Hysteresis
Kai Ryssdal and Maria Hollenhorst, "What a magnetized iron screwdriver can teach us about the post-COVID economy", NPR Marketplace 12/2/2020:
There’s a scientific demonstration you may have seen in grade school, in which an iron nail or screwdriver is transformed into a temporary magnet by striking it repeatedly with an actual magnet. It demonstrates “hysteresis,” a term meaning “delayed” or “lagging behind” in both physics and economics. We won’t delve deeply into the science of why that works here, but the concept itself is important to understanding the economic damage caused by COVID-19.
“If you’re holding a piece of iron, the piece of iron itself is not likely to be magnetized,” said Elizabeth Green, a condensed-matter physicist at the National High Magnetic Field Laboratory in Tallahassee, Florida. “But when we do start applying an external parameter, in this case, an external magnetic field, we can start magnetizing this piece of iron, and if we remove that external field, the iron will still remain magnetized.”
It shows how once a piece of iron has been magnetized by something external, it will remain magnetized even when the thing that caused the change is gone. “Nature is lazy,” Green said. “It likes to stay in a particular state, it does not like change. So when we change the direction of this external field, what happens is everything lags behind.”
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