physics question #155



Anna, a 14 year old female from the Internet asks on March 17, 2000,

Q:

Does the temperature of a magnet affect its strength?

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the answer

James Livingston answered on March 16, 2000, A:

[Editor: The simple answer is that if you heat a magnet it will lose its magnetism. Now here are the details from Dr. Livingston.]

I discuss this in my book Driving Force: The Natural Magic of Magnets. The Curie temperature is defined as the temperature above which a ferromagnet loses its magnetism. The strength of any magnet will decrease as the temperature approaches the Curie temperature.

The Curie temperature is very high for some magnets, such as alnicos and cobalt-samariums, which retain much of their magnetic strength to temperatures as high as 500 degrees Centigrade. The neo (neodymium-iron-boron) magnets that are the strongest at room temperature, however, have a relatively low Curie temperature, and some start losing some of their strength at temperatures as low as 150 degrees C. Ferrite magnets, the most commonly used permanent magnets, anomalously gain a little strength with modest increases in temperature, but lose magnetic strength starting at about 200 degrees.

For each material, alloying additions can be made to decrease sensitivity to temperature, an important feature in some applications. Most companies that sell magnets provide in their data sheets information about the temperature sensitivity of properties, usually in the form of a temperature coefficient or some similar term.

Small changes in temperature (say up to the boiling point of water) don't have much effect on today's magnets, but large temperature increases will eventually decrease their strength, in some magnets more than in others.

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