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# ch21_print - Magnetic Forces and Magnetic Fields 1 Magnets...

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Magnetic Forces and Magnetic Fields 1 – Magnets Magnets are metallic objects, mostly made out of iron, which attract other iron containing objects (nails) etc. Magnets orient themselves in roughly a north - south direction if they are allowed to rotate freely ( compass ). Assume that a magnet has bar form. Objects are attracted most strongly to the ends of the magnet called poles . There are two poles: north pole and south pole Magnetic poles exert attractive or repulsive forces on each other similar to electric forces between charged objects. Dr.D.Wackeroth Spring 2005 PHY102A Magnetic Forces and Magnetic Fields Like poles repel each other and unlike poles attract each other Important difference to electric charges: Electric charges can be isolated (proton, electron), but magnetic poles can- not be isolated magnetic poles always occur in pairs! By placing iron containing objects close to a magnet, these objects become magnetized, ie. they develop magnetic poles. To describe the interaction of magnets and magnetized materials, it is con- venient to introduce the concept of the magnetic field , analogous to the electric field. Dr.D.Wackeroth Spring 2005 PHY102A

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Magnetic Forces and Magnetic Fields 2 – Magnetic Fields Experiments demonstrate that a stationary (non-moving) particle does not interact with a static magnetic field. However, whenmovingthroughamagneticfieldachargedparticleexpe- riencesaforce. Properties: The force has its maximum value when the charge moves perpendic- ular to the magnetic field lines. The force is zero when the particle moves along the field lines. The magnetic force exerted on a test charge q 0 , moving with velocity ~ v can be used to describe the properties of the magnetic field, ~ B . Dr.D.Wackeroth Spring 2005 PHY102A Magnetic Forces and Magnetic Fields From experiment we know: The force is proportional to the strength of the external magnetic field, B . It is proportional to the sine of the angle θ between the direction of ~ v and the direction of ~ B . It is proportional to the charge q 0 . It is proportional to the magnitude of the velocity, v . F = q 0 vB sin θ (1) The magnitude of the magnetic field is then defined as B = F q 0 v sin θ (2) SI unit of ~ B : Tesla 1 T = 1 = 1 . In practice one often uses the gauss as an unit: 1 T = 10 4 G Dr.D.Wackeroth Spring 2005 PHY102A
Magnetic Forces and Magnetic Fields Direction of the magnetic force: Experiments show that the direction of the magnetic force is always per- pendicular to both ~ v

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ch21_print - Magnetic Forces and Magnetic Fields 1 Magnets...

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