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Equipartition of energy

  1. Mar 27, 2012 #1
    I'm not sure that I understand the equipartition theorem intuitively. In general the thermal energy for a body is:

    U = Nf/2kT

    So more degrees of freedom, means that the temperature raises less for a fixed energy input:

    dU = N f/2 k dT

    Now temperature is a measure of an objects willingness to give up energy. Why would this depend on how many degrees of freedom a system has. I.e. why would a system with only one degree of freedom receiving a fixed amount of energy get more willing to give up this energy than a system with 100 degrees of freedom receiving the same amount of energy?
     
  2. jcsd
  3. Mar 27, 2012 #2
    This might be a stupid analogy, but imagine two restaurants. A has 10 tables, and B has 20 tables. Now, assume a group of 60 people go to lunch at these two restaurants. Now these people don't like to sit together at a table, so they tend to spread evenly. Then in thermal equilibrium, you expect 20 people at restaurant A and 40 at restaurant B.

    Of course, people represent units of energy, and tables are degrees of freedom, and temperature is the average amount of energy in a degree of freedom.

    Why make the stipulation that units of energy do not like to sit together? It comes down to statistics. There are more configurations where energy is mostly spread out than with energy bunched up in one place.
     
  4. Mar 27, 2012 #3

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    Temperature is not a measure of the willingness to give up energy. Standing in 0 degree air, you do not give up as much energy as standing in 0 degree water. The difference is density and specific heats. A block of aluminum at 0 degrees doesn't feel as cold as a block of iron at 0 degrees.

    I think the question you want to ask is why does adding a certain amount of energy to a 1 DOF material raise the temperature more than adding that same energy to a 100 DOF material. Its because that energy will spread evenly over those 100 DOF's. It spreads because of statistics, not by some spreading force. If you shuffle a deck of cards, the red suits and the black suits will spread more or less evenly throughout the deck.
     
  5. Mar 28, 2012 #4
    Actually, I rather like the definition of temperature as the willingness to give up energy. That's actually a pretty intuitive translation of the more rigorous but abstruse definition:
    [itex]\frac{1}{T}=\frac{dS}{dU}[/itex]
    The second law of thermodynamics says that entropy increases with time, so an increase of entropy with energy can be thought of as a willingness to grab energy from the surroundings. And this is equated to reciprocol of temperature. So higher inverse temperature means a higher willingness to grab energy from the surroundings. In other words, lower temperature is a stronger willingness to grab energy from things with higher temperature.
     
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