Measuring Work 1

[alansimpson]

I wish to measure the work generated from the force exerted by permanent magnet. Using the simple basis of force by distance I set up a static test to measure the load on an object as I moved it away from the magnet incrementally. By summing all the load X increment of movement I should should get an idea of work being done by the magent over total distance moved. Something tells me my thinking is sloppy on this issue.

Any comments?

Is there a better way of measuring work?

 

[JStephen]

The magnet isn't doing any work, you are. You're measuring a quantity, but what that measurement tells you, I have no idea. Using a different size or shape "object" will give different answers, will it not?

You could measure the "work done by friction" or "the work done by molasses" in the same way, for what that's worth.

And given that you're measuring the work in that way, you can use approximate integral methods instead of just adding it up. IE, Simpson's rule or something similar.

I'm not sure what you're getting at, but I have seen articles in the past in less-than-reliable publications about the endless energy stored in magnets. There isn't any endless energy stored there. They do MAKE magnets and whatever little bit of energy is stored in them is just a remnnant of what went into the things at the factory.

 

[TheTick]

Treat magnetism it the same way you would treat gravity. [Δ]PE = mg[Δ]h for gravity. On a larger scale, this would be an integral with g being a function of h, as well.

Perhaps take some force measurements at discreet points and get a working curve of force vs. distance. I believe field strength is generally inverse to square of distance.

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[alansimpson]

I am not trying to extract energy or work from a magnet. I am trying to measure the work done by the force exerted by the magnet over a distance. Just as a coiled spring does work. I am assuming work is done to move object into position from which I start test. I could do test in reverse and obtain data.

 

[JStephen]

In that case, sounds like you're doing it right.

Some magnet configurations have very strong attraction when the parts are very close together, and that may be difficult to deal with.

 

[IRstuff]

Depending on the desired accuracy, a fisherman's spring scale and some sort of controlled linear motion like a gear-drive slide could get you there.

TTFN

 

[Cockroach]

"TheTick" has the correct analogy. A magnetic field can be treated exactly as it's gravitational counterpart once "mass" and "pole strength" are changed. The gravitational constant "G" simply becomes the inverse to permeability of free space.

In terms of mathematics, you get F = (P1 P2)/(u d^2). As in classical physics, integrating force over distance results in energy, so that work and energy are the same thing. You can now obtain the mechanical potential for your system, then dividing by distance would be "magnetic strength".

Note you could also thing of things in terms of torsion, since maganetic moment and work have the same units, force X distance. Check out your elementary electrical books, this is how an electrical motor works. Surfing the web, I imagine, would probably gleam good results.

Keep it simple. I don't think applying the various line integrations with Gauss Law would be necessary in your case, unless of course you are into self abuse.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada