When a current-carrying conductor is placed in a magnetic Field, it experiences a force. Experiment shows that the magnitude of the force depends directly on the current in the wire, and the strength of the magnetic Field, and that the force is greatest when the magnetic Field is perpendicular to the conductor.
In the set-up shown in Figure 1.1, the source of the magnetic field is a bar magnet, which produces a magnetic Field as shown in Figure 1.2.
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| Figure 1.1 Mechanical force produced on a current-carrying wire in a magnetic Weld |
The notion of a ‘magnetic Field’ surrounding a magnet is an abstract idea that helps us to come to grips with the mysterious phenomenon of magnetism: it not only provides us with a convenient pictorial way of picturing the directional eVects, but it also allows us to quantify the ‘strength’ of the magnetism and hence permits us to predict the various eVects produced by it.
The dotted lines in Figure 1.2 are referred to as magnetic Flux lines, or simply Flux lines. They indicate the direction along which iron Wlings (or small steel pins) would align themselves when placed in the Field of the bar magnet. Steel pins have no initial magnetic Field of their own, so there is no reason why one end or the other of the pins should point to a particular pole of the bar magnet.
However, when we put a compass needle (which is itself a permanent magnet) in the Field we Wnd that it aligns itself as shown in Figure 1.2. In the upper half of the Wgure, the S end of the diamond-shaped compass settles closest to the N pole of the magnet, while in the lower half of the Wgure, the N end of the compass seeks the S of the magnet. This immediately suggests that there is a direction associated with the lines of Flux, as shown by the arrows on the Flux lines, which conventionally are taken as positively directed from the N to the S pole of the bar magnet.
The sketch in Figure 1.2 might suggest that there is a ‘source’ near the top of the bar magnet, from which Flux lines emanate before making their way to a corresponding ‘sink’ at the bottom. However, if we were to look at the Flux lines inside the magnet, we would Wnd that they were continuous, with no ‘start’ or ‘Wnish’. (In Figure 1.2 the internal Flux lines have been omitted for the sake of clarity, but a very similar Field pattern is produced by a circular coil of wire carrying a d.c. See Figure 1.6 where the continuity of the Flux lines is clear.). Magnetic Flux lines always form closed paths, as we will see when we look at the ‘magnetic circuit’, and draw a parallel with the electric circuit, in which the current is also a continuous quantity. (There must be a ‘cause’ of the magnetic flux, of course, and in a permanent magnet this is usually pictured in terms of atomic-level circulating currents within the magnet material. Fortunately, discussion at this physical level is not necessary for our purpose.)
