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The first crude thermometer was invented in the 1600s by Galileo. Accurate thermometers arrived some 200 years later. It wasn’t until this time, in the mid-1800s, that the concept of energy was widely accepted. It is now clear that temperature is a measurable indicator of energy. In the last 150 years since the discovery of energy, fundamental laws governing the transport, conversion, and storage of energy have been developed. These laws are the basis for the science we call thermodynamics.
The First Law of Thermodynamics
Simply stated, the first law of thermodynamics says that energy cannot be created nor destroyed. However, energy can be transformed and transferred. The main ways in which energy is manipulated are heat and work. It must be understood that heat and work are not properties of matter, but rather they are the routes by which energy is moved or converted.
When energy moves from areas of higher temperatures to areas of lower temperatures this is called heat transfer. If you touch a hot stove, large amounts of energy are transferred from the hot stove to your colder hand. Alternatively, if you make a snowball with your bare hands, energy moves from your warmer hands to the colder snow. The flow of energy in each of these situations is in the form of heat.
Work, as defined in physics, is force applied over a distance. In thermodynamics, work is a means of transferring and storing energy. If you pull a wagon up a hill, you are doing work, using your energy to move something over a distance. You have also stored some of your energy in the wagon. To see that energy, push the wagon off the top of the hill and it will race to the bottom, using the energy you gave it through work.
The first law says that no matter how energy is transferred, transformed, or stored through heat or work, the same total amount of energy is always present. For this reason, energy is said to be conserved. In other words, whenever energy decreases in one place, it must increase by an equal amount somewhere else.
The Second Law of Thermodynamics
The second law has been defined in many ways over the years. Heat cannot flow from areas of lower temperature to those of higher temperature. Creating order in one system must create equal or greater disorder in the surroundings. Perpetual motion machines are an impossibility. No process can convert heat completely to work. These are all valid statements of the second law.
The second law is concerned with the relationship between heat and work. Work can be completely converted into heat with no losses. For example, if you rub your hands together on a cold day, all the work you do is converted to heat. However, all of the energy in heat cannot be converted to work. Some of the heat will always be dissipated to the surroundings. In industry, hot steam is often used to drive work-producing turbines. This is a way of converting heat to work. However, some of the heat energy will be dissipated to the surroundings and not converted to work. The disparity between heat and work is filled by the concept of entropy.
Entropy is classically defined as disorder or randomness. It is also said that entropy tends to increase with time in natural environments. For example, pretend you have a large box with 100 rabbits in it. Fifty of the rabbits have black fur, and the other 50 have white fur. You put all of the rabbits with white fur on the far right hand side of the box, and all of the rabbits with black fur on the far left hand side.
At this point, the box of rabbits is very ordered and has very little randomness, hence low entropy. If you then leave and come back one minute later, it is likely that most of the white rabbits will still be on the right and most of the black on the left, with only a few of them mixing in the middle. So after one minute, there is a little more disorder in the box, therefore the entropy has increased slightly. If you then leave the box and come back several hours later, it is likely that the rabbits will be thoroughly mixed with black and white rabbits in all parts of the box. The box of rabbits now has a large degree of disorder and randomness, meaning very high entropy. To return to the original situation with all of the white rabbits on one side and all of the black rabbits on the other, you will have to do a significant amount of work to move and separate the rabbits.
Thermodynamic entropy works in a similar way. When heat is used to create work, some of the heat is “lost.” This lost heat contributes to the entropy of the system. In other words, the extra heat makes the system more disordered and random. If you want to restore the original order to the system, you will have to add more work, similar to the rabbits in the box.
The Third Law of Thermodynamics
The third law also deals with entropy. It states that a system with a temperature of absolute zero (-273 degrees C or -459 degrees F or 0 degrees K) will have no entropy. Going back to our rabbit example, if the temperature in the box of rabbits is lowered, the rabbits will move around more slowly and the entropy of the box will increase much more slowly. However, if the temperature is made so low that the rabbits freeze in place, the entropy of the system will be at a minimum because the rabbits cannot mix, cannot increase their disorder.
It is the same with energy systems. As the temperature approaches absolute zero, the system becomes more and more sluggish, preventing any disorder from developing. The third law says that entropy will approach a limit of zero as the temperature approaches zero. These laws of thermodynamics are enough for a detailed energy analysis in most situations. Additional laws that deal with self-organizing and nonequilibrium systems may also exist.
Uses of Thermodynamics
The practical uses of thermodynamics are limitless. Because energy is all around us, the laws of thermodynamics can be applied to almost anything. The practical application on which thermodynamics was founded is the engine. Studying and improving the engine were the real motivations for studying thermodynamics and developing the laws we have today. Throughout the last century, the engine has been the primary device for converting heat to work. Through thermodynamics, we have made engines more efficient and found ways that we can use engines to make our lives easier. Thermodynamics is also essential for understanding and designing air conditioning and heating systems. Understanding the flow of energy is pivotal to technology like refrigeration.
The transfer of energy in the body also follows the laws of thermodynamics. For this reason, thermodynamics is important for the medical field as well. Medical researchers use thermodynamics to develop medical equipment used for diagnosis and treatment of patients. Thermodynamics is also at the root of drug delivery systems, which govern how the medicine you take gets to the part of the body where it is needed.
- Roger Kinsky, Heat Engineering: An Introduction to Thermodynamics (McGraw-Hill Education, 1989);
- Anastasios Tsonis, An Introduction to Atmospheric Thermodynamics (Cambridge University Press, 2007).