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Interesting question, puzzling me

  1. May 1, 2005 #1
    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?
     
  2. jcsd
  3. May 1, 2005 #2

    Danger

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    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:
     
  4. May 1, 2005 #3
    I've taken drugs like that too..... :yuck:

    :rofl:
     
  5. May 1, 2005 #4

    Danger

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    Get thee to GD! We can use the likes of you down there to help supress the evil Artman.
     
  6. May 2, 2005 #5
    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
     
  7. May 2, 2005 #6

    arildno

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    Cool!
    Is it repeatable, or was it a freak occurrence?
     
  8. May 2, 2005 #7

    brewnog

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    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: May 2, 2005
  9. May 2, 2005 #8

    brewnog

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    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.
     
  10. May 2, 2005 #9

    SpaceTiger

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    Perhaps we should consider a "don't try this at home" addendum... :uhh:
     
  11. May 2, 2005 #10

    brewnog

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    Absolutely not. You can learn a lot from scars...
     
  12. May 2, 2005 #11
    Wow! Wowowow! I burnt my finger on the light bulb but it was worth it. I'm testing a penny now.
     
  13. May 2, 2005 #12

    Danger

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    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:
     
  14. May 2, 2005 #13

    Integral

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    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.
     
  15. May 2, 2005 #14

    Danger

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    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.
     
  16. May 2, 2005 #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?
     
  17. May 2, 2005 #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.
     
  18. May 2, 2005 #17

    Danger

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    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.
     
  19. May 2, 2005 #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
     
  20. May 2, 2005 #19

    Danger

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    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.
     
  21. May 2, 2005 #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.
     
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