# Strength of an Electromagnet

1. Mar 6, 2005

### mrjeffy321

I think I remember there being a formula to determine the Strength of an Electromagnet, but I just cant remember what it is, and i cant find it any where.

of course the strength will depend on many things, # of coils, current, voltage?, Power? distance away from magnet and other things.

2. Mar 6, 2005

### spender

strenght of electromagnet will also depend on core.

3. Mar 6, 2005

### mrjeffy321

I am specifically looking for the formula I refered to earlier actually if anyone knows it.

While were talking about the core of the magnet,
how will the diameter of the (round) core effect the strength?
Lets give 2 extremes, a very small diameter, wouldnt that concentrate the magnetic field and make it very strong all the way through, as compared to a very wide diamter, which will only be semi-strong around the edges.
but if you "add" up the strength on both, would they be equal, it is just that on one it is more concentrated and another it is more spread out?

4. Mar 7, 2005

### vinter

Before getting a mathematical formula, you need to 'define' the strength of an electromagnet. If the definition is clear, the formula will follow immediately...

5. Mar 7, 2005

### Andrew Mason

The field strength depends upon the current and the turn density as well as the permeability of the core. It does not depend on voltage, # of coils, or power. Sustaining a magnetic field does not require energy because no work is being done in maintaining the field. The power to a superconducting magnet should be very close to 0 but the magnetic field can be tremendous because of the current.

AM

6. Mar 7, 2005

### mrjeffy321

When I was refering to strength of a magnetic field , I am really refering to the force that it exerts on an objecft (like a metal plate for example), but I know this is wrong, that is the force of the field, I need to find the intensity/whatever you call it, which has a unit of Tesla
B=N/A*m
where B is the magnetic field, N is the force in newtons the onject experiences, A is the amperage, and m is the length in meters if the wire that is experiencing the force.

although this isnt quite what I was looking for, it is a start.

Can you tell me what to look for specifically, so that I can narrow down my search and find the info I am looking for.

7. Mar 7, 2005

### Crosson

mrjeffy321, I would be happy to solve this magnetostatic problem, involving a description of forces (newtons) and fields (Teslas).

The field is highly dependent on the geometry so please describe precisely the geometry of the situation (or draw a paint picture. Also, the effect of the electromagnet on an external current (object) depends on the orientation of that current, so include that also.

Is it a solenoid (A long coil of wire wrapped around a ferromagnetic {iron} core)?
If it is, than I can tell you that the field H will go as:

$$H = \frac{\mu_0 I}{2\pi r}$$

Where r is the perpendicular distance from the solenoid. H is not B, and is not F.

Then you need to know the magnetization of your material, which is difficult for ferromagnets, I will see if you have details.

8. Mar 7, 2005

### mrjeffy321

Well in my situation, I would have a solid Iron core, cyllendrical, wrapped with wires.
the object to be attracted would be something like a flat piece of sheet metal (to make it easy) so that they would rest against eachother easily.

example picture, top object (minus the periods) is the magnet, the bottom is a flat piece of metal.

................| |
................| |
................| |
................| |
................|_|

.........______________

9. Mar 8, 2005

### Crosson

Then:

$$B = \mu I N$$

where mu is the permeability of your metal, I is the current in the wire, and N is the number of turns per unit length ( valid if the object is near the core).

10. Mar 8, 2005

### mrjeffy321

so mu is a constant that I have to find for my partular material.
I found some constants dealing with this, but they are given diferently,
I found 3 different things, mu i/mu not, mu m/mu not, H c/A/m,
it says that mu i, is the initial permeability, mu m is the maximum permeabilty, and mu not is the magnetic permeabilty in free space.

is this number, say for Iron, generally very big, like 200 or 6000?

I is the current in amps.
N is the # of coils, (per unit length?)

is this "per unit length" part refering to how many coils I have per meter?
say I had a long (1 meter long) cyllendrical metal core, the N would be the number of coils I have total around the whole thing.
but lets say I had a 10 cm (.1 m) long core, so N would be equal to the number of coild I have * 10? or is this "unit" being refered to not the meter?

also, say I had a really wide (in diameter) core, would this matter any? as compared to a thin core?