Your entire post [below] demonstrates exactly what I mean. If you
transform equations willy-nilly you can end up very confused about
the direction of causality in a given practical context.

See Gerald's excellent illustrative example.

cheers, Ian

"Jim Adney" wrote in message
...
On Sun, 6 Jul 2003 16:33:27 +0100 "Ian Iveson"
wrote:

Equations can be rearranged. You can just as easily say that
voltage heats resistors but it would be a highly misleading
confusion of dependent and independent variables in most
contexts.

You seem to be arguing that

P = Isquared * R

is a more correct form and that

P = Vsquared/R is less correct.

How about P = I * V?

So it is with cores. They are power rated, not voltage rated.

Yes, cores themselves are often sold with a power rating, but this
has
everything to do with the space available for copper around the
core
(the winding window) and nothing to do with the flux density in
the
core.

For example, if I want to design a transformer using standard E-I
cores I would first pick a size that would allow a large enough
winding window. Then I have to use a thick enough stack of these
pieces to support the flux that I will get from my voltage, number
of
turns and frequency. I may have to adjust N, or the stack
thickness to
get what I need, and I may even have to pick a different size E-I
part
if things get out of hand. It's an iterative process if you want
to
get the most economical finished part. In practice this isn't done
very well unless the production volume is large.

In fact, if you could wind your transformer with superconducting
wire,
you could put ANY amount of power thru it, as long as the windings
and
the core were properly designed with respect to the voltage and
frequency.

The core is there to handle the flux. The copper is there to
handle
the current.

What most people don't get, is that in a real transformer the flux
peaks are the highest when there is NO LOAD on the transformer.

-
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Jim Adney
Madison, WI 53711 USA
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