0310 Notes

# 0310 Notes - The First Law of Thermodynamics(re-stated You...

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3/10/10 Oregon State University PH 202, Lecture 28 1 The First Law of Thermodynamics (re-stated) You can’t win: To do 1 Joule of work on an object you cannot spend less than 1 Joule of energy.” Energy is neither created nor destroyed in the universe. The Second Law of Thermodynamics (re-stated) You can’t even break even: To do 1 Joule of work on an object you must spend more than 1 Joule of energy.” Every interaction in the universe changes at least a little energy from some other form into thermal energy . OK… so what? What’s so signi fi cant about this?

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3/10/10 Oregon State University PH 202, Lecture 28 2 Q: What is energy? A: The usual de fi nition in physics is that it’s the ability to do work . Q: All energy has the ability to do work, but is it available to do it? A: No. This is the key to understanding thermal energy. Again, think of the problem of herding cats (or kindergarteners). It’s possible that they could all align and use their motions (spend their kinetic energy) doing work in the same direction. But it’s not likely to happen —no more likely than, say, to fl ip a fair coin 1 million times and have it turn up heads every time . Thus we say that the energy of the random thermal motion is not all available to do work; it would cost us more work to align all those motions than the work they could then do for us in return.
3/10/10

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