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Re: Bar magnets



Wow, did I goof!
The x in the F_n magnetic force term is NOT the same as the x in the Hooke's
law term!
Fix it, Ludwik!

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor
----- Original Message -----
From: "Bob Sciamanda" <trebor@VELOCITY.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Saturday, February 17, 2001 12:46 AM
Subject: Re: Bar magnets


Look to see if there is an equilibrium point.

As a beginning stab, model the magnetic force as a power law: F_n=C/x^n :

Then F(x) = mg -kx +C/x^n.

Set dF(x)/dx equal to zero to look for an eqilibrium position.
(I think you'll find a negative value of x for positive n.)

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor
----- Original Message -----
From: "Ludwik Kowalski" <KowalskiL@MAIL.MONTCLAIR.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, February 16, 2001 10:20 PM
Subject: Bar magnets


Without thinking too much I attempted an experiment
which failed, as described below. Comments and
suggestions will be appreciated.

I wanted to measure the attractive force between the
poles of two bar magnets (each 13 cm long and 1 cm
in diameter. Keeping these magnets by hand I started
to feel the attraction when the distance between the
coaxially positioned bars was 1 cm or so.)

I suspended one magnet from a brass spring whose
k was 10 N/m. The mass of the spring was 175 g
while the mass of the magnet was 61 g. Naturally,
the spring stretched a little. Then holding another
magnet by hand I started approaching its pole toward
the suspended magnet from below. I expected to see
additional stretching of the spring for each distance
between the poles, for example 1 cm, 0.75 cm and
0.5 cm and 0.25 cm. But this did not happen.

The suspended magnet moved suddenly to the hand-
held magnet to establish the contact (when the distance
between the poles was less than about 1 cm). This
happened regardless of how careful I was in trying
to prevent natural oscillations. What is wrong with
this attempt to measure the magnetic force Fm?

I expected the equilibrium at: k*x=mg+Fm.The
Fm increases when the distance between the poles
becomes smaller and I expected larger x for smaller
distances between the poles. What should I do?
I know that Coulomb used a torsion spring to
establish the relation between Fm and the distance
between the two poles. Why is my linear spring
different? Why was my equilibrium unstable?
Ludwik Kowalski