Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Adam Murphy on 15/06/2020 16:44:48

Title: Do magnets weaken?
Post by: Adam Murphy on 15/06/2020 16:44:48

Rob got in touch to ask:

"Having bought a camera tripod with some strong rare earth magnets on the feet, I’ve been wondering - do magnets weaken over time?"

What do people reckon?

Title: Re: Do magnets weaken?
Post by: evan_au on 15/06/2020 23:40:25

A magnet with a strong overall magnetic field is in a higher energy state than the same material with many small magnetic domains which point in opposite directions, giving no overall magnetic field.
- So "permanent" magnets are thermodynamically unstable, and can be expected to weaken over time (in theory)
- But this could be a very long time; natural lodestones (magnetite minerals) have lasted for eons
See: https://en.wikipedia.org/wiki/Magnetite#Magnetic_properties

This theoretical instability can be accelerated by:
- Severe vibration: eg hit repeatedly with a hammer
- High temperatures: Approaching the Curie Temperature. This varies with material, but for commercial magnets, usually well over 300C
- Strong external magnetic field which opposes the existing magnetic field.
- Oscillating magnetic fields, decaying away to 0

See: https://en.wikipedia.org/wiki/Curie_temperature

Title: Re: Do magnets weaken?
Post by: evan_au on 16/06/2020 08:20:48

PS: On Lodestones
It seems that not all magnetite has a strong magnetic field (ie stronger than the Earth's magnetic field).
- it is thought that magnetite gets locally magnetized by the powerful electrical current from a nearby lightning stroke.
- It should be possible to detect this in lodestones discovered in-situ, as the direction of the magnetic field should rotate when moving around the location of the most recent lightning strike - and the strength of the field should decrease with distance from the lightning strike...

Magnetite has a low coercivity, so it can get demagnetized very easily
- So it's not clear exactly how long lodestones have held their magnetic field....

Commercial permanent magnets have high coercivity, so they should retain their field for longer (but beware of using your tripod on the top of a high mountain in a thunderstorm...)

See: https://en.wikipedia.org/wiki/Lodestone