In general, thermodynamics is the branch of science that describes the relationship between heat, work, temperature, and energy. In thermodynamics, we divide the universe into two different parts: system and surroundings. A surrounding is the region outside of the system and interacts with it. A system is a closed region where we observe the movement of energy and matter between substances. There are three types of systems: open, isolated, and closed systems. First of all, an open system is where there is an exchange of both energy and matter between the system and surroundings like a pot with boiling water without a cover. An isolated system is where there is no such an exchange of both energy and matter between those two. Lastly, a closed system is when there is only an exchange of heat/energy but no exchange of matter between system and surroundings. Now, I will briefly talk about the history of thermodynamics in the next paragraph.
History of Thermodynamics
There are 10 different scientists who contributed to the theory of thermodynamics. First of all, Robert Boyle used an air pump to make the relationship between pressure and volume. He proved that pV (Boyle’s Law) is constant where p is pressure and V is volume. Second of all, Jacques Charles proved that volume and temperature are proportional to each other in a constant pressure. Third of all, Louis Gay-Lussac predicted that at 0 kelvin, the volume of the gas would be 0 by using Charles’s Law. Fourth of all, Sadi Carnot, known as the “father of thermodynamics”, introduced the concept of work in his book called “Reflections on the Motive Power of Fire.” Fifth of all, James Joule proved that heat and work are essentially the same after conducting experiments. Sixth of all, Rudolf Clausius introduced the concept of entropy in order to show that not all heat is converted to work but wasted. Seventh of all, Lord Kelvin . Eighth of all, James Clerk Maxwell introduced the kinetic theory of gas that explains the velocity distribution of gas particles. Ninth of all, Ludwig Boltzmann explained the energy distribution of gas molecules and derived the formula S = klnΩ. Last of all, Josiah Williard Gibbs used Gibbs Free Energy theory to derive the mathematical formula and introduced the concept of enthalpy.
Thermodynamic Variables
Before we talk about the three different laws of thermodynamics, we need to understand various thermodynamic variables.
Temperature is the measurement of intensity of an object’s heat. In thermodynamics, we do not use Celsius and Fahrenheit but use Kelvin, which is an indicator of absolute temperature. 0 degrees Celsius is equal to 273.15 Kelvin.
Entropy measures the disorder or randomness of the system and uses ‘S’ to symbolize it. The unit of it is J/K and uses this concept to describe the direction of the spontaneous reaction as entropy always increases when there is a change in an isolated system.
Pressure is a force exerted on the area of the wall by the gas molecules. We use ‘p’ to symbolize it and there are lots of units but the SI unit is Pa (N/m2).
Volume is the amount of space that the system occupies. We use ‘V’ and SI unit m3 but we also use L (liter) frequently.
Mole is the amount of atoms and molecules in the system, uses ‘n’, and SI unit is mol.
Law of Thermodynamics
There are three major laws of thermodynamics we need to understand
First Law of Thermodynamics
It states that energy can neither be created nor destroyed in the universe but can be transformed into other forms. This law is formulated with ∆U = Q - W where U is the change in internal energy, Q is heat added to the system, and W is work done by the system. The great application of this law is the heat engine. Heat engine uses heat energy to convert it to mechanical energy (work). It does not convert all of the heat energy to mechanical energy but releases some fraction of heat to the surroundings. In this case, energy is conserved because the energy input is equal to the output energy (heat + work).
Second Law of Thermodynamics
In nature, heat flows from high temperature to low temperature. Similarly, the ball goes from high position to low position. We call this scenario a spontaneous reaction. On the other hand, in order to reverse this motion, we need to put energy into the system. We call this case a non-spontaneous reaction. Now, in the ideal reversible reaction, the change in entropy can be 0, but since most systems are irreversible, the change in entropy is greater than 0, which means that the natural process tends to go to disorder. For example, after we clean our room, the room may look really clean at that moment. However, without any effort, the room will be really messy and disorganized. This means that all of the spontaneous reactions happening all around the world are increasing the entropy. If the entropy eventually reaches the maximum value, the heat death will occur.
Heat death by definition is that all lives on Earth will die due to the extremely hot temperatures. When this happens, all of the world would have the same temperature. Nowadays, there are many ongoing technical developments and innovations around the world. I do not mean that there must not be any industrialization, but we need to use renewable energy sources like solar energy and wind energy as much as possible. We are currently wasting lots of energy. Energy is conserved, but heat energy (wasted energy) is really hard to use again. Everything including turning on the light and charging the phone is wasting energy. Therefore, we need to find the solution to convert the heat energy to the available energy. However, energy cannot be captured and seen by naked eyes, which means that it is almost impossible to convert the wasted energy into usable energy. However, as technology advances, we might be able to find some solutions. For now, the best way to save energy is to turn off the unnecessary lights and buy a car that uses renewable energy.
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Third Law of Thermodynamics
Temperature is the measure of intensity of heat energy. Heat energy is also the same as kinetic energy of the system. The relationship between temperature and kinetic energy is proportional. As the temperature increases, the kinetic energy increases vice versa. In this sense, when there is no kinetic energy, there is no temperature. This temperature is called absolute zero kelvin or -273.15 degree Celsius. At this temperature, all of the substances including helium are in solid phase. With this knowledge, the third law of thermodynamics states that at absolute zero temperature, the entropy of any crystalline substance (solid phase) will be equal to zero. This can be explained by the Boltzmann’s formula: S = klnW. The value k is just Boltzmann’s constant with the value of 1.380649 × 10-23 m2 kg s-2 K-1, but W is quite special. It represents the number of possible arrangements of atoms or molecules in the system. Since, at absolute zero temperature, atoms/molecules can only be in one arrangement, W must equal 1. Since ln 1 is equal to zero, the entropy becomes 0.
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