# Electromagnets Attraction/Repulsion force rate.

Discussion in 'General Electronics Discussion' started by Dretron, Feb 21, 2013.

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1. ### Dretron

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Jun 9, 2012
When a electromagnet has rated attraction force of 200N
is it possibile to have the same equal repulsion force rate?

I mean, lets say the electromagnet was set at the N pole, and attracted a magnet with a force of 200N.
And then reversed the input current flow of that electromagnet thus changing the pole from N to S, would it generate an equal repulsion force?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
My gut feeling says yes, but I have no particular way of proving it.

3. ### Dretron

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Jun 9, 2012
I was debating with a guy about this. Says its impossibile to have the same force for repulsion/attraction.

4. ### john monks

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Mar 9, 2012
Let me answer it in a slightly different way.
If you have a coil facing a north pole of a magnet and the attractive force is 200N then reversing the current in the coil will cause a repulsive force of equal magnitude provided that the molecules in the magnet to no change position. Now here is the twist. In an iron magnet the molecules do change position and you will not get a repulsive force of equal magnitude. In fact the coil may still attract the magnet and your iron magnet will become remagnetized in the opposite direction.
Think of it this way. An iron magnet is actually comprised of a whole lot of little electromagnets because the iron particles are atoms with electrons orbiting around them. And the direction of this orbit can be altered thereby shifting the entire orientation of the iron molecule by subjecting the molecule by a very strong external magnetic field. In fact this is how some permanent magnets are made.

Magnetism is simply the relativistic affect of a moving electric charge as explained by Albert Einstein in his special theory of relativity.

5. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Initially I was not in agreement with your friend until I considered replacing one of the magnets with an equal weight/size iron bar. It will be attracted by either pole of the magnet but not repulsed. Can you surmise where I'm going with this?

CYA: On the other hand I might be all wet!

Chris

6. ### Dretron

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Jun 9, 2012

Thank you John,Chris

I think of it in terms of logic, if you have an electromagnet that has two poles, everything is equal between the poles. The North pole = The S pole, thus the force is equal.

I never heard of a dipole having different factors in their poles that would lead to them being non-equal.
So logically when the electromagnet attracts the magnet(Neodymium) at 200N of force, if I switch the current the other way around I believe the magnet well be repelled at the same rate of force attracting it.

Hope that makes sense...
But I tired understand you points of view but its not giving me a proper kick in the noggin.

The thing is, I have no evidence to prove my point. Other than common sense
What do you all think?

7. ### john monks

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Mar 9, 2012
A neodymium is a very good magnet. And if the magnet was perfect you would be correct. But the magnet is not perfect and some perturbations do take place when subjected to a strong external magnetic field. I suspect that if you tried this experiment with a neodymium magnet you would find your conclusion to be very close. And if you did this experiment with a plane old iron magnet your result would be poorer. Think of it this way. If you tried the experiment wil a non- magnetized iron bar the coil would pull the bar either way you have the current flowing. This is possible only because the iron molecules do move to some degree. The laws of electromagnetics seem to be fixed in nature and there is nothing we can do about it.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Damn shame about that.

9. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I think you can measure this. The attached sketch is very basic but it gives the concept.

Chris

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10. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I forgot to mention that the spacer is non magnetic.

Chris

11. ### Dretron

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Jun 9, 2012
Having it close is good enough.
most systems are 90 - 99% approximate but not exact.
But I think the two forces are equal dont you all agree?

12. ### Dretron

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Jun 9, 2012
John a simple example to prove my point is an electric motor.
I believe the electromagnet repels and attracts the magnet with equality of force rates.

13. ### davennModerator

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Sep 5, 2009
Why?? what was his reasoning ?

Dave

14. ### Dretron

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Jun 9, 2012
Chris thank you for the method

15. ### Dretron

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Jun 9, 2012
Well
This guys was... a sales man for a website selling
electromagnets...

16. ### Dretron

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Jun 9, 2012
And argued its impossible to have equal repulsion and attraction forces...
I said that logically makes no sense.
He said its the design!

I was like... What?...

17. ### john monks

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Mar 9, 2012
Dreton, I agree that with a neodymium magnet the attraction and repulsion is the same but perfectly the same because of the way the molecules move around in the magnet. and I believe the attraction will be greater that the repulsion by at least a little bit.
In a motor there certainly is attraction and repulsion. But this is a poor example because even if there were no repulsion the motor would still turn. It would turn with less torque. It turns because the armature is attracted to the stator during part of the travel and is repelled during another part of the travel.
If there were only coils in a motor at all, which is possible, then I would agree with you. The wild card is the iron or other magnetic parts.

18. ### Dretron

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Jun 9, 2012

I agree.
Iron or any other magnetic parts would do poorly.
But with Iron or any other magnetic part attracting it would be efficient?

Other thing in case where a motor is being designed or you're playing around with these forces what force is best?
Attraction?
Repulsion?
Both?
I believe we can design a motor with electromagnets that only attract a magnet in a circular motion style! I'm pretty sure something like this exists.

19. ### john monks

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Mar 9, 2012
Dretron, I basically agree that the repulsive force and attractive force are equal. Magnetic material, like iron, confuse things because iron is normally only attracted to a magnet regardless of the pole. This is because iron is comprised of many little electro-magnets creating dipoles that can be rotated around easily. In fact this is why iron is magnetic. This tends to confuse the basic underlining physics. But your basic premise is correct. That is the repulsive force is equal to the attractive force when dipoles confront each other. It's just that the dipoles in iron can be pertibated.

Last edited: Feb 23, 2013

101
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Jun 9, 2012

Fair enough
Thanks John