# Electromagnets range

1. May 10, 2008

### Cocacolacan

Hello,
Let's say I have a neodymium magnet with 20 lbs of force, and an electromagnet with 200 lbs of force. Is there a way I could figure out the maximum distance that they would attract each other?

2. May 10, 2008

### TVP45

You would have to specify the shapes and dimensions and specify a threshold for the attraction.

3. May 10, 2008

### Cocacolacan

circular magnets, 2" diameter (both of them) and air is in between them, and what is the threshold of attraction?

4. May 10, 2008

### TVP45

Threshold of attraction is the very low number you arbitrarily pick to be zero attraction. Are you an EE student?

5. May 10, 2008

### Cocacolacan

No I'm not, I will be studying to be a physicist or biomedical engineer next year though (going into college next year).

And I'm still not getting this threshold of attraction, have any links?

6. May 11, 2008

### buddingscientist

What do you mean the maximum distance they attract each other? The range is infinite. Do you mean when Fmag can overcome the friction of the magnet on the table?

7. May 11, 2008

### Cocacolacan

okay, let's rephrase. So I have two permanent magnets, one is pulling 30 lbs and the other is pulling 20 lbs. There's a .5 coefficient of friction, and they're opposite poles. How would I figure out the distance at which the magnets would overcome the coefficient of friction and pull each other together?

8. May 13, 2008

### Cocacolacan

No, I did not. I'm hoping my AP Physics teacher might have an answer, he is usually so busy, but now after the test he might be free.

9. May 14, 2008

### optrix

As I understand it, magnet/electromagnet calculations can get extremely complicated, as they depend on the shapes of the poles, the angle they make with each other, the distribution of magnetism over the surface of the objects, and other things too. I believe there are some simplified scenarios for which you can get an orcder of magnitude approximation, as explained by the wiki article (google 'Force between two magnets'), but beyond that, you might have to resort to Finite Element Analysis, or something equivalent.

10. May 15, 2008

### Cocacolacan

Okay, I have an equation.

If you're going through air between the magnets,
the equation will be

F=Magnetic Permability constant*S1*S2/r2

That is, (4*pi)×10−7 N·A−2 times the strength of the pole of interest number one times the strength of the pole of interest number two divided by the distance that separates them squared.

You can put it onto a calculator, set R as X, and watch the force drop off into oblivion.

Hope this helps someone in the future.

11. May 15, 2008

### Phrak

There is nothing clear about any of this. Magnets, for one, do not obey an inverse square law. And you've said nothing about this source of friction.

12. May 16, 2008

### Cocacolacan

Alright then, what do magnets follow? And wouldn't friction be something that would be dealt with later? As I would find a force on the graph that would equal the normal force of the magnet?

Does anyone know an answer to this question, it is driving me nuts. (How to find the force between two magnets across a distance that are interacting in air?)

The magnets are both circular, one is an electromagnet and the other is a permanent magnet.

Would this question be better in the physics section of the forum?

Last edited: May 16, 2008
13. May 26, 2008

agreed