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Thermodynamics is a branch of chemistry and physics that explores thermal energy (heat energy) and how this relates to other forms of energy. The four laws of thermodynamics explain how this thermal energy converts into other energy forms and how matter is affected by thermal energy. Thermal energy is the energy produced when something (a system or a substance) becomes warmer and the molecules and atoms it is made up of move increasingly quickly and vibrate. 

The four laws of thermodynamics also deal with the theory of entropy. Entropy is a very important concept when looking at thermodynamics, and indeed for the understanding of many other branches of physics and chemistry. A definition of entropy from https://www.infoplease.com/dictionary/entropy follows;

Entropy (n)

  1. (on a macroscopic scale) a function of thermodynamic variables, as temperature, pressure, or composition, that is a measure of the energy that is not available for work during a thermodynamic process. A closed system evolves toward a state of maximum entropy.
  2. (in statistical mechanics) a measure of the randomness of the microscopic constituents of a thermodynamic system.”

Essentially, entropy is a measure of disorder. As a system or a substance becomes less balanced, less ordered, and more random, we say that the entropy has increased.

 

What are the Four Laws of Thermodynamics?

It is interesting to note that the four laws of thermodynamics begin with the number zero, then one, two, and three – but there are still four laws. The reason there is a zeroth law is that the other three were established first and named. So when another law was established that underpins the others, it had to be named the zeroth law. The four thermodynamics laws are as follows:

The Zeroth Law

Zeroth Law of Thermodynamics (Thermal equilibrium)

The Zeroth of the four laws of thermodynamics is that if two thermodynamic systems are in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

This might seem complicated or confusing, but it is actually a very simple concept that we all already know. It means that if two things are balanced with a third thing, they are also balanced with one another. Essentially, this can be described as if X=Y and Z = Y, then X=Z. It might sound very obvious when it is explained, but this is a fundamental principle of science and forms the basis for many more complex theories.

The First Law

Heat transfer, and the first law of thermodynamics

The First Law of Thermodynamics states that energy can neither be created nor can it be destroyed. It can only change forms. In any process, the total ampount of energy of the universe remains the same. For a thermodynamic cycle, the net amount of heat supplied to the system equals the net amount of work done by the system.

This means that if you begin with a certain amount of energy, and then you later have less, it has not been destroyed. It may have changed form, but it still exists somewhere. Equally, energy cannot be created. It can only change from one form to another. To take a very everyday example of this, if you are cold and need to produce heat to warm yourself, and you decide to light a fire, you cannot create energy out of nothing. When you light a fire, you are releasing stored energy in the fuel and converting this to heat. A human being is another example; we require fuel to function. We consume energy in the form of the food we eat. It is not enough for us to just simply look at the food; our bodies must break it down through digestion and release the calories from it. We lose some of the energy we consume through the heat our bodies produce; this energy is simply converted into another form. It is still never destroyed.

The Second Law

Second Law of Thermodynamics

The Second Law of Thermodynamics says that the entropy of an isolated system that is not in equilibrium will over time tend to increase, approaching a maximum value at equilibrium. This is also expressed as the entropy of the universe tends to a maximum.

This means that the universe is essentially becoming more disordered. Entropy in any closed system will either stay the same or increase; it doesn’t go backwards to less entropy. Entropy happens as energy moves or changes form and is wasted. As more and more energy is wasted, entropy (disorder) increases.

This also explains why it is impossible to convert energy from one form to another without some of that energy being wasted (remember it is never lost). This is why there has never been a perpetual motion machine invented. Nothing is 100% energy efficient. There is always some energy wastage, and as the wastage occurs, entropy increases.

The Third Law

The third law of thermodynamics

The Third Law of Thermodynamics states that as the temperature approaches absolute zero, the entropy of a system approaches a constant minimum. This means that in theory, at absolute zero (0K), there would be zero entropy. This is theoretical because the system would have to be perfect, and absolute zero would have to be reached. A perfectly ordered substance (the example always used is a crystal) could be formed, but it is not possible to reach absolute zero. The third law is essentially an extension of the entropy theory that completes the laws of thermodynamics, but it is a theoretical concept rather than a demonstrable effect.

The Laws of Thermodynamics are the culmination of the work of many physicists and chemists, building on the work of many studies into thermal energy that have taken place since the early days of exploratory science. The four thermodynamics laws have been established and accepted, and while the study of thermodynamics is an ongoing scientific endeavor, the four laws of thermodynamics underpin our understanding of thermal energy and form the foundation of much of our understanding of the worlds of both physics and chemistry. You can read more about thermodynamics and the history of its study here.