> 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
