1Unit 16 Electrodynamics I 16.1 Permanent magnets 16.2 The magnetic force on moving charge 16.3 The motion of charged particles in a magnetic field 16.4 The magnetic force exerted on a current-carrying wire 16.5 Current loops and magnetic torque 16.6 Biot and Savart’s law 16.1 Permanent magnets A bar magnet can attract another magnet or repel it, depending on which ends of the magnets are brought together. One end of a magnet is referred to as its north pole; the other end is its south pole. The rule for whether two magnets attract or repel each other: opposites attract; likes repel. Breaking a magnet in half results in the appearance of two new poles on either side of the break. This behavior is fundamentally different from that in electricity, where the two types of charge can exist separately. We saw a visual indication of the electric field Eof a point charge using grass seed suspended in oil. Similarly, the magnetic field Bcan be visualized using small iron filings sprinkled onto a smooth surface. The filings are bunched together near the poles of the magnets. This is where the magnetic field is most intense. The direction of the magnetic field, B, at a given location is the direction in which the north pole of a compass points when placed at that location. In general, magnetic field lines exit from the north pole of a magnet and enter at the south pole.
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