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
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  • 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.

h0dgey84bc
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Recently for some unknown reason, I placed a two pence coin (I don't know if you are from the UK or not, but this is about 2.5cm in diameter, I think its made of copper plated steel or bronze, and weighs around 6-7 grams) on top of a 60w light bulb, in a standard bedside lamp, to my surprise after about 10 minutes of the light bulb being on, the coin suddenly lept about 3 feet in the air, weird. Any theories on the physics behind this?
 
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h0dgey84bc said:
Recently for some unknown reason, I placed a two pence coin (I don't know if you are from the UK or not, but this is about 2.5cm in diameter, I think its made of copper plated steel or bronze, and weighs around 6-7 grams) on top of a 60w light bulb, in a standard bedside lamp, to my surprise after about 10 minutes of the light bulb being on, the coin suddenly lept about 3 feet in the air, weird. Any theories on the physics behind this?
That has got to be one of the weirdest things I've ever heard of. To start with, you should probably double-post this in the psych section to find out why you put the coin there in the first place. All that I can think of, if it happened just the way you stated, is that it must have become magnetized by the alternating current in proximity to it, and was then repelled by the same field that created it. It's hard to imagine here, but you guys use 220AC, right? More current... maybe... I dunno, but that's my best guess. :redface:
 
I've taken drugs like that too...

:smile:
 
Adrian Baker said:
I've taken drugs like that too...

:smile:
Get thee to GD! We can use the likes of you down there to help supress the evil Artman.
 
haha, yeah, I have no idea why I initially did it, was just messing around, but was quite shocked when that happened.
The voltage we use in the UK in 230V AC at 50hz
Magnization is an interesting theory, it's just the spontoneity of it that shocked me, like you might expect it to be gradually maganitzed and pushed slowly off the bulb for example... or something like that
but it was like a certain critical value had been reached, and then wham, shot up...crazy stuff
 
Cool!
Is it repeatable, or was it a freak occurrence?
 
Damn right, try it again! Does the light bulb still work?


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








Edit: Ok I'm trying it myself. 5 minutes and it hasn't done anything yet.

Edit 2: The coin is still there, but 10 minutes ago it was lovely and sunny outside, and now there's an absolutely horrific storm! There's something going on here...

Edit 3: The rain is now coming down sideways. I really do think I've warped the gravitational field around here or something. Getting scared...
 
Last edited:
HOLY CRAP!

I was just about to give up on the experiment, the storm has stopped and I got bored. I went to turn off the lamp, and just as I disturbed it the coin went shooting off the light bulb!

I'm sorry if I sounded like I was mocking in my last post, but this really does work! The coin pinged off the lampshade (I was using an AnglePoise type lamp), shot across my room, landed on the floor and melted a hole into the carpet!

I'm moderately weirded out by this.
 
brewnog said:
I'm sorry if I sounded like I was mocking in my last post, but this really does work! The coin pinged off the lampshade (I was using an AnglePoise type lamp), shot across my room, landed on the floor and melted a hole into the carpet!

Perhaps we should consider a "don't try this at home" addendum... :rolleyes:
 
  • #10
SpaceTiger said:
Perhaps we should consider a "don't try this at home" addendum... :rolleyes:

Absolutely not. You can learn a lot from scars...
 
  • #11
Wow! Wowowow! I burnt my finger on the light bulb but it was worth it. I'm testing a penny now.
 
  • #12
h0dgey84bc said:
but it was like a certain critical value had been reached, and then wham, shot up...crazy stuff
Maybe there's a hysterisis point where random magnetic eddies suddenly snap into alignment. I wonder if the bimetallic nature of the coin has something to do with it. Too bad I can't try it here. It might work with our voltage and frequency, but we don't have similar coins. :frown:
 
  • #13
How about this.

Nonuniform heating. The center of the coin is in contact with the bulb, while the outer edge is not. The center will be considerably hotter, that is nearly at the bulb temperature, then the edge which will be much closer to air temp. Now either spontaneously or upon disturbance, the coins suddenly snaps to a concave (vex, depending on your view point) shape due thermal expansion of the center, which propels it across the room.
 
  • #14
Integral said:
How about this.

Nonuniform heating.
Sounds reasonable, but I'm wondering how practical it really is. Copper is something like second or third best thermal conductor of all metals. Wouldn't a copper jacket over steel distribute the heat evenly around the surface and so uniformly heat the interior? And how easily would that chunk of steel bend? I don't think that you could do it by hand, and the heating shouldn't produce that much force.
 
  • #15
Thermal stress is much more reasonable than magnetic effects... I'm wondering about the expansion difference between the steel core and the copper cladding. If the copper expands more (does it? I don't know), then will the two part company?
 
  • #16
Perhaps the lower copper coating eventually pops off the steel core to bulge downward...

The upper surface of the coin will be cooler than the lower as well, but I don't know if that makes much difference.
 
  • #17
At least it's really easy to determine whether or not thermal effects are the culprit. Just get a metal ball of the same radius as the bulb, put the coin on top, and heat the thing with a torch. If the coin jumps, then problem solved.
 
  • #18
I think it has to be a combined effect of the eddy currents and the heat current set up in the coin which though both must be very small but their maxima of combined effect after some time,due to increase of resistance be responsible for the event
 
  • #19
nishant said:
I think it has to be a combined effect of the eddy currents and the heat current set up in the coin which though both must be very small but their maxima of combined effect after some time,due to increase of resistance be responsible for the event
Yeah. Synergistic effects can be outright spooky at times. The other thing that crossed my mind is that if there are decent eddy currents involved, they would probably cause even more heat than the bulb. That is, after all, pretty much how an induction heater works.
 
  • #20
see,one more thing that gives u some idea that here there is a chance that thernal currentmay be involvedis the fact that the author has mantioned that when it was sunnyoutside it took considerably less time for the coin to go up than when it was stormy.
 
  • #21
PeteSF said:
Perhaps the lower copper coating eventually pops off the steel core to bulge downward...

The upper surface of the coin will be cooler than the lower as well, but I don't know if that makes much difference.
Seems like this will guarantee the direction of the bulge. That is the hot side will see an increase in radius, that must be corrected for by bulging toward the bulb. If the change happens slowly there would be no jump. However, if it suddenly changes state, from flat to bulged it would/could jump. Unfortunately I do not have ready access to a bulb or I would try this with an U.S. quarter. which is a similar size.
 
  • #22
what if you do this with a iron coin
 
  • #23
The penny didn't jump and it was on the lamp all night. The side that was pointing up has lost all it's copper coating but the other side's just slightly discoloured. (Pennies are like 2ps but about half the size.)
 
  • #24
All this theory is a bit premature. For all we know the hodgey's brother was upstairs, and walked past with a giant magnet.

Hodgey, repeat the experiment to verify the result.
 
  • #25
no,it is true I have just tried it,it goes up!
 
  • #26
I've repeated it two or three times. I can't get it to flip off on its own, but with a very slight disturbance it pings off with quite some force.

I will entertain a thermal (rather than electromagnetic) explanation since it takes quite a while (around 10 minutes I've found) before the coin will do this, so I'm guessing that the coin has to reach a certain temperature before it will do it. However, I don't like the explanation of thermal deformation of the coin since they're pretty substantial lumps of metal, and the coating is so thin that I doubt it would bear enough of a load to act as a kind of bimetallic strip. The geometry of the coin is such that I'd expect the heat to be pretty uniformly distributed through the thickness of the coin, especially given the long heating period involved.

I've only been able to get this to work with a 2p coin, - copper plated steel. 'Silver' doesn't work, but these coins aren't bimetallic OR ferromagnetic so that doens't help.

The problem is that the only ferromagnetic coins are also the only copper-plated ones; all the other British coins are the same alloy throughout their cross section.


If I get really bored over exams I might try and model this in Ansys.
 
  • #27
it should be a combined effect of the two
 
  • #28
nishant said:
see,one more thing that gives u some idea that here there is a chance that thernal currentmay be involvedis the fact that the author has mantioned that when it was sunnyoutside it took considerably less time for the coin to go up than when it was stormy.

I don't think the storm made a difference mate. At first, when I was being sceptical, I just thought it was funny that as soon as I started my experiment the clouds instantly closed in, and there was a monster storm which only lasted about 10 minutes. :smile:

My tongue was in my cheek right up until the moment the coin pinged off the light bulb, and from then onwards my jaw was on the floor.
 
  • #29
brewnog said:
I will entertain a thermal (rather than electromagnetic) explanation since it takes quite a while (around 10 minutes I've found) before the coin will do this, so I'm guessing that the coin has to reach a certain temperature before it will do it. However, I don't like the explanation of thermal deformation of the coin since they're pretty substantial lumps of metal, and the coating is so thin that I doubt it would bear enough of a load to act as a kind of bimetallic strip. The geometry of the coin is such that I'd expect the heat to be pretty uniformly distributed through the thickness of the coin, especially given the long heating period involved.
I hadn't thought of this before, but using the term 'hysterisis' in reference to the magnetism brought something else to mind. Is there any chance that rather than perfectly normal steel, there's some altered alloy with a shape-memory effect such as in NiTiNOL? Maybe when it gets hot enough it tries to resume the shape that it had before it was minted. Have any of you closely examined the shape immediately after the jump, before it has a chance to cool? What about if you just heat it any old how, such as on a stove?
 
  • #30
Hey Nishant, did u actually try an iron coin? And is the orientation of the coin important ? I'm asking all those whose coins went up! Coz mine didnt. I'd like to know if any other coins apart from a 2p coin works, and is it only a 60 W bulb which did the trick?
My guess for the explanation is that the tungsten filament of a bulb is a solenoid so it produces a magnetic field. and the coin will generate eddy currents due to the constantly oscillating field. The currents will keep increasing and also some sort of polarity develops in the coin, so when it reaches a certain critical value and the field changes again, the repulsive force is enough to dislodge it.
This is also a workable theory because, when it is slightly disturbed as someone said it goes flying off. That maybe so if the coin is disturbed when it is about to reach its critical value and is unstable.
Whatsay?