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Author
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Topic: Magnetic field of permanent Magnet (Read 3147 times)
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D. Boire
Newbie
Posts: 1
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I tried this lab for the first time this year and here are my/students typical results.
Using analyze curve fit, variable power.
note: collecting data between 5 cm and 10 cm resulted in RMSE value closer to 0.0000 for all curve fits.
d to -3 power, mu = 0.4943 , RMSE = .02572
d to -2 power, mu = 8.18 , RMSE = .001636
d to -4 power, mu = 0.004528 , RMSE = .481936
The only reference I found on typical values of mu were that a bar magnet has a mu ~ 5-ish.
conclusion based upon data, inverse square is the model that fits.
- RMSE is closest to 0.0000, and mu value is near expected.
thoughts?
Dames
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gastineau
Vernier Specialist
Full Member
Posts: 128
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The field from a true dipole is inverse-cube. This applies for distances far enough from the magnet so that the physical dimension of the magnet is small compared to the measurement distances. If you're using a standard bar magnet that is 20 cm long or so, you're violating that assumption.
The field pattern from a non-dipole will vary from inverse-cube. It may be that your particular magnet does more closely match an inverse-square field pattern, but in that case you can't really use the values to find the magnetic moment of the magnet.
Values for mu will vary widely for various magnets and materials. When I took the sample data for this activity years ago, I got barely over 0.1 A m^2 for one magnet, and perhaps 5X that for another.
In both cases I got a good inverse-cube fit, but the magnet thickness and diameter were very small compared to the measurement distances. Magnets were only a few mm in diameter and thickness. Larger magnets would require getting farther away to obtain dipole-like behavior.
For an elementary discussion of the inverse-cube nature of a dipole magnetic field, see the excellent derivation of magnetic moment in Electric and Magnetic Interactions, by Chabay and Sherwood, published by Wiley. This is a calculus-based introductory physics book.
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