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Electric circuits vs. material current density

To: femm@xxxxxxxxxxxxxxx
Subject: Electric circuits vs. material current density
From: Fernando Seco <fseco@xxxxxxxxxxx>
Date: Fri, 30 Mar 2001 12:08:37 +0200
Organization: Instituto de Automática Industrial

Dear David & FEMM users,

I'm trying to simulate the magnetic fields in an iron tube
created by a solenoid wrapped around it, which carries a total
current of 40 A @ 1 kHz. FEMM provides two ways to simulate this:

a) Put the copper coil in a circuit and specify a total
current of 40 A; edit the copper material properties and set
J = 0 MA/m^2. This is done in file DYNFE05.FEM.

b) Do not make an electric circuit, but put a current
density of J = 8 MA/m^2 in the copper (times 5 mm^2 cross section
should give the total 40 A current). This is in file DYNFE06.FEM.

(I'm attaching both files if you want to try it)

When I actually perform the simulations, the result is
that the magnetic field is twice larger in the first case (I was
expecting roughly the same values). If you use a block integral to
compute the total current in the solenoid you get I~=40 A in the
first case, but only I=~14+j*14 A (|I|=20 A) in the second. That
seems to explain the different computed magnetic fields.

Could it be that in the first case you give the peak
current and in the second the RMS current? But that would give
only a difference of sqrt(2), not 2, I think.

My question is: what is the physical meaning of both
types of simulation, and which is the "correct" one to actually
describe a real solenoid?

Thanks for your help,

P.S.: Dave, yellow meshes are cool!

--

Fernando Seco
fseco@xxxxxxxxxxx
http://www.iai.csic.es/users/fseco

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<c0> = 8000000.000000
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<BeginBdry>
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