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# measure magnetic attraction versus distance

## measure magnetic attraction versus distance

(OP)
I mave measured attraction versus distance, my results are a little unexpected and I want to compare mine with someone else's. But I do not know he did similar experiments. If someone could give me a person's name, I will be grateful.

### RE: measure magnetic attraction versus distance

In the meantime, perhaps you can describe your results and what you expected

### RE: measure magnetic attraction versus distance

(OP)
What I expected: what you find in literature: like Newton's gravity law : F :: 1/r^2. Or if you prefer to express that different: In a rotatory symmetric system {\nabla\cdot B == 0} everywhere, the same since there is no source or sink of B outside the magnets.

On log-log paper that gives a line with slope 2:1. For all 6 different sets of magnets the points lie an a perfect straight line, but their slope is alike, but far from 1:2.

What I am after is confirmation that more people have measured that, and if so, how they explain it.
----

By the way: When you let the distance decrease slowly, the magnets (which one?) do the work F * \delta l. Where does that energy come from? The only thing I can think of is internal energy, which means that they grow colder. Has anybody measured that? Does there exist a book on the thermodynamic theory of magnetism?

One more, probably stupid question:
Suppose I have a copper wire from me to the other end of the table, 5 cm above the table. Current from me to the other end, 15 - 20 Amperes.

I move a compass across the table under the wire. Which way can I expect the pointer marked "N" of my compass will point to? Left or Right?

To make sure that my compass 'feels' the circular field I move it to above the wire. As expected the compasneedle turns to its opposite.

It should be a simple question: What do you expect and why:

a. When below the wire, the pointer marked "N" points to my left, when above to my right
or.
b. below to my right right, when above left?

And I will be very grateful to the one who makes me understand the meaning of "Permeability" of "Nothing".

N

### RE: measure magnetic attraction versus distance

You word this like it's homework. If it is, it is not allowed.

Otherwise, if you go to the mechanical engineering other topics, you'll find a post on movement of overhead conductors which relates to your post.

### RE: measure magnetic attraction versus distance

This is all stuff that's evident upon studying an elementary magnetics text. One thing to bear in mind is that a real magnet cannot behave with 1/r^2 behavior, because it's not a point source.

### RE: measure magnetic attraction versus distance

(OP)
Yes, you are right, it is.

But a nobody can build a safe, reliable house, let it be a skyscraper, on a lousy foundation. So before I can do anything I must make sure I understand the foundations in all details.

So please answer the simple questions, you will see they are not as 'silly' as you think they are.

### RE: measure magnetic attraction versus distance

This is neither a homework nor tutoring site. And this does not appear to be work related.

### RE: measure magnetic attraction versus distance

On what basis do you expect there will be a force varying as 1/R^2.

It's not a trick question, it's not obvious to me how we would come to that conclusion.

I think you start by drawing an analogy to electrical circuit.

We have two dipoles separated by a distance R >> r where r is the characteristic distance of the dipole.

The far-field behavior for a dipole is 1/R^3 (drops off faster than the point charge 1/R^2 since the effects of the equal/opposite charges of a dipole tend to cancel each other at a distance).

How does the other dipole react to the field from the first? Its reaction can be predicted in terms of a torque based on field strength, dipole strenght, and angle relative to the field. How are we tranlating that torque to a force?

=====================================
(2B)+(2B)' ?

### RE: measure magnetic attraction versus distance

How many poles are the magnets? For a two-pole/thru-magnetization (point source) the exponent is 3, but generally the exponent is the number of pole-pairs plus 2.

https://en.wikipedia.org/wiki/Dipole#Magnitude

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