The Second Law of Thermodynamics

             A lot of engineering students wonder about the Second Law of Thermodynamics before taking the thermodynamics class, and many are still wondering after the class is over. Even though they can use the equations as directed, some students still wish that they had a better “feel” for the Second Law. In this post and the next couple, I’ll try to describe the Second Law of Thermodynamics in a qualitative way in order to help develop that intuitive feeling for this important fundamental and ubiquitous physical principle. 
            The Second Law shows up all around us. It appears in everyday experiences such as the cooling of a hot bowl of soup at breakfast and in more abstract applications such as establishing a theoretical upper limit on the efficiency of internal combustion engines, turbine engines, and steam power plants. In what follows, we’ll explore several equivalent statements of the second law and talk about some implications of those statements. 


Basis of the First and Second Laws
            Most people feel like they have an intuitive feel for the First Law of Thermodynamics which simply states that energy can neither be created nor destroyed. Energy can change forms; we often buy energy from a utility in the form of electricity, then convert that electrical energy into light, or heat, or vibrations of a stereo speaker, etc. However, a careful accounting of all of the energy involved in any such transformation will show that no energy appears or disappears in the process. This “law” is simply an expression of many observations: in all of history no one has ever been able to show that energy could be annihilated or created out of nothing. Those cases where somebody claimed such a process have, under careful scrutiny, always been shown to involve an error or a trick, but never a violation of the First Law of Thermodynamics. The Second Law of Thermodynamics is based upon a similar body of experience but for some reason it is much more common to find people proposing hypothetical devices or processes that would violate the second law than the first law. However, in all of history, spanning scales from sub-cellular processes to processes inside suns, nobody has ever found an exception to the Second Law of Thermodynamics. As with the First Law, it is really just a very distilled statement of an enormous number of observations, and seems to just be the way that the universe works.


One Statement of the Second Law
            There are many different ways of expressing the second law but in general, any one of them can be shown to logically imply all others. Each different expression usually has one or more classes of situations to which it is most applicable. In order to begin to get a feel for the second law we will consider several of these expressions and their implications.

            One way of stating the Second Law is to say that not all heat energy can be transformed into work. Before exploring that statement further, it is necessary to establish some nomenclature and definitions. In the language of thermodynamics, heat is energy that moves due to a temperature difference and work is energy associated with things like moving a force through a distance, a torque through an angular displacement or equivalent processes. (In fact, the fundamental distinction between heat and work is made on the basis of the second law.) We have use for both types of energy: we use heat to cook our food, warm our houses, sterilize medical instruments, etc., and we use work to turn the wheels on a car, or to turn the shaft of an electrical generator to produce electricity. Electrical energy, which is a form of work, can be used to power motors and electronic appliances, or it can be converted into heat as it is in toasters and electric ovens. A heat engine is any device that operates continuously to turn heat into work. Internal combustion engines, Stirling engines, and steam power plants all function as heat engines. Returning now to our statement of the second law, we see that this statement is saying that no heat engine can turn all heat into work. 
Some of the heat that is put into the heat engine from a high temperature source can be converted into work, but some (non-zero) fraction must be sent on as heat to some lower temperature sink. The amount of heat that is turned into work divided by the total amount of heat that goes into the heat engine is termed the thermal efficiency. The second law can be used to determine the maximum possible thermal efficiency for a heat engine. This maximum depends on both the high temperature at which heat is supplied and the low temperature at which it is rejected. Depending on how well it is designed, an actual engine might have any efficiency from zero up to the maximum, but no engine can have an efficiency better than the maximum efficiency determined by the second law. This statement of the second law also implies that different forms of energy have different values. That is, if all work can be converted into heat, but not all heat can be converted into work, then clearly work is more valuable to us than heat. This distinction is never made by the first law. The first law only accounts for total amounts of energy and ignores the value of different forms. The two laws are each useful in their own way.
 An illustrative comparison can be drawn by considering the way that we keep track of automobiles. For some purposes, such a sizing a parking lot, we are only interested in the number of cars. An expensive car does not take any more spaces than a cheap one. For other purposes, such as budgeting, the price of each car is every bit as important as the number of cars. It would be impossible to ignore automobile values and always deal only in numbers of cars. Similarly, for engineering purposes the second law is as important as the first law since it establishes the value of different forms of energy.

            Finally, the second law can be used to make a quantitative valuation of different forms of energy. For our purposes it will suffice to say that the valuation is made based on the percentage of energy that can be changed into work, and the higher the temperature, the bigger percentage and hence, the higher the value. Thus work is always more valuable than heat, and high temperature heat is more valuable than low temperature heat. This aspect of the second law is easily accessible from common experience. For example, there is an enormous amount of heat contained in warm ocean water, but because it is only slightly warmer than the air, it is very “low grade” heat, and it is very difficult to harvest it economically. On the other hand, a hot geothermal spring might contain much less total heat than, say, the gulf stream, but because it is at a higher temperature, it can be converted to work at a much higher efficiency.

Second Law, part 2

Second Law, part 3