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Free Energy Thought Experiment Question

  1. Nov 25, 2011 #1
    I have a thought experiment I don't understand.
    Imagine there's a grain of salt that weighs 5 mg and it's 1mm above above a pool of water and it's on a very thing piece of flimsy material that takes about 1*10^-5 Newtons of force to break. Using Newton's 2nd law, I get the grain of salt is pushing downwards at a force of ~4*10^-5 Newtons. So it needs another 3*10^-5 Newtons to break the material. Since it is 1mm above the water, the work needed is 3*10^-8 Joules. ( Correct me if my math/formulas are wrong).
    Using the ∆G= ∆H-T∆S. ∆H is 3*10^-8 let's say the temperature is 330 K. Assume that T∆S>3*10^-8. The entropy would be increased, I think, b/c the breaking of salt into Na and Cl- ions increases the possible microstates of the solution.

    My question is, in the real world would the material break spontaneously? It seems counterintuitive. Thanks
     
  2. jcsd
  3. Nov 25, 2011 #2
    Your numbers make no sense to me. 5 mg would be pushing down with 5*10^-5 N, wich is already 5 times as big as the force you said was needed to break the flimsy material.
    The number you give for work needed makes no sense at all. You would need the elastic properties of the object to compute the work needed to break it.

    The problems is that the flimsy object must break before any of the salt comes into contact with the water, so the entropy increase will play no role in breaking the material.
     
  4. Nov 25, 2011 #3
    Ok Thanks. Yeah I meant 1*10^-4 for the force needed to break it. I don't know about elastic properties so I just invented a number.
    But you answered my question. I guess events in the future can't affect the past.
     
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