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# Magnetic Force Help (page 2)

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By McGraw-Hill Professional
Updated on Sep 7, 2011

## Lines Of Flux

Physicists consider magnetic fields to be comprised of flux lines , or lines of flux . The intensity of the field is determined according to the number of flux lines passing through a certain cross section, such as a centimeter squared (cm2) or a meter squared (m2). The lines are not actual threads in space, but it is intuitively appealing to imagine them this way, and their presence can be shown by simple experimentation.

Have you seen the classical demonstration in which iron filings are placed on a sheet of paper, and then a magnet is placed underneath the paper? The filings arrange themselves in a pattern that shows, roughly, the “shape” of the magnetic field in the vicinity of the magnet. A bar magnet has a field whose lines of flux have a characteristic pattern (Fig. 14-1).

Fig. 14-1 . Magnetic flux around a bar magnet.

Another experiment involves passing a current-carrying wire through the paper at a right angle. The iron filings become grouped along circles centered at the point where the wire passes through the paper. This shows that the lines of flux are circular as viewed through any plane passing through the wire at a right angle. The flux circles are centered on the axis of the wire, or the axis along which the charge carriers move (Fig. 14-2).

Fig. 14-2 . Magnetic flux produced by charge carriers traveling in a straight line.

## Polarity

A magnetic field has a direction, or orientation, at any point in space near a current-carrying wire or a permanent magnet. The flux lines run parallel to the direction of the field. A magnetic field is considered to begin, or originate, at a north pole and to end, or terminate, at a south pole . These poles are not the same as the geomagnetic poles; in fact, they are precisely the opposite! The north geomagnetic pole is in reality a south pole because it attracts the north poles of magnetic compasses. Similarly, the south geomagnetic pole is a north pole because it attracts the south poles of compasses. In the case of a permanent magnet, it is usually, but not always, apparent where the magnetic poles are located. With a current-carrying wire, the magnetic field goes around and around endlessly, like a dog chasing its own tail.

A charged electric particle, such as a proton, hovering in space, is an electric monopole , and the electrical flux lines around it aren’t closed. A positive charge does not have to be mated with a negative charge. The electrical flux lines around any stationary charged particle run outward in all directions for a theoretically infinite distance. However, a magnetic field is different. Under normal circumstances, all magnetic flux lines are closed loops. With permanent magnets, there is always a starting point (the north pole) and an ending point (the south pole). Around the current-carrying wire, the loops are circles. This can be seen plainly in experiments with iron filings on paper.

## Dipoles And Monopoles

You might at first think that the magnetic field around a current-carrying wire is caused by a monopole or that there aren’t any poles at all because the concentric circles apparently don’t originate or terminate anywhere. However, think of any geometric plane containing the wire. A magnetic dipole , or pair of opposite magnetic poles, is formed by the lines of flux going halfway around on either side. There in effect are two such “magnets” stuck together. The north poles and the south poles are thus not points but rather faces of the plane backed right up against each other.

The lines of flux in the vicinity of a magnetic dipole always connect the two poles. Some flux lines are straight in a local sense, but in a larger sense they are always curves. The greatest magnetic field strength around a bar magnet is near the poles, where the flux lines converge. Around a current-carrying wire, the greatest field strength is near the wire.

Practice problems of these concepts can be found at: Magnetism Practice Test

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