Thanks, Bill, that's a much more detailed description than I gave in
USMA:9124. However, rather than discussing it in terms of the energy
required to rotate the dipole out of the field, I would have been
inclined to describe it in terms of the torque exerted on the dipole by
an external field. Same horse, different end.
Jim
"Hooper, Bill and or Barbara" wrote:
>
> > From: [EMAIL PROTECTED] (John Tauxe)
> > Subject: [USMA:9119] RE: SI units test!
> >
> > What does the following expression represent:
> >
> > 80 ZA-m^2
>
> Note that the expression
>
> 80 ZA-m^2
>
> (which means zetamperes TIMES square metres). It is not zettamperes PER
> square metre, which some of you have mistakenly written.
>
> However, the expression is not the earth's magnetic field. it is a value
> known as the earth's magnetic dipole moment.
>
> The earth's magnetic filed is quite small and measured in different units
> (because it's an entirely different thing). The earth's dipole moment is
> very large and the prefix zetta, "Z" for 10 to the PLUS 21, is correct. It
> should not be a lower case "z" which is zepto, for 10 to the MINUS 21
>
> Following are a few more technical details that some of you may not be
> interested in.
>
> The expression 80 ZA-m^2 gives the earth's "magnetic dipole moment". That's
> a technical term for a characteristic of our planet that is very important
> in geophysics but probably not very important to anyone else. It is not the
> magnetic field at the earth's surface; that is expressed in entirely
> different units. The average value of the magnetic field is about 10^-4 T
> (ten-to-the-minus-four tesla) which could of course be conveniently
> expressed in millitesla (mT) or microtesla (�T) if desired. (But zeptotesla,
> zT, would certainly be an inconveniently small unit.)
>
> The magnetic dipole of the earth is simply the value of an imaginary
> electromagnet that would give the same pattern and strength of magnetic
> field as is found surrounding the earth. The dipole moment (m) of an air
> core electomagnet is defined as the current (I) flowing through the wires
> wound around the air core multiplied by the cross sectional area (A) of the
> core and multiplied by the number of turns of wire (N) wound around the
> core;
> m = NIA
> (An electromagnet with an iron or other core will have a significantly
> different dipole moment due to the effect of the material in the core, but
> the basic relationship is still the same.).
>
> In SI, the expression NIA has units of amperes (from the current, I)
> multiplied by square metres (from the area, A). (N is a pure number with no
> units.) Thus the units are A-m^2 (amperes times square metres) which can be
> shown to be equivalent to J/T (joules of energy per tesla of magnetic field
> strength). In the latter form (J/T), it tells how much energy in joules is
> needed to rotate a magnet which is acted upon by an external magnetic field
> in tesla. (Note that the field is an external field acting on the magnet,
> not the magnetic field of the magnet itself).
>
> Regards,
> Bill Hooper
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
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