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Unformatted text preview: HB 11-26-07 Magnetic Field of a Circular Coil Lab 12 1 Magnetic Field of a Circular Coil Lab 12 Equipment- coil apparatus, BK Precision 2120B oscilloscope, Fluke multimeter, Wavetek FG3C function generator, 3 leads Comment The computer monitor should be off to a void picking up an interference signal with the search coil. Reading Electrical Safety at the beginning of this manual. 1 Introduction A wire coil that is carrying a current produces a magnetic field ~ B ( ~r ), where ~r is the distance from the center of the coil to the field point. The strength of the field ~ B is proportional to the current I in the coil. The strength and direction of the field depend on ~r . For large distances from the coil ( r a , where a is the radius of the coil), the shape of the magnetic field of a coil is identical to the electric field produced by a point electric dipole. For large distances both fields fall off as 1 /r 3 . In this experiment you will measure the magnetic field of a circular coil at distances that are fairly close to the coil. The large distance approximation is not valid. A constant magnetic field can be measured in many ways. You can use a compass, a Hall Probe, a rotating coil of wire, or nuclear magnetic resonance. In this experiment the magnetic field will not be constant but will vary sinusoidally with time. Such a time varying magnetic field will induce a time varying voltage in a small coil which will be called the “search coil.” The search coil will be used to measure the magnetic field produced by a larger coil called the “field” coil. The current in the field coil will be varied sinusoidally with time and produce a sinusoidally varying magnetic field. 2 Electromagnetic Induction A magnetic field can be described at each point in space and time by a vector ~ B whose di- rection coincides with the direction of the field and whose length is proportional to the mag- nitude B of the field. A magnetic field that changes with time produces a non-conservative electric field. This phenomenon, called electromagnetic induction, was discovered by Fara- day, Henry, and others. The non-conservative electric field will produce a current and voltage in the search coil. By measuring this voltage for different positions and orientations of the small “search” coil the time varying magnetic field produced by the large field coil can be mapped out. The frequency of the sinusoidal current will be low enough so that the mag- netic field mapped out by the search coil will be essentially identical to the magnetic field produced by a field coil carrying a constant current. 3 The Magnetic Field A wire carrying a current generates a magnetic field ~ B whose magnitude and direction at each point in space depend on the length and shape of the wire, the current ﬂowing through the wire, and the location of the point at which the field is determined. A convenient way to depict the pattern of the magnetic field is to draw a lines such that each line is always...
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- Magnetic Field, search coil