These conversions are the International Steam Table (IT) values, and variations up to 0.5% will be found since they were originally based on energy for temperature changes in water, and that varies a small amount based on what temperature of water is used to establish it. The British Thermal Unit was also introduced. At that time calories were the accepted unit of heat and joules became the accepted unit of mechanical energy.
Careful measurements showed the increase in the temperature of the water to be proportional to the mechanical energy used to stir the water. In a classic experiment in 1843, James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles. As illustrated in the heat and work example, the temperature of a gas can be raised either by heating it, by doing work on it, or a combination of the two. Heat flow and work are both ways of transferring energy. After that, neither the words work or heat have any usefulness in describing the final state of the sytem - we can speak only of the internal energy of the system. The First Law identifies both heat and work as methods of energy transfer which can bring about a change in the internal energy of a system. Zemansky points to the First Law of Thermodynamics as a clarifying relationship. He would counsel the introduction and use of the concept of internal energy as quickly as possible.
He also objects to the use of the vague term "thermal energy" and to the use of the word "heat" as a verb, because they feed the misconceptions, but it is hard to avoid those terms. In warning teachers and students alike about the pitfalls of misusing the word "heat", Mark Zemansky advises reflecting on the jingle:ĭon't refer to the "heat in a body", or say "this object has twice as much heat as that body".
But you can also increase its internal energy by doing work on it, and since the internal energy of a high temperature object resides in random motion of the molecules, you can't tell which mechanism was used to give it that energy. Surely you can take an object at low internal energy and raise it to higher internal energy by heating it. The word heat is better reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one. To describe the energy that a high temperature object has, it is not a correct use of the word heat to say that the object "possesses heat" - it is better to say that it possesses internal energy as a result of its molecular motion. One key idea from this example is that if you are presented with a high temperature gas, you cannot tell whether it reached that high temperature by being heated, or by having work done on it, or a combination of the two. I found the idea in a little article by Mark Zemansky entitled "The Use and Misuse of the Word 'Heat' in Physics Teaching". This example of the interchangeability of heat and work as agents for adding energy to a system can help to dispel some misconceptions about heat. An object does not possess "heat" the appropriate term for the microscopic energy in an object is internal energy.The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is properly called heating. Heat may be defined as energy in transit from a high temperature object to a lower temperature object.
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