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Magnetic Field Strength and Low Temperature
Name: Lisa S.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 10/16/2004
Question:
Does decreasing the temperature of a magnet effect its
magnetism?
Replies:
Hi, Lisa-
All permanent magnet materials have a high temperature at which
magnetism is extinguished.
At room temperature, increasing temperature usually causes a small
decrease in field strength.
As temperature increases further, this slope gradually gets larger, until,
approaching the Curie temperature of the material, magnetism dives to zero
and is extinguished.
I have a "Levitron" kit, a large square magnetic pedestal with a small
magnetic spinning top.
If you spin the top and lift it into position over the pedestal correctly,
repulsion between the two magnets keeps the spinning top hovering in mid-air.
(After about 5 minutes, air drag slows down the spinning, so the top flips
over and falls.)
Well, on a cold day, the magnets are a few percent stronger and the top
needs more weight to balance at the right height.
Then when my hand warms it up, the top gets weaker and some of the weight
needs to be taken off.
I believe the top has a Neodymium-Iron-Boron type magnet, for which the
dependence on temperature is notoriously strong.
This is simply because the Curie temperature of neodymium magnets is
unusually low.
If iron-based permanent magnets have any serious, sudden changes at
cryogenic (very cold) temperatures,
I do not know much about them.
Electromagnets are also improved by cold, but in a different way.
Low temperature makes the resistance of the copper wire smaller,
so one can get a stronger field by applying higher currents, without
overheating the wire.
An extreme but similar case is electromagnets made of super-conducting wire,
which has zero resistance when it is below it's superconducting transition
temperature.
So one can power up the magnetic field much farther than one could with
copper wire,
to a field about ten times stronger than the best rare-earth permanent
magnets.
Then the limits are:
a) the coil might break from the wires on opposite sides of the coil
magnetically repelling each other.
It is really that strong.
b) superconductivity is extinguished by sufficient magnetic field
strength.
This allowable strength is higher when it is colder.
Warming up to the superconductive transiton temperature,
the allowable field declines more and more steeply, finally
diving to zero.
c) you do not want the stored current to melt the whole thing if the
superconductivity suddenly stops.
Seems like cold is always your ally when making magnetic fields.
Jim Swenson
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