Physics-Thermodynamics, Lesson 1

Heat

Heat may be defined as energy in transit from a high temperature object to a lower temperature object. 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.

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. 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. 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. 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.

Temperature

A convenient operational definition of temperature is that it is a measure of the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules. The flow of heat is from a high temperature region toward a lower temperature region. The details of the relationship to molecular motion are described in kinetic theory. The temperature defined from kinetic theory is called the kinetic temperature. Temperature is not directly proportional to internal energy since temperature measures only the kinetic energy part of the internal energy, so two objects with the same temperature do not in general have the same internal energy (see water-metal example). Temperatures are measured in one of the three standard temperature scales (Celsius, Kelvin, and Fahrenheit).

The triple point of water is 273.16 K, and that is an international standard temperature point. The freezing point of water at one atmosphere pressure, 0.00°C, is 0.01K below that at 273.15 K. If you want to be really precise about it, the boiling point is 373.125 K, or 99.75 °C. But for general purposes, just 0 °C and 100 °C are precise enough.
While the typical treatment of temperature scales takes the freezing point of water to be 0C and the boiling point at standard pressure to be 100C, there are more precise treatments of standard points for defining temperatures. one standard point is the triple point of water which has been defined to be 273.16K. The freezing point of water at atmospheric pressure is .01K below this at 273.15K.

That will be all now, the next lesson will be on the laws of thermodynamics