                     1 tesla = 1          = 1
                                              ampere-meter    (meter) 2

        The gauss (G), equal to 10 −4  T, is another unit of magnetic field sometimes used. The magnetic field near a
        strong permanent magnet is about 0.1 T.


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