Re: [femm] Ampere's law

[David Meeker <dmeeker@xxxxxxxx>]

tano1938 wrote:

> Now that I've modeled my my first design, i.e. FRT_OLD.fem in files
> database, I've been using the result to reinforce some of the basic
> laws in magnetics. In FRT_OLD.fem, I drew a closed contour through
> the magnet, steel, air gap and back to the magnet. Then, using the
> H.t integral, I expected to get an answer near zero, but got a huge
> number of ampere turns instead. I'm obviously doing something wrong.
> Can someone help?

The answer that you got is correct. Remember the model of permanent 
magnets that FEMM uses--a non-magnetic material wrapped by a current 
sheet. (See http://groups.yahoo.com/group/femm/message/1865 ) When you 
draw a closed contour through magnet, steel, air gap, and back through 
the magnet, you are enclosing the current sheet "flowing" at the edge of 
the magnet. The since the density of current on the equivalent current 
sheet is equal to Hc, result for enclosed current which one would expect 
is Hc*(length of magnet). In your case, Hc=50963 A/m. The length of 
the magnet in the magnetized direction is 1.6" (0.04064 m). The product 
of these two quantities is 2071 A. One one particular loop (linking the 
points (0.2,2.2), (1.4,2.2), (1.4,-0.1), (0.2,-0.1) into a closed loop), 
the H.t integral result that I get is 2069 A.

Dave.
--
David Meeker
email: dmeeker@xxxxxxxx
www: http://femm.berlios.de/dmeeker