We explain what the laws of thermodynamics are, what is the origin of these principles and the main characteristics of each one.
What are the laws of thermodynamics?
The laws of thermodynamics (or the principles of thermodynamics) describe the behavior of three fundamental physical quantities, temperature, energy and entropy, that characterize thermodynamic systems. The term “thermodynamics” comes from the Greek thermoswhich means “heat,” and dynamoswhich means “strength.”
Mathematically, these principles are described by a set of equations that explain the behavior of thermodynamic systems defined as any object of study (from a molecule or a human being, to the atmosphere or boiling water in a saucepan).
There are four laws of thermodynamics and are crucial to understanding the physical laws of the universe and the impossibility of certain phenomena such as perpetual motion.
See also: Principle of conservation of energy
Origin of the laws of thermodynamics
The four principles of thermodynamics They have different origins, and some were formulated from the previous ones. The first to be established, in fact, was the second, the work of the French physicist and engineer Nicolás Léonard Sadi Carnot in 1824.
However, in 1860 this principle was formulated again by Rudolf Clausius and William Thompson, then adding what we today call the First Law of Thermodynamics. The third appeared later, also known as “Nerst's postulate” because it arose thanks to the studies of Walther Nernst between 1906 and 1912.
Finally, The so-called “zero law” appeared in 1930 proposed by Guggenheim and Fowler. It should be said that it is not recognized as a true law in all areas.
First Law of Thermodynamics
The first law is called “Law of Conservation of Energy” because it dictates that in any physical system isolated from its environment, the total amount of energy will always be the same even though it can be transformed from one form of energy to different ones. Or in other words: energy can neither be created nor destroyed, only transformed.
In this way, by supplying a certain amount of heat (Q) to a physical system, its total amount of energy can be calculated as the heat supplied minus the work (W) done by the system on its surroundings. Expressed in a formula: ΔU = Q – W.
As an example of this law, let's imagine an airplane engine. It is a thermodynamic system that consists of fuel that reacts chemically during the combustion process, releasing heat and doing work (which makes the plane move). So: if we could measure the amount of work done and heat released, we could calculate the total energy of the system and conclude that the energy in the engine remained constant during the flight: energy was neither created nor destroyed, but rather made change from chemical energy to heat energy and kinetic energy (motion, that is, work).
Second Law of Thermodynamics
The second law, also called “Law of Entropy”, can be summarized as follows: the amount of entropy in the universe tends to increase over time. This means that the degree of disorder of the systems increases until reaching an equilibrium point, which is the state of greatest disorder of the system.
This law introduces a fundamental concept in physics: the concept of entropy (represented by the letter S), which in the case of physical systems represents the degree of disorder. It turns out that in every physical process in which there is a transformation of energy, a certain amount of energy is not usable, that is, it cannot do work. If it cannot do work, in most cases that energy is heat. That heat that the system releases, what it does is increase the disorder of the system, its entropy. Entropy is a measure of the disorder of a system.
The formulation of this law establishes that The change in entropy (dS) will always be equal to or greater than the heat transfer (dQ) divided by the temperature (T) of the system. That is, that: dS ≥ dQ / T.
To understand this with an example, it is enough to burn a certain amount of matter and then collect the resulting ashes. When we weigh them, we will verify that it is less matter than what was in its initial state: part of the matter was converted into heat in the form of gases that cannot do work on the system and that contribute to its disorder.
Third Law of Thermodynamics
The third law states that The entropy of a system that is taken to absolute zero will be a defined constant. In other words:
- Upon reaching absolute zero (zero in units of Kelvin), the processes of physical systems stop.
- Upon reaching absolute zero (zero in units of Kelvin), the entropy has a constant minimum value.
It is difficult to reach the so-called absolute zero on a daily basis (-273.15 ° C), but we can think about this law by analyzing what happens in a freezer: the food we place there will cool so much that the biochemical processes inside will slow down or even stop. That is why its decomposition is delayed and it will be suitable for consumption for much longer.
“Zero” Law of Thermodynamics
The “zero law” is known by that name although it was the last to be proposed. Also known as Law of Thermal Balancethis principle dictates that: “If two systems are in thermal equilibrium independently with a third system, they must also be in thermal equilibrium with each other.” It can be expressed logically as follows: if A = C and B = C, then A = B.
This law allows us to compare the thermal energy of three different bodies A, B, and C. If body A is in thermal equilibrium with body C (they have the same temperature) and B also has the same temperature as C, then A and B have the same temperature.
Another way of stating this principle is to argue that when two bodies with different temperatures are brought into contact, they exchange heat until their temperatures become equal.
Everyday examples of this law are easy to find. When we put ourselves in cold or hot water, we will notice the difference in temperature only during the first minutes since our body will then enter thermal equilibrium with the water and we will no longer notice the difference. The same thing happens when we enter a hot or cold room: we will notice the temperature at first, but then we will stop noticing the difference because we will enter thermal equilibrium with it.
References
- “Principles of thermodynamics” on Wikipedia.
- “Laws of thermodynamics” on Geofrik's Blog.
- “The laws of thermodynamics” in Khan Academy.
- “Thermodynamics” in The Encyclopaedia Britannica.
- “The laws of thermodynamics in 5 minutes” (Video) in Quantum Fracture.
