Theories and questions about Magnetism
 

Ok, a good friend of mine and I are trying an experiment with magnetism, electromagnetism in general. Unfortunately, this is something neither of us have too much experience in.

We need to move a permanent magnet along a path, say a cylinder. We want to position the electromagnet at one end of the path, and pull the permanent magnet by applying power to the electromagnet.

I have several questions about the practicallity of what we are trying to do. First, how strong can the fields in a practical (I.E. - homemade) electromagnet get?

Second, I know it isn't the best resource, but Wikipedia says if you use an electromagnet with a soft iron core, it retains that magnetic charge (hysteresis), and if so can that charge be dissappated or even reversed (to force the permanent magnet away from the electromagnet).

My understanding is that if you reverse the current going through the wire around the electromagnet, you change the polarity. Can you overcome the existing polarity that the iron core is retaining? Is much lost in overcoming and changing that polarity?

Last, what would be a practical way of making a homemade electromagnet, preferably something circular, and that can maintain a bit of a kick. We're planning to use a 12 volt motorcycle battery to power the magnet, with a variable resistor to moderate the amount of charge going to the magnet from the battery. If it wasn't obvious, we also want to put a switch somewhere in the electrical system to try to use the existing wire around the electromagnet to reverse polarity in the magnet quickly. That way we could go from attracting the permanent magnet to "pushing it away". Is this practical, or even possible?

Any questions/thoughts/criticisms would be appreciated.

.....I am no genius but...are you trying to get anwsers for your homework? If so then this proubly is the place.

and yes I belive it could be possiable done. But once again I am no genuis. But I am pretty sure you could do that.

Nope, this was for an experiment. I would like to create a certain kind of device that utilizes electromagnetism, and am looking for anything that might help, that's all.

As I understand it, the strenght depends how much energy you put into the wires. I'd imagine that a homemade one without an excessive powersource wouldn't be too strong, but I don't know.


with an EM with iron inside, there will be some left over magnetism even when it is off due to the iron being magnetized. I think if you changed it's polarity it wouldn't be as effective - because of the previous effect the magnet had on the metal.


Again, with an iron core it might be difficult to reverse polarity because of the iron becoming magnatized, but I'm not sure on this matter - I used a refrigerator battery with less of a charge than that, it wasn't intended to be very powerful and I used very little energy.

This has everything to do with the type of metal in the core the number of turns or wire and the power you are pumping through it. In short use the soft iron, there are other reasons for this, and a lot of turns. Also, the variable resister will give you the option of fine tunning the strength. It will be much stranger than natural magnets though.

The soft iron is going to retain a very small portion of the magnetism and its very easy to get rid of, having been very easy to put there. Now the hardish steels will do the same thing if you run an electromagnet around them long enough and will take just as long to reverse. In short, all the soft iron core is going to do is give you a bit of lag time on the switch from on polarity to the next, which will happen anyways due to the induction in the coil. There isn't all that much loss in doing this and its rather unavoidable.


That right there sounds good just find some sort of soft iron ring. As for wire I suggest going to Radio shack walking in and asking for "magnet wire" and then saying something along the lines of "that wire they wrap electromagnets in". Thats all I did to get the wire I use. Then you wrap the iron core, making sure all the turns go one way and remembering that if you curl your right hand around the wraped wire in the direction current flows, negative to postive, your thumb points to where the magnets north pole should be.

Do you lose anything in the process by adding more turns to the wire? And could you do this on a circular piece of iron? Now I'm just asking out of curiosity, because my original idea has already been done...

My original idea was to create an engine that used a permanent magnet as a piston. The electromagnet could be mounted to the top (where the head would normally be), and if you cycled current to the electromagnet, you could turn the engine.

Then I checked the U.S. Patent and Trademark website. Sucks to be me.

Someone has already created such a device, although they're device deviates from what my idea is a bit. I have other theories that I'm running with, so we'll see what happens.

http://patft.uspto.gov/netacgi/nph-P...&RS=PN/5219034

By the way, I'm still pursuing this, and would be interested in any ideas.

About the only thing you lose with more turns is time. The more turns you have the longer it will take to reverse the polarity if I remember my induction right, but that only comes into play when you reverse current direction and first turn the thing on.

The shape of the iron doesn't matter a great deal. Although if its a ring and you wrap the wire so it passes into and out of the hole in the ring I'm not sure what'll happen. My guess not a very strong magnet as magnetic north would change directions as you moved around the ring. For best results you want the wire wrapped in more or less a straight cylinder.

Another comment on the core issue; yes, depending on the material you use, it can be difficult to fork up the domains enough to reorganize them completely. This is uber-true with stuff like.. neodymium!

Generally, though, the only thing you need to depolarize an electromagnet is a hammer; shut off the current, and either heat the material (not to melting point, but get it hot) or smack it as much as it can stand with a hammer or drop it if your material will stand that. Voila! Total demagnetization.

Hell, you don't even need a current loop. A cylinder of neodymium/iron/boron and a coil of wire will do. You can power a flashlight with just that stuff and a string to hold the magnet.

As for natural magnets and homemade electromagnets... no. NIB (again) magnets are strong enough to break fingers... can you find me a household electromagnet that can do that?

You need a true superconducting magnet or a powerful electromagnet to outstrip rare earth magnets. Mine are rated at 12,500 Gauss; I don't think many household EM devices can generate that much juice.

Or you could always use a star, whose magnetic field can stop a freight train from 250,000 miles away. o_O

Hmmm.....

Some really good thoughts, I appreciate the responses. The magnetic field for the device I'd like to create wouldn't have to be too strong because the piston travel will probably be less than 6 inches from bottom dead center to top dead center. I've thought about rare earth magnets, but am at a loss as to where I can obtain some that are at least 1 1/8 in. (the size of my cylinder) and preferably circular.

Try the link in my sig. :cool: