Thermodynamics

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Introduction to Thermodynamics

Thermodynamics is said to be a branch of physics that deals with heat, work, and temperature and their relation with the energy and radiation and physical properties of matter. The behaviour that is of these quantities is governed by the four laws that are of thermodynamics which convey a quantitative description using measurable macroscopic quantities in physics. But these all can also be explained in terms of microscopic constituents by statistical mechanics. The phenomenon of thermodynamics applies to a wide variety of topics in science and engineering, especially in physical chemistry, chemical engineering and mechanical engineering as well in other complex fields such as meteorology. Here, we will discuss thermodynamics in detail.

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Laws of Thermodynamics PDF

  • The First Law Thermodynamics

Law of Conservation of Energy states that energy can neither be created nor can it be destroyed. Energy can only be transferred or changed that too from one form to another. For example, we can say that turning on a light would seem to produce energy, however, it is the electrical energy which is converted.

A way of expressing the first law of thermodynamics is that any change which is in the internal energy denoted by ∆E of a system is given by the sum of the heat that is denoted by q that flows across its boundaries and the work (w) done on the system by the surroundings:

We write it as : ∆E = q + w

This law tells us that there are two kinds of processes: the work process and the heat process that can lead to a change in the internal energy of a system. Since we can see that both heat and work can be measured and quantified so this is the same as saying that any change which is occurring in the energy of a system. In other words, we can say that energy cannot be created or destroyed. If heat usually flows into a system or we can say that the surroundings do work on it then the internal energy increases and the sign q and w are positive.  Conversely, we can say that the heat flowing out of the system or work which is done by the system that too on the surroundings will be at the expense of the internal energy and q and w will therefore be negative.

  • The Second Law of Thermodynamics

The second law of thermodynamics says that the entropy of any system which is isolated always increases. The system which is the isolated system spontaneously evolves towards a thermal equilibrium that is the state of entropy which is the maximum of the system. We can say that the entropy of the universe which is of the ultimate isolated system only increases and never decreases.

A simple way in which we can think of the second law of thermodynamics is that of a room. If not cleaned and tidied, then the room will invariably become more messy and disorderly with time – that is regardless of how careful one is to keep it clean. When the room is cleaned then its entropy decreases but the effort to clean it has resulted in an increase in entropy outside the room that exceeds the loss of the entropy.

  • The Third Law of Thermodynamics

The entropy that is of a system at absolute zero is typically said to be zero. And we can say that in all cases it is determined only by the number of different ground states it has. Specifically, the entropy of a pure crystalline substance in the perfect order which is at absolute zero temperature is zero. This statement is to hold true if the perfect crystal has only one state with energy that is minimum. 

  • The Zeroth Law of Thermodynamics

This law identifies the thermal equilibrium and introduces temperature as a tool for identifying equilibrium. According to this law, “we can say that if two systems are in thermal equilibrium with a third system then those two systems themselves are in equilibrium.”

An assembly that is of a very large number of particles whose state can be expressed in terms of pressure and volume and temperature, is known as a thermodynamic system.


The Thermodynamic System is Said to Be Classified Into the Following Three Systems

(i) Open System: It exchanges both energy and matter with the surrounding.

(ii) Closed System: It exchanges only energy but not matter with surroundings.

(iii) Isolated System: It exchanges neither energy nor matter with the surrounding.


Thermodynamics Notes PDF

The branch of physics which deals with the study of the transformation of heat into other different forms of energy and vice-versa is known as thermodynamics.

The phenomenon of thermodynamics is macroscopic science. It generally deals with bulk systems and does not go into the molecular constitution of the matter.

A collection which is of an extremely large number of molecules or atoms is said to be confined within certain boundaries

such that it has a certain value of pressure denoted by P volume denoted by V and temperature denoted by T is known as a thermodynamic system.


The Thermal Equilibrium

A system of thermodynamics is in a state of equilibrium if the macroscopic variables such as pressure and volume and temperature, mass composition etc. that generally characterise the system do not change in time. In equilibrium that is thermal equilibrium, the temperature of the two systems is said to be equal.


FAQ (Frequently Asked Questions)

1. Explain what thermodynamics is.

The phenomenon of thermodynamics is the study of relationships involving heat, and the mechanical work and other aspects of energy transfer that takes place in devices such as refrigerators and the heat pumps as well as the internal combustion engines etc.

2. Explain what is the thermodynamic system in physics.

A thermodynamic system is a body of matter and/or radiation, confined in space by walls that are with defined permeabilities which generally separate it from its surroundings. The surroundings may also include other thermodynamic systems or we can say the physical systems that are not thermodynamic systems.

3. What are the 3 laws of thermodynamics?

Traditionally we can say that thermodynamics has stated three fundamental laws: that is the first law and then the second law and the third law. The first law of thermodynamics states that when energy passes into or out of a system that is as work, heat, or matter then the system's internal energy changes.

4. What is the best definition of heat?

We can say that heat is the form of energy that is transferred between two substances that too at different temperatures. The direction which is of energy flow is from the substance of higher temperature to the substance of temperature which is lower. Heat is said to be measured in units of energy that is usually calories or joules.