Re: Magnetic field energy loss

From: George Murphy (
Date: Tue Sep 30 2003 - 20:44:27 EDT

Donald Nield wrote:
> George Murphy wrote:
> >
> >
> > Thanks for the references. I suspect that one reason why Barnes & other YECs
> > became so smitten with the idea that the decay of the earth's field proved YEC was not
> > any observational data but the simple theoretical result that the decay time for a B
> > field initially frozen in a highly conducting sphere with dimensions on the order of the
> > earth's core is a few thousand years. Of course whether such an elementary model
> > represents the real earth very well is another matter!
> >
> > Shalom,
> > George
> Could George please check the estimate of the decay time? My back-of-the-envelope
> calculation based on data in H. K. Moffat, "Magnetic Field Generation in Electrically
> Conducting Fluids", CUP, 1978 (core radius 3500 km, magnetic diffusvity 2.6 m^2 /s) yielded
> a time of the order of 10^5 years, which is a bit too long to be useful for the YECs.

        You're right - it's larger than I said, which shows the danger of working by
memory. On checking back I realized that I probably had in mind the statement in
Jackson's _Classical Electrodynamics" that "the time is "of the order of 10^4 years for
the molten core of the earth."

        Of course the result depends on what one uses for the conductivity (or magnetic
diffusivity) for the core material _and_ for the initial field configuration. The
diffusion time is proportional to the square of a characteristic distance within which B
changes significantly [L^2 ~ B/(del^2 B)] and that characteristic distance isn't
necessarily on the order of the dimensions of the core. It isn't hard to construct
artificial initial field configurations for which the characteristic distance is much
smaller than the core radius so that the diffusion time is much smaller than the above
estimates. But this gets beyond the question of what YECs may have been thinking and
into constructing another model. (& I hope I haven't given YECs any ideas!)


George L. Murphy

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