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Re: [Phys-L] heat content



It would probably do us all much good to go (re)read what Francis Sears has written in his text, An Introduction to Thermodynamics and .....I have the 1953 edition. Chapter 4, The First Law of Thermodynamics has some fairly strong statements about heat, work, and energy and the use of those terms. A few quotes (you should go read the whole chapter yourself):

Page 44: "We have shown that the work W may be very different when a given system is taken by different processes from one state to another. Since the change in internal energy is independent of the process, it follow that the heat Q is different for different processes also and that as a consequence Q is _not_ a function of the state of the system and it is meaningless to speak of the 'heat in a system,' or the 'heat of a system.'" He then addresses the absolute zero question.

Page 46: "It is important to note that the internal energy of a system cannot be identified with either work or heat. The terms heat and work are used only in connection with a flow or interchange of energy between a system and its surroundings." He then discusses how to increase the temperature of a resistor: in an electrical circuit, energy is deposited in the resistor by connecting it to an electrical circuit (Sears uses the term "electrical energy"). He next places a disconnected resistor in a flame. First resistor changes state because of W. Second changes state because of Q, Each results in the same final internal energy state U. He then says,"The end state of the resistor is exactly the same in both processes and it is meaningless to assert that as a result of the first process the resistor contains more "work," while at the end of the second process it contains more "heat." What is does contain is more _energy_ [Sears's emphasis] and say that the increase in its internal energy is the same in both processes if its initial and final states are the same in both."

You could do the same thing with 3" wood screws, a 4x4 block of wood, a drill, and a heat lamp. Increase the temp of the screws using either work or radiation. In the end, it's the internal energy that has changed, not the "thermal energy content." Your students might be amazed at how hot a screw will get simply by running it in and out of the block.

While this is no appeal to authority, I believe Sears gives us well thought words for serious consideration. I hope this is an accurate summary: work and heat are means of changing the internal energy, and once the internal energy is changed, the cause of the change is irrelevant.

It kind of sounds like a general relativity principle for thermodynamics (gravity should be considered indistinguishable from an accelerated reference frame).

NASA words (heat content) are bad. But thermal energy content is bad, too. Maybe they should say the internal energy of the oceans is increasing primarily due to heat flowing into it from the (warming) atmosphere.

Find a copy of Sears if you can. It is beautifully and thoughtfully written.