Magnetism and Concrete

Most recent answer: 07/01/2016

I have a question on permanent magnets losing their magnetism. We have large magnets we use in our shop. These magnets have been losing their strength rather quickly. I have always heard that leaving a magnet on concrete would cause the magnet to lose its strength similar to leaving a car battery on concrete. Is this true or an old wives tale?
- Ed (age 47)
Kodak Tn USA

I believe the concrete has essentially no direct effect. If, however, the concrete is out in the sun it can get very hot. Heat definitely speeds up the decay of permanent magnets a lot. Also, if the magnets are subject to mechanical shocks that can speed up the decay. 

Mike W.

(published on 07/01/2016)

Follow-Up #1: magnetism and steel bars

Most concrete structures are reinforced with steel bars. I wonder if the effect (the questioner asks) have to do with these bars!
- Mehran (age 65)

Probably not, since locating a steel bar near a magnet actually lowers the magnetc free energy a bit and thus stabilizes the magnetized state.

Mike W.

(published on 07/06/2016)

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