Phys16 - Unit 16 Electrodynamics I 16.1 16.2 16.3 16.4 16.5 16.6 Permanent magnets The magnetic force on moving charge The motion of charged

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1 Unit 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 E of a point charge using grass seed suspended in oil. Similarly, the magnetic field B can 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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2 16.2 The magnetic force on moving charge The magnetic force depends on several factors: The charge of the particle, q ; The speed of the particle, v ; The magnitude of the magnetic field, B ; The angle between the velocity vector and the magnetic field vector, θ . The mathematical relation of them in vector form is ) ( B v q F G G G × = . One can rewrite it as a scalar expression, e.g. sin qvB F = . The maximum force is obtained when = 90 o . The force vanishes when = 0 o . Now we define the magnetic field B as sin qv F B = . The SI unit is 1 tesla = 1 T = 1 N/(A m). The tesla is a fairly large unit of magnetic strength, especially when compared with the magnetic field at the surface of the Earth, which is roughly T 5 10 0 . 5 × . Thus, another commonly used unit of magnetism is the gauss (G), defined as follows: T G 4 10 1 = . In terms of the South magnetic pole of Earth
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3 gauss, the Earth’s magnetic field on the surface of the Earth is approximately 0.5 G. A bar magnet has a magnetic field of roughly 100 G. Remark: Magnetic field lines never cross one another. As the direction in which a compass points at any given location is the direction of the magnetic field at that point. Since a compass can point in one direction, there must be only one direction for the field B . If field lines were to cross, however, there would be two directions for B at the crossing point, and this is not allowed.
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This note was uploaded on 09/06/2010 for the course BSC PHY1417 taught by Professor Prof during the Spring '08 term at HKU.

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Phys16 - Unit 16 Electrodynamics I 16.1 16.2 16.3 16.4 16.5 16.6 Permanent magnets The magnetic force on moving charge The motion of charged

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