Why Did a Coin Jump Off a Heated Light Bulb?

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SUMMARY

The phenomenon of a two pence coin jumping off a heated 60W light bulb is attributed to a combination of thermal expansion and electromagnetic effects. The coin, made of copper-plated steel, experiences nonuniform heating, causing it to potentially deform and create a critical point where it is propelled off the bulb. The discussion suggests that eddy currents generated by the alternating current in the bulb may also play a role in this event. Experimentation indicates that the coin's behavior is repeatable under similar conditions, particularly with the 2p coin.

PREREQUISITES
  • Understanding of thermal expansion and its effects on materials
  • Basic knowledge of electromagnetic theory, particularly eddy currents
  • Familiarity with the properties of copper-plated steel
  • Knowledge of AC voltage and frequency, specifically 230V AC at 50Hz
NEXT STEPS
  • Research the effects of thermal expansion in metals and how it can cause deformation
  • Explore the principles of eddy currents and their applications in electromagnetic fields
  • Investigate the properties of different coin materials and their thermal and magnetic characteristics
  • Conduct controlled experiments to test the phenomenon with various coins and light bulb wattages
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Physics enthusiasts, experimental scientists, and anyone interested in the intersection of thermal dynamics and electromagnetism will benefit from this discussion.

  • #31
is there any thermal aspect of this event?
 
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  • #32
I'm going with Integral on this one. The thermals are causing the coint to, ever so slightly, conform to the shape of the bulb. It forms into a shape similar to a wave spring or wave washer. As soon as you disturb it, the center of heating moves away from one side which cools quickly, causing the coin to reshape itself.

Does the coin flip in the air when it goes flying?

What's the dimensions of the coin? I'll see if I can squeeze in a model.
 
  • #33
I'm more inclined towards something along the lines of what danger just mentioned. I see this as a likely result of a phase transition at high temperature, metastable state. That even explains why the perturbation is required.

The alloy, after being heated, should have entered a new phase, but appears to still exist metastably in the low temperature phase. A small perturbation causes rapid transformation into the high temperature phase. The attendant rearrangement of the crystal structure leads to a small but macroscopic deformation...and viola ! You have the flight of the 2p.

As an experimentalist, I have a few questions/suggestions.

Nishant : You claim to have gotten this same result. Was it also with a 2p coin ? (ie : 2p or not 2p, that is the question.)

Can someone (brewnog ? others ?) find out the alloy composition, and try and hunt down a phase diagram for that family of alloys ?

To test Integral's (revised by Fred) theory : Fred, can you try a crude simulation ? I find it hard to believe that you can build up large enough thermal gradients in a metal coin that you create a shock-wave from thermal relaxation. I'll see if a back of the enveloper gives me anything...

We need data...this looks like fun !
 
  • #34
FredGarvin said:
What's the dimensions of the coin? I'll see if I can squeeze in a model.

brewnog said:
2p coin specifications: Copper plated steel, 7.12 grams, 25.9mm diameter, 1.85mm edge thickness.

The copper coating will sustain very little thermal gradient (even at small thicknesses, I guess...haven't calculated anything yet), so may essentially be isothermal. Stell has much poorer thermal conductivity, though, in this case, has a larger A/L, so the conductance may be similar.

Any numbers on the alloys or coating thickness, brew ?
 
  • #35
Danger said:
What about if you just heat it any old how, such as on a stove?

If the 2p jumped after being heated by the stove or by a flame (I'm not sure on the electrical properties of a stove), that would show that the heat and not the electric was the cause of the jump, correct?
 
  • #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.
 
  • #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...