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Unformatted text preview: Chapter 20 sources of magnetic field there are two ways to produce a magnetic field: a permanent magnet and an electric current. A bar magnet and a current loop produce similar patterns of magnetic field lines permanent magnets and magnetic plates the atoms in a permanent magnet material act as small bar magnets; their magnetic fields are produced by the circulating current of the atomic electrons and the spin magnetism of the electrons. A bar magnet contains a north magnetic pole and a south magnetic pole Amperes law Amperes law can be used to calculate the magnetic field produced by a current. This law relates the field directed along a closed path on the edge of a surface with the current that passes through the surface:  (closed path) B ll L = I enclosed B is measured in teslas (T). The constant is the permeability of free space and has the value 0 = 4 x 107 T x m/A when applied to a long, straight wire carrying a current I, Amperes law gives the field a distance r from the wire as B= I / 2 r right hand rule number 1: to find the direction of the field produced by a current, 1. place the thumb of your right hand along the direction of the current 2. curl your fingers; they will then give the direction of B as the field lines encircle the current magnetic force on a moving charge the magnitude of the magnetic force exerted on a charge moving with velocity v is given by: F B = qvB sin ( is angle between v and B) right hand rule number 2: to find direction of the magnetic force on a moving charge 1. point the fingers of your right hand along the direction of v 2. curl your fingers in the direction of B. Always curl through the smallest angle that connects v and B 3. if q is positive, the magnetic force on q is parallel to your thumb. If q is negative, the magnetic force is in the opposite direction magnetic fields due to a current the magnetic field at the center of a current loop is: B = I / 2R a solenoid consists of N tightly wound turns of wire of finished length L. The field inside a solenoid is equal to: B solenoid = NI/L a permanent magnet material is sometimes inserted inside a solenoid; in this case, the total field will be the field due to the current plus the field from the permanent magnet Hall effect the Hall effect involves a currentcarrying wire that is placed in a magnetic field perpendicular to the current. The magnetic force on the current leads to an electric potential difference between the edges of the wire, which can be used to determine the sign (positive or negative) of the charges that carry the current...
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 Fall '11
 dasu/karle
 Physics, Current

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