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Thermodynamics isue not homework but debate

  1. May 29, 2008 #1
    Thermodynamics isue... not homework but debate....

    Hi, i have visited the forum for an explanation in cuantum theories (curious) and now i need other help.

    In my school (in Chile, we speak spanish, forgive my english, please), my teacher gave a test, im really good at physics but this time i got wrong an answer that i think that is wrigh, post your opinions and give VALID answers, not just a yes, maybe mathematical answers would be better.

    the problem is this If you leave a baloon exposed to the sun, ¿what will happen?
    (Really easy so far, right)
    -My teacher says that it will explote (¡¡¡ NO WAY¡¡¡, really??') because of the presure increase.
    -I say it will explote because of the volume increase.

    This are my argumens
    -When you say something (in cience) you have to make an hipothesis, to make a theory you have to reduce (or eliminate) any other variable or noise (Right).
    So far so good....
    so i started to think, the sun heats the balloon, that makes presure increase that makes volume increase that makes the ballon explote.... so the presure is linked to the volume (in the baloon) .
    -To eliminate all variables i simulated 3 experiments
    1--Volume variable (all constants)
    2--Presure variable (all constants)
    3--Temperature variable (all constants)

    In 1--Volume variable i give the next "hipothetical" scenario:
    -In order to preserve presure and temperature constant i will decrease the "outside baloon" presure, so the presure differences will make the ballon expand without changing inside presure (At least in very little) nor temperature.... (in theory) what im doing is to expand the baloon, Will it explode at some point??....YES.

    In 2--Presure variable i give the next "hipothetical" scenario:
    -In order to preserve volume and temperature constant i will increase the "outside baloon" presure, and at the same time increase the "inside baloon" presure (either with magic or adding more gas) so the presure differences is 0 that will make the ballon to increase (inside) presure without changing volume nor temperature.... (in theory) Will it explode at some point??....NO.... because the presure inside and outside will eliminate each other (First law of Newton).

    In 3--Temperature variable i give the next "hipothetical" scenario:
    -Honestly, i tried to think this one, and i just couldnt figure it out....

    SO what do you think??, my teacher is right or i am right, because if you thing about it, in all cases, volume is what makes a baloon explode, right??

    PS: A tie is also allowed, that means say that neither the presure nor the volume is what cause the baloon to explote, that means saying that BOTH are linked so both answers are right.
  2. jcsd
  3. May 29, 2008 #2


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    Fundamentally, the balloon fails because it takes less energy to break the atomic bonds of its constituent material than it does to stretch them any farther.

    I agree with you. The balloon would not fail if the pressure were increased at constant volume and temperature (for example, by enclosing the balloon or by increasing external pressure too, as you pointed out). But it would fail if the volume were increased at constant pressure and temperature (for example, by manually stretching the material). It would also fail if the temperature were increased at constant pressure and volume (for example, by heating an open balloon). In the temperature case, the balloon would fail because the solid plastic would start to flow.

    So in my opinion it seems more reasonable to say that a balloon fails by volume increase than by pressure increase. But these are coupled in the closed-balloon-in-the-sun scenario, so it seems like a minor issue. I think it is much more useful to describe the failure mechanism, the fundamental idea of atomic bonds breaking, than to argue about the cause.
  4. May 29, 2008 #3
    A rubber balloon filled with AIR will leak, because air can pass through the rubber. Perhaps the leakage rate compensates for thermal expansion from heat from the sun (PV = nRT). Over time, the sun degrades the elasticity and quality of the rubber, possibly causing failure.

    If the balloon is filled with gas lighter than air rises indefinitely: external pressure decreases towards a vacuum. An increasing difference between internal and external pressure adds stress to the balloon's rubber, possibly causing failure.
  5. May 29, 2008 #4
    first time i am hearing that. IF it does leak, in what quantities does air leak through rubber?
  6. May 30, 2008 #5
    Balloon wall thickness and material are not standardized, thus vary greatly. Mylar balloons usually have less air leakage than rubber, but are usually more expensive.

    The greater the difference between inside and outside pressure, the greater the leakage rate. A freshly filled helium filled rubber balloon could leak one fourth of its gas overnight.
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