Old 09-21-2012, 01:26 PM #1
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Default Calculate Electromagnetic Field Strength

Hi!

This forum has been extremely helpful to me in the past, and I am hoping I can get a little more benefit from all of your collective experiences.

I have recently become interested in electromagnetic fields generated by electromagnets.

I have not, however, had any luck at all figuring out how to calculate the strength of a field produced by a given electromagnet.

For example:
If I have a 1/16 of an inch iron rod, with a single layer of 36 gauge wire wrapped around it (the number of coils will vary with length of course) and I am running (v) volts through it with (I) amps, how strong will the field be at (x) inches from the coil? How do I do this with metric?

And once I have the answer, I am sure the units will be of no use to me, so how do I use that to work out how much actual weight the thing can move of different materials at different distances?

And does the shape of the object being moved matter?

What if Im using a magnet as the "object being moved"?

What shape will the field be?

This is literally so far beyond me to work out on my own that I dont even know where to begin. I tried the usual googleing, but everything I came up with seems to assume that I already have an electrical engineering degree.

Even a simplified kids version that just gave me a rough idea would be great!

Thanks so much guys,
-Liam.

Edit 9/21/12, 9:22am :

What happens when I change the diameter of the wire or the core? What about different insulators around the wire? What happens if I change the pattern of wraps, or change direction at the end of the core and come back? Does is help or hurt?

Thanks!

End of Edit 9/21/12, 9:23 am



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Old 09-21-2012, 02:31 PM #2
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Default Re: Calculate Electromagnetic Field Strength

Well to some extent, you do need to understand why and how the magnetic field being generated is going to develop for around a wire. For there you can continue with solenoids which are only a little more complex.

If you hang tight, I should have more time later today to post some formulas for you. I will have to break out my physics book as I only spent a few weeks on it a couple years ago.

So what I may need from you is:

What is the setup of your magnet? Do you want it to pull objects upward against gravity to hold them? Or were you trying to push objects upward with the magnet being underneath?

The core of your solenoid will have a different permeability than open air, so we'll need to account for that in the calculations.

Determining the forces that result from one end of a solenoid may be more difficult to calculate than for a straight or a single-loop current, but I'll explore it so to see what we can know.

Shape? Shape might have an effect on the orientation in which the object will want to align itself, once its in the B field. Object volume and/or surface area will also determine the flux that can reach the object. Weight of the object of course important also.

If you're using your standard solenoid coil, the B field lines are shown here: Solenoid - Wikipedia, the free encyclopedia

Again, I should be able to help you find the field inside the center of the coil, but as for the outside, I would probably have to break about my calculus book to review vector field calculus. I think for now, you can just start small, and work your way up to a better understanding. Get me some of this information above, and I'll try and collect some useful formulas and puts them in a useful form for you.
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Old 09-21-2012, 04:15 PM #3
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Default Re: Calculate Electromagnetic Field Strength

Thank god for this forum and its incredibly competent members.

Ok, so the goal is to set up a coil that will vibrate a small magnet from a few inches away. It only has to move it the slightest amount.

I dont have any of the equipment yet, so I am looking for suggestions. Wire? Core?

After reading that article, I have a few questions about electromagnets in general.

When an electromagnet "atracts" something, the solonoid is trying to move it into the center of the coil, far enough toward the middle of the solonoid to avoid fringe activity... right?

Assuming that the above is correct, what is it trying to do when it has a core? Is it trying to put the attracted object into the middle of the core?

And how do I interpret those force lines around the solonoid that everyone shows in their drawings? Does the "looping" line mean that if I put an object outside a solonoid it will whip around the edge and do its best to get iinto the middle?

In the formula for the magnetic constant, what does the "L" in the denomenator represent?


What the H does "magnetic flux density" mean?

Again, thank you so much for helping me with this. I really need to find a way to give back to this comunity, because you guys have helped me with basically every succesful project I have ever completed.

Thanks,
-Liam.

Last edited by Lascannon; 09-21-2012 at 04:25 PM.
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Old 09-21-2012, 08:36 PM #4
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Default Re: Calculate Electromagnetic Field Strength

Wow, too many questions to answer in one post. I'll do one or two for now, and try to fill in any gaps for what meatball is cooking up.

The wire used only determines the max current you can send through it without it melting. The field strength is dependent on the current, not the wire diameter. Not directly anyway. If you have a set voltage, smaller wire -> more resistance -> less current -> weaker field. It also means more heat. I think that means ideally you want larger wire with current control, rather than voltage control since this means much less heat than relying on the resistance of the wire to limit the current. IF you want something to vibrate at a small distance, that means feeding the coil AC (like a speaker) instead of DC. With AC, the coil's inherent inductance will help limit the current further.

The simplest way to get a magnet to vibrate is probably to get the voice coil out of an old speaker, and power it with a typical home stereo amplifier.
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Old 09-21-2012, 09:58 PM #5
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Default Re: Calculate Electromagnetic Field Strength

Can I get a large enough coil? I was looking for somewhere in the 4 inches range. What sort of speaker has a larger enough coil for that? Does the size even matter? Or is it just number of coils? Or current?

Oh dear I'm afraid I am a complete newby.

Thanks so much for helping me out, cypragon!

-Liam.
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Old 09-22-2012, 02:47 AM #6
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Default Re: Calculate Electromagnetic Field Strength

It depends on the permeability of the core (air is less, iron/ferrite etc is more), number of turns, current, and distance to the coil. Woofers will have a larger diameter voice coil, but that is mostly just to even out the force on the larger cone from what I understand.
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Old 09-22-2012, 03:06 AM #7
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Default Re: Calculate Electromagnetic Field Strength

So do you have a power supply ?

Do you have any of these components, wire, iron core (nail)

I would suggest you get some wire and a big nail and just try it out experiment even if you don't have power supply
you could use a "D" cell those are pretty common and easy to get and wire is not hard to find, you could cut open
an old computer cable and use one of the conductors from that.

Try it out experimenting is fun why let every one else have all the fun when you could do it your self

good luck
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Old 10-10-2012, 03:37 PM #8
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Default Re: Calculate Electromagnetic Field Strength

Ok, so I have done some experimentation. It turns out to be supprisingly easy to do.

However, I still do not understand the rules which govern this system. Basically, what I need it this:

A complete guide to calculating electromagnetic field strength and shape, that takes into account all of the relevent variables, such as, is there a core, where is it relative to the wires, how are the wires wound, what is everything made of, how many times did i wind it, how many volts and amps do i have from my power supply... I don't even know where to begin.

If anyone knows of such a book or webpage, I would be incredibly grateful.

Thanks!
-Liam.
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Old 10-10-2012, 04:15 PM #9
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Default Re: Calculate Electromagnetic Field Strength

See lectures 11, 14, 15, 20, 21, 22.

I recall lecture 20 as having some detail about the standard solenoid and its properties.

The more inductance a coil has, the more energy it can store in a magnetic field.

MIT OpenCourseWare | Physics | 8.02 Electricity and Magnetism, Spring 2002 | Video Lectures
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Old 10-11-2012, 02:44 AM #10
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Default Re: Calculate Electromagnetic Field Strength

I love you. I mean that in the least creepy way possible. But I love you.
Thread closed. Thanks guys. Done deal.

This forum is amazing.

If I could give reps, I would, but I cant yet. I'll throw some photos up when I have my project finished.

Thanks everyone!
-Liam

Edit: Turns out I can rep. Hu-rah!

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Old 12-02-2012, 02:11 AM #11
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Default Re: Calculate Electromagnetic Field Strength

Quote:
Originally Posted by Lascannon View Post
In the formula for the magnetic constant, what does the "L" in the denomenator represent?


What the H does "magnetic flux density" mean?
H doesn't have any specific name but it is useful in the lab because when you build an electromagnet you run a certain free current through the coil. And the current is what you can read directly on the dial and that determines H. Whereas B (what is usually called the 'magnetic field') depends on the specific material used.

H is really an expression that does in magnetostatics what D (electric displacement) does in elecrostatics. It allows you to write Ampere's law just in terms of free current. Since H = B/μ0 - M, ∇ B => ∇ H = J, or in the integral form ∮ H . dl = I, which is what's useful to us.

To find out the strength, you need a Gaussmeter with Hall probe to be placed between the pole pieces of your electromagnet and it will give you a reading. Here is a pic:


The thing with the yellow handle is the Hall probe oriented perpendicular to the magnetic field emanating from the north pole of the electromagnet (which in this case produces a nonhomogeneous field). The pic btw is a setup of 'Quincke's method' for measuring the magnetic susceptibility of certain liquid.
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