Physics: Principles and Problems

Chapter 25: Electromagnetic Induction

Problem of the Week

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Lunar Prospector
Launched January 6, 1998, the Lunar Prospector was designed to focus on three goals:
  • Determine the resources of the moon, especially water.
  • Map the structure of the moon, including core and crust.
  • Gather evidence on the evolutionary history of the moon.
This 650 pound craft carries five scientific instruments to obtain data to answer many long-standing questions about the moon.
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Lunar Prospector craft
The magnetic field surrounding the moon is actually a combination of three fields: a 35 000 nT Earth B-field, a 10 nT Sun B-field, and finally a 1000 nT Moon B-field.

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Triaxial Fluxgate Magnetometer
Measurement of the magnetic field is done by the triaxial fluxgate magnetometer. Triaxial means that it can measure the strength of the magnetic field in three different directions. Fluxgate is a wire coil through which a magnetic field can pass. As the Lunar Prospector orbits the moon, the coil travels through a continuously changing magnetic field. Consequently, in accordance with Lenz's law, a current is produced in the coil. By measuring the variation of the current passing through the coil, the magnetometer makes a map of the magnetic field surrounding the moon.
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Close-up of the magnetometer
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/POWproblem1.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> Suppose the triaxial fluxgate magnetometer measures three components of the B-field as Bx = 14 830 nT, By = 8 092 nT, and Bz = 30 340 nT. Determine the magnitude and direction of the magnetic field.
 
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/POWproblem2.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> The triaxial fluxgate magnetometer is mounted at the end of a 2.6 meter boom. Why might this configuration be necessary?
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