Heat and Thermodynamics
When we say "Heat up the soup.", what do we mean? Well, we all know what to do, put the soup in a pot, put the pot on the stove, and turn on the flame (or burner, or electric heating element, etc.). How much heat do we provide (add to) our soup? 'Till its at the right temperature, of course. We all know about the general idea of hot and cold, but to be precise we need some conventions. (Sometimes hot and cold are not so obvious - here's an experiment: Take a container of hot water and one of cold water. Put one hand in the hot container and the other in the cold container and let them stay for a while. Now put both hands in a third container of water at room temperature. What do you find? (Make your prediction first, then try it).
Temperature
Physicians recognized that a patent's temperature reflected his/her state of health, and made thermometers to measure it, but no standard existed until Gabriel Fahrenheit provided one in 1709. (Earlier, Newton had proposed a similar device, from which Fahrenheit's was derived.) The two reference points were chosen as a salt and water mixture (0 degrees) and the temperature of the human body (100 degrees). On this scale, still used today in the United States (but hardly anywhere else), water freezes at 32 degrees and boils at 212 degrees. In 1742 Anders Celsius proposed a scale with 0 set at the freezing point of water and 100 set at the boiling point of water. As this was much easier to duplicate it quickly became the standard.
How do you build a thermometer? Find a material whose properties change with temperature (Fahrenheit used alcohol and later mercury). Bimetals, thermocouples, semiconductors and chemicals can also be used.
Heat
How are heat and temperature related? Here's the formal way: If you add heat to a substance, its temperature will rise. Specifically, if you raise the temperature of 1 gram of water 1 degree Celsius, you have added 1 calorie of heat to the substance. An important point: You can add heat to, or take heat away, from something, but you can't measure how much heat that something has. Heat after all is a form of energy, not a substance. While we can't tell how much heat an object (say a bucket of water) has, we can tell how much heat one needs to add to raise its temperature by some amount, and that amount of heat is strongly dependent on the material being heated. If you have ever heated water in a pot you know that the pot quickly gets hot, but the water seems to take forever. This character of different substances is called "heat capacity" (the capacity of the substance to absorb heat). Water has a very large heat capacity - about a large as anything (1 calorie will raise the temperature of 1 gram of water 1 degree C). Metals have small heat capacities (1 calorie of heat will raise the temperature of 1 gram of copper 10.6 degrees C). Because water has such a high heat capacity (and because its so available), we often use "specific heat" instead of heat capacity. Specific heat relates the heat capacity of various materials to that of water, which is given the specific heat value of 1. Thus, copper has specific heat 0.094 ( 1/0.094 provides the temperature rise of 10.6o C given above). Of course, it takes more heat to raise the temperature of a lake than of a cup of water - the amount of material (i.e. the mass) is important. Putting this together, the mathematical relation between heat added and resultant temperature increase is given by )Q = m c )T. In words, adding heat ()Q) to a substance will cause a corresponding raise in its temperature ()T). How much heat it takes is proportional (depends on) how much material you have (m), and the heat capacity (c) of the material.
Thermodynamics
The First Law:
We have already seen the first law: It is the law of conservation of energy. It states that energy can neither be created nor destroyed - only changed in form. As with all the laws of thermodynamics the first law can be stated in many ways, but the message is always the same. a mathematical statement (the kind physicists like) is dE = dQ - dW. It says, in very compact form, that the change in internal energy of a system (dE) is the difference between the heat added to the system (dQ) and the work done by the system (dW) on the outside world. Of course, this equation is just another way of saying the total energy is conserved.
The Second Law:
Steam engines were first put to practical use in England to pump ware out of mines. While early models were designed by Thomas Savory and Thomas Newcomen, James Watt made great improvements on their design and is popularly credited with their invention. Inventing them is one thing - understanding the principles of how they work is another. It was Sadi Carnot who provided the tools for understanding why they work. He generalized the steam engine into any engine and set the limits of its efficiency. The generalized engine of Carnot is called a heat engine. It takes heat in at a high temperature, uses the heat energy to do some work, and then expels the remaining heat at a lower temperature. Carnot measured the maximum efficiency of an ideal heat engine and his result is a statement of the second law of thermodynamics: A heat engine cannot be built with 100% efficiency. That's why there is no such thing as a perpetual motion machine.
Entropy:
It was Rudolph Clausius who introduced the concept of entropy. Entropy is a puzzling thing to grasp, but an idea of its meaning can be gained by considering the degree of disorder a system might have. As the disorder of a system increases (it becomes more disordered), its entropy increases, and it is the nature of things to become disordered. Anyone who has left a child in a neatly arranged room for any length of time is aware of this. Here's an example using coins: Take four coins and toss them. How will they land? Well, in some combination of heads and tails. If you write down all possible arrangements you will find that there is only 1 way they can be all heads and 1 way all tails. There are 4 ways to have 3 heads and 1 tail, 4 ways to have 3 tails and 1 head, and 6 ways to have 2 of each. Conclusion: The more disordered state of the system (of 4 coins) is the most likely. Another example: Take a container with a partition down the middle. Fill one half with red gas molecules and the other with blue ones. This is a highly ordered system - the different molecules are neatly separated. Now remove the partition and wait a short while. The gasses have each filled the container and you have a purple gas. Why did this happen? Because, given the chance, all systems tend to a state of maximum disorder. Entropy is a measure of the number of "available states" of a physical system. For the coins, the maximum number of available states (6) corresponds to the most likely (and most disordered) situation. The gas is the same way: Most ordered has only 2 states (red on left, blue on right). Least ordered, millions of states (various combinations of reds and blues on left and right).
As things become more disordered their energy becomes lass available. Gasoline, for example, is a highly concentrated form of energy. That's why its so useful. Once its burned in our cars however, the energy is still the same but virtually useless to us - it is in the form of warmed air from the exhaust, heat of friction within the engine, etc. As entropy increases, energy becomes less concentrated (less ordered), and harder to put to use. This disquieting fact leads eventually to a universe devoid of useful energy (but just as much) - to the "heat death of the universe".