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Q & A: magnetic stripe damage

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Most recent answer: 01/14/2013
Q:
When a magnet damages a magnetic stripe on a credit card, why does the length of time of exposure matter more than the strength of the magnet?
- Evan (age 12)
Ellicott City, Maryland, USA
A:
That claim is surprising. I bet the opposite is true. The stripes are rated by "coercivity", the approximate magnitude of the magnetic field needed to flip their little magnetic domains. The dependence on the exposure time is much weaker than the dependence on the field, so the ratings don't usually even mention the exposure time. There's a good physical reason for this. The domain alignments are strongly held in place by interactions with nearby structures. To flip on its own, the alignment has to briefly pick up energy much bigger than the typical scale for the thermal (heat) energy at room temperature. That happens very rarely. Magnetic fields bigger than the coercive field push so hard on the domain alignments that they can flip without having to pick up any extra energy. Basically, in those big fields the flip is "downhill" all the way. There is a narrow range of fields a little less than the coercive field that lets the domains flip by picking up only a modest amount of thermal energy, and within that narrow range exposure time matters. As a first estimate, though, it's pretty close to say that fields less than the coercive field don't matter much for reasonable times and fields greater than the coercive field flip the domains very quickly. Mike W.

(published on 01/14/2013)

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