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Q & A: gravity from magnetism

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Most recent answer: 12/08/2010
Q:
Have there been any expirements to check if the gravitational force is increased or decreased when an object is magnetized? For instance has anyone expirementally verified that G=kmm/r^2 is changed when an object ( and it would probably have to be a massive one because of how small gravitational forces are) is magnetized?
- Alex Bennett (age 17)
El Paso,TX, El Paso county
A:
I don't know of any. At least for ordinary materials, this experiment would seem nearly impossible. Take some material with a mass density of 1 gm/cm3. Assume that it has a big internal magnetic field, say B=3000 G.  The internal field energy density (in cgs units) is just B2/8π, or about 400,000 erg/cm3. That corresponds (via E=mc2) to a mass density of about 4 10-16 gm/cm3. That's just way too small compared to the ordinary mass density to measure. (Sticklers will note that for a ferromagnetic material the net energy change may be negative rather than positive, etc., but this calculation gives the approximate scale correctly.)

Perhaps there are cases of extremely magnetic stars in which the magnetic field mass isn't negligible, so we'll update this if an astrophysicist knows of such a case.

Mike W.

(published on 12/08/2010)

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