# Thread: cancelling magnetic fields with current...

1. ## cancelling magnetic fields with current...

got to thinking bout something and I came across a problem that could be solved with a "reverse electro magnet".

since you can make a bar magnet with a iron rod, DC current and a coil of copper wire, is it possible to cancel a permanent bar magnet with a coil of copper wire and a current? Can one in essence shut off a neodymium magnet temporary with an application of current?

hmm...
thanks

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I don't see why not. Essentially, you are 'bucking' the magnet(google bucking magnet) through an electromagnet. But there will be physical forces between the coil and magnet.

Not to mention it won't be perfect. I doubt you'll ever get perfect cancellation. If you take a stereotypical cylinder of a bar magnet and wrap wire only around the curved suface, the magnetic field touching the poles will be even stronger with the current on while objects further away see the cancellation.

Hi!
You're welcome.
Last edited by alainprice; 2008-Jul-23 at 03:57 AM.

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Originally Posted by sabianq
got to thinking bout something and I came across a problem that could be solved with a "reverse electro magnet".

since you can make a bar magnet with a iron rod, DC current and a coil of copper wire, is it possible to cancel a permanent bar magnet with a coil of copper wire and a current? Can one in essence shut off a neodymium magnet temporary with an application of current?

hmm...
thanks
Yes, in principle you can do this but:
1) you are better off using an iron magnet. The field of a neodymium magnet is so high it would be hard to make a coil strong enough.

2) Perfect cancellation would be very difficult. The fast drop off of the magnetic field, coupled with imperfect field orientation, would make it hard to get to zero.

3) it would be a big pain in the backside to do. Hysteresis would make trying to get the field to zero very time consuming as you would have to slowly reduce the magnitude of the field.

4. If you apply enough energy to melt the bar, sure. Randomization FTW!

5. Originally Posted by sabianq
is it possible to cancel a permanent bar magnet with a coil of copper wire and a current?
Cancel would be the non-technical description. An opposing magnetic field does not actually cancel the original field, but rather warps the field lines to a tighter configuration so they are not available at some distance. Some leakage is inevitable so the field is only weakened.

Using a Mu metal shroud will also warp field lines and reduce a magnets sphere of influence.

6. thanks for the repply.
I am looking for a very high strength magnetic field that can be turned on and off as needed.

Neodymium magnet strength is perfect, electro magnets are to week, so i was hoping there was a way to "turn of" a permanent magnet temporary in much the same way that one can turn on an electro magnet.

thanks

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You need a superconducting electromagnet.

Problem is, they are limited in current to remain superconducting.

8. Originally Posted by sabianq
thanks for the repply.
I am looking for a very high strength magnetic field that can be turned on and off as needed.

Neodymium magnet strength is perfect, electro magnets are to week, so i was hoping there was a way to "turn of" a permanent magnet temporary in much the same way that one can turn on an electro magnet.

thanks
You may want to research, superconductor magnetic cranes. The technology is developed for you and many issues resolved already. For example you can not instantly cut off the current in a coil (due to impedance). What they do instead is to flow the current through a 2nd coil.

Super conducting magnets have limitations based on coil strength which is subject to extreme mechanical forces.

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Originally Posted by sabianq
Neodymium magnet strength is perfect, electro magnets are to week...
If you want to cancel a strong magnetic field, you need to use another strong magnetic field to do it. If your magnet needs to be on longer than it needs to be off, then you'd want both the permanent magnet and the electromagnet. But if it needs to be off more than on, then you just need the electromagnet.

10. OK, my strength has never been in EM physics, but how does this relate to a faraday cage?

11. Perhaps it makes sense to use a weaker iron bar magnet and wrap it with an electro-magnet. When you want to pick something up, flow the current in the coil so that it is additive with the bar magnet and reverse the current when you want to buck the bar magnet. The combined strength of the EM and the PM may be enough for the task at hand and you may still have the advantage of "switching off" the PM.

12. Originally Posted by Veeger
Perhaps it makes sense to use a weaker iron bar magnet and wrap it with an electro-magnet. When you want to pick something up, flow the current in the coil so that it is additive with the bar magnet and reverse the current when you want to buck the bar magnet. The combined strength of the EM and the PM may be enough for the task at hand and you may still have the advantage of "switching off" the PM.

nice!
for this application, the system needs to be small and very strong.
i like everyones insight!
thanks
cheers

13. Another piece of trivia that you may or may not find useful:

A superconductor is the only known material that will completely stop a magnetic field.

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It excludes fields from within the conductor, more specifically.

This is not true for both types of superconductors, though.

15. I'm sure I have read that piece of trivia somewhere, but what you are saying does make sense.

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