# attraction force on electromagnets

Discussion in 'Electronic Basics' started by Camilo Andres Gil Cardona, Oct 1, 2004.

1. ### Camilo Andres Gil CardonaGuest

hi everybody. the question is simple: what's the formula to calculate
attraction force of an electromagnet (in newtons) ? other question:
has repulsion force between two magnets, the same value that
attraction force between those same magnets?

2. ### Paul DraperGuest

The answer to the second question is yes, provided the N and S poles
of a given magnet are shaped the same way.

The answer to the first question is not as simple as you would hope.
It depends one whether the magnet is a horseshoe magnet, a bar magnet,
a solenoid, a toroid, whether there's a ferric core,...

PD

3. ### Paul DraperGuest

As a personally interesting anecdote, I was once responsible for the
design, construction, and operation of a 250,000 lb iron-core toroidal
magnet. I chose to instrument it with a Hall probe, embedded in a thin
slot in one side. But the slot itself would disturb the field, which I
had to account for. I proposed a linear model to calculate that,
superimposing a piece of "negative iron" where the slot was, and
calculating the total field of the magnet, against which I could
compare the Hall probe measurement. This approach was deemed clever by
my professors until a young post-doc pointed out to me, in a fatherly
conversation in the hallway, that iron core magnets are nonlinear
beasts and superposition doesn't work, a revelation that left me sick
to my stomach. Sure enough, the Hall probe read values that were
inconsistent with calculations and with the backup instrumentation. I
ended up doing a very complicated lattice-relaxation calculation on a
mainframe computer to compute the field.

PD