Magnet question

[Sylvia Else]

Just to be perfectly clear, we cannot shield one end of a magnet. The
covered pole just gives a magnet with one end bigger than the other.
In other words, more screws will be attracted to the growing blob of
screws until it's large enough to enclose the other pole and all the
flux in between.

However, the addition of each screw changes the shape of the field. It
will not be always possible to add a new screw at an arbitrary place on
the outside of the growing blob, even when there remain places where it
is possible to add a screw.

Sylvia.

 

[Sylvia Else]

Interesting COE issue:

As the first screw is yanked up to the magnet, a certain amount of
work is done. That work could be recovered by having the screw drive a
tiny winch and generator on its way up. If that's not done, the screw
will slam into the magnet and waste that energy as noise and heat and
deformation and EM radiation and magnetization of the screw itself.

Any eddy currents will also heat the screw on its way up, again
wasting the energy.

Since the magnet was magnetized using some finite amount of energy, no
more than that amount is available to lift screws. So the number of
screws that can be lifted is limited by some sneaky geometrical
considerations. COE is often sneaky.

Nothing inherently limits the number of screws that will stick to the
magnet, provided the magnet doesn't have to do work to add more
screws.

Still, if a screw will stick to the magnet, or the growing blob of
screws at some point, then there must be at least some force holding it
there, and some work would be done on the screw as it approaches.

But I agree about COE being sneaky. It's often far from obvious how the
Universe keeps track so as to balance its books.

Sylvia.

 

[Robert Monsen]

Ok so I have a permanent magnet which I fix say 1 inch above the
table.
I then slide a screw under it and it pulls it up. It has done mgh
amount of potential energy. If I keep doing this will the magnet
eventually de-magnetise? ie the energy in the field has all gone in
doing this work,


Hardy

I wonder if the earth loses gravitational field when meteors fall into
it?



When the screw 'falls into' the magnet, it is giving up potential
energy... you get energy when you move a screw from infinity to the
magnet. The screw will exchange its potential energy for kinetic
energy, first in the form of motion, and then in the form of heat in
the magnet.

Note that the screw won't move unless the net energy of the system is
less than before, meaning that the potential energy loss the screw wrt
the magenet is greater than the potential energy gain of the screw wrt
the earth's magnetic field. So, some energy is given off in this
exchange, in the form of heat.

Regards,
Bob Monsen