# Formula for the strength of an electromagnet

1. Dec 23, 2008

### Mindstormed

Trying to determine the strength of an electromagnet in Teslas, with an iron core.
Some sites have the same basic formula but with different units after B=

Some use μ0, μr, or μ. Which one to use?

For N (the number of turns), is this the general number of turns accounting for multiple layers, or turns per meter (or inches), or what?

And L (the length), is it the length of the iron core or the length of the wire used, and should it be in meters or inches?

According to this site (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html), a 0.1 meter (4") length solenoid (I'm assuming their talking about the iron core?) with 200 turns, 1 amp, and an iron core with 200 relative permeability is 0.5 Tesla. With 3 amps, this is about 1.5 Tesla and that doesn't sound right.

Any help is appreciated, thanks!

2. Dec 23, 2008

### mgb_phys

μ = μ0 * μr
μ0 is a fundemental constant, you also need μr for your core material, or the book may just list μ and do the multiplication for you.

Just the total number of turns.

It's the length of the core that has coils wrapped around it.

You can use any units you like, as long as you have matching units for μ.
It's probably safest to use metres for everything.

Nope that sounds a bit high.

μr iron = 5000 μr steel = 200 (varies a lot!)
B = μ N I / h = 200 * 4pi*10^-7 * 200 * 1 / 0.1 = 0.5T
which still sound s a lot - but this is the field inside the core - the field outside the coil is much lower.

Last edited: Dec 23, 2008