Why Did a Coin Jump Off a Heated Light Bulb?

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In summary: You should definitely try this at home! It's really weird and cool.In summary, a two pence coin placed on top of a 60 watt lightbulb in a standard bedside lamp shot up 3 feet in the air, presumably because of the magnetic field created by the current running through the lightbulb. It is possible to try this at home, although it is probably not something you would want to do.
  • #36
Gokul43201 said:
Any numbers on the alloys or coating thickness, brew ?

Until 1992, the coins were solid Bronze (97% copper, 2.5% zinc, 0.5% tin).
Since 1992, the only data I can find is "copper plated steel".

In 1998, both variations were minted.

I've tried it with a 1971 coin (as well as post-92 ones), and surprisingly it worked.


As for coating thickness, I can't find any data for that either, but on the plated coins it can easily be scratched off by a sharp implement. I guess they've been electroplated; the coating can't be more than a few microns thick.
 
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  • #37
brewnog said:
Until 1992, the coins were solid Bronze (97% copper, 2.5% zinc, 0.5% tin)...

I've tried it with a 1971 coin (as well as post-92 ones), and surprisingly it worked.
Out goes my theory ! No phase transitions in 3% brass till 1100C.

But, the whole thing being essentially copper, I doubt you can get more than a few degress of thermal gradient across it.

EDIT : A rough calculation says that you can get no more than 5K of temperature difference between the center and the rim, in the steady state.
 
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  • #38
What about the glass of the bulb?
Perhaps the shock is in the glass, not the coin... or some combination of the two.

Is the plating intact after the jump? Could the shock be caused by the plating catastrophic detaching from the core?

Where are the serious physicists? There's a paper in here somewhere...
 

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