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Coin heated

  1. Jul 22, 2007 #1
    I have a copper coin with a hole in the centre. The hole is a circle. If the coin is heated, the metal will expand towards all directions in and out, so the radius of the circle will be decreased. Correct?
  2. jcsd
  3. Jul 22, 2007 #2


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    Well if you had an infinitely thin ring of copper and heated
    it, it could only increase in circumference, so the hole
    would actually get bigger as the radius of the ring increased.

    I believe that your statement that it expands in every
    direction is more or less true. What has to be considered,
    though, is the real world behavior of such an object where
    every atom is expanding just a little bit. The expansion
    will create a somewhat complicated stress/strain response
    in the metal, and the particular amount of displacement
    in a given area would be the result of lessening the overall
    stress energy of the metal.

    Though expanding from metal into free air is a pretty low
    energy thing to do since there wouldn't be back-pressure
    from the air in the hole, you must also consider that
    there would be a compression of the metal encircling the
    air-hole since more metal would have to be squeezed into
    a smaller space circumferentially too.

    I think that mainly it'd follow the path of expanding
    outward radially since that'd seem to lessen strees on the
    whole of the piece.

    Certainly if it became really hot to liquefy, though, you'd
    expect that it'd start to slump/flow/creep to fill in a small

    It'd make an interesting FEM analysis.

    Empirically I've never seen a hot 'nut' or 'washer' become
    more difficult to put around a cold bolt, or a hot jar lid
    become harder to screw around a cold jar. That would
    tend to support the theory that mostly it'd just expand
    outward as the lowest energy lowest stress option.
  4. Jul 22, 2007 #3
    Divide the ring into 16 segments. Now make one of those segments twice as large. What inner diameter will 16 of those give rise to?
  5. Jul 22, 2007 #4

    Doc Al

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    Staff: Mentor

    Incorrect. Do the exercise that cesiumfrog suggests. All linear dimensions will expand, including that of the hole.
  6. Jul 22, 2007 #5
    Or, just draw two concentric circles on your computer screen and then zoom in on the image. The geometry is the same.
  7. Jul 22, 2007 #6


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    Gold Member

    I did this experiment in Grade 7.

    You have a metal ball and a metal ring. The ball is slightly too large to fit through the ring.

    Heat the ring for a few minutes over a Bunsen burner and now the ball can slip through the ring.

  8. Jul 22, 2007 #7


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    Staff Emeritus
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    If the radius decreased, what could you say about the interatomic spacing in the tangential direction, along this inner wall? Would it have increased or decreased? What would you expect heating to produce (an increase or a decrease in the interatomic spacing)?
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