NOTHING can be more important than the development of thermodynamics during the Industrial Revolution in the late 18th and early 19th centuries in Europe, at least, from a scientist's point view. From James Watt to Carnot, from Eric B. Davidson and Felice Matteucci to Rudolph Diesel; from the old days steam locomotives to nowadays Formula 1 racing cars; from Kelvin and Planck to Clausiusall these are connected to our very law of thermodynamics. Like what Mr. Feynman said: "if I can express my idea which earns me the Nobel Prize in 5 minutes, then it is not worth of it anymore." Analogy, defining thermodynamics in some few hundred words would have been a difficult taskit takes centuries to sprout, growth and bear.
But let's try to simplify thermodynamics in a much easier and simpler way, such that everyone understands what it talks about; as this is the main purpose of physicsresolving and categorising complex things into a simpler, unified theory and explanation or law.
Thermodynamics, in strict, comprises of 4 law, namely the first, second, third and zeroth law. Unlike Newton's third law of motion, thermodynamics law developed in a more interesting mannerfrom started from first till third, then back to zeroth. Each law has its own statement, which we shall see now. From the work "thermo", we can expect that it is about heat, and the "dynamic" implies that there is change or motion of heat, and perhaps some other things, for the particular system.
The first law, states that when heat is added to a system and the system does work, then the internal energy of the system changes by the amount equal to the heat added minus the work done. This is a very important statement, which implies: heat is a form of energy, which can be used to do work. In other words, when heat is added into a system, then it is to increase the internal energy of the system, besides doing work. The first thermodynamics is really the fundamental for the second thermodynamics law, which is more precise and elegant.
So now we move on to second law of thermodynamics. This law further declares the relationship between heat increased (or taken), the internal energy changes and the capable work done by a system. From the 1st law, we can argue that: we can keep the internal energy of the system constant, so that we can have 100% work done by heat. But the 2nd law denied this statementno engines can convert heat completely into work, and no refrigerator can transfer heat from a colder place to a warmer place without doing work. It would be harder to understand that "no engine can convert heat completely into work" without further discussion, but it is very easy to appreciate that heat can flows from a cold region to a warm region.
By then, a more general statement is required for the 2nd law. Therefore, the concept of entropy, introduced by Rudolf Clausius, is used to generalize the 2nd lawwhen all system involved in a process are counted into, the entropy either remains unchanged or increased. It is impossible to have decreased. Entropy can be defined as "the heat that cannot be used to do work".
We then go further to the third law of thermodynamics, which is seldom discussed and understood. The third law states that absolute zero temperature cannot be reached; or if it was, then there would be not only no changes of entropy for a process, but also the system has no entropy. This is somehow interesting, which we found that it is true that we are harder and harder in lowering the temperature, as it is closer to zero absolute temperature.
And now we reach the zeroth law of thermodynamics, which one can appreciate easily by his/her living experiencesif two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. Systems in thermal equilibrium means the systems are at same temperature. Therefore, no net heat is transferred among these systems. A very simple example, thermometer is work on this principle/law.
Now we have four thermodynamics, which all are self-determining yet mutually dependence. Therefore, in order to define thermodynamics by combining 4 laws, it would be better and more appropriate to use the 4 laws together to describe this physical world. This is always the rules for physics.
Therefore, by laws of thermodynamics, we can conclude that time is not reversible, or there is the "time arrow" to follow. This is a result from the second law, which states that the entropy can either remain or increase. Perhaps we can take a simple example. A coal, with a lot of heat stored inside (high heat capacity), can be used to do work by releasing the heat stored, mainly by burning it. Then, it becomes ash and gases. All these are at higher entropy, which they cannot do much work as same amount of coal. And we know we can have coal changed into ash and gases, but we cannot do the reverse! Thus, in other wordsin natural, processes tend to a higher degree of disorder. Note that we use "disorder" instead of "entropy", but they are leading to the same concept. This would probably the essence of thermodynamics, as it describes the flow of time is in only one direction, and is irreversible.
Another important conclusion or implement of thermodynamics would have been the unreachable of zero absolute temperature. This comes from the third law. Since no zero absolute temperature can be reached, therefore it would have been impossible to produce a 100% efficient engine. This is because the efficiency of an engine, at best, according to the French physicist Carnot, is one minus the fraction of the temperature at cold reservoir to the temperature at hot reservoir, can never be one. In other words, the fraction of the two temperatures cannot be zero. Therefore, zero absolute temperature is impossible. In fact, this is also the double-sided proveboth for efficiency of engines and zero absolute temperature.
Last but not least, is the so-called heat death of the universe. Although energy cannot be destroyed and created, and is conserved in any process, but from thermodynamics, we can argue that the energy that can be used to do work is decreasing in the universe. Just like the example of burning the coalat final the system is death: cannot do any work. And for sure, taking the whole universe as the system, then we can predict that at last, though it lies far in the future, the universe is at its maximum disorder, and no work can be done. This is the heat death.
So, how are you looking at thermodynamics now? It is hard to define thermodynamics in very simple words or sentences, right? Like what I have said, the best to define the law, is to apply it to this physical universe.