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Thermodynamics is a branch of Physics which deals with the equilibrium, energy and its transformation from one form to another. It deals with the relationships between heat and work, and the properties of the system in equilibrium. Thermodynamics comes from the word thermo that means temperature and dynamics means change over time. It gives an idea about temperature change, energy transformation and work of a system. Thermodynamics came into being as man wanted to convert the heat into useful work. 
Thermodynamics is the field of physics that deals with the relationship between heat and other properties such as pressure, density, temperature, etc. in a substance. A system undergoes a thermodynamic process when there is some kind of energetic change within the system, usually associated with changes in pressure, volume, temperature, or any sort of heat transfer. These concepts helped the scientists a lot in inventing efficient steam engines in 19th century.

What is Thermodynamics?

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Thermodynamics is the branch of science or physics that studies various forms of energies and their conversion from one form to the other like heat to electrical, electrical energy to mechanical energy, chemical to mechanical, wind to electrical etc.
Thermodynamics is that which tells about how work is done due to energy transfer between system and surrounding. It gives the relationship between heat and work. Heat transfer takes place till thermal equilibrium is achieved in any system. 
The system is one enclosed by a space within the boundary. The rest space outside it is the 
It gives the relationship between heat and work. Heat transfer takes place till thermal equilibrium is achieved in any system. The system is one enclosed by a space within the boundary. The rest space outside it is the surrounding.
The laws of thermodynamics were established over the years as some of the most fundamental rules which are followed when a thermodynamic system goes through some sort of energy change.
There are four laws called as laws of thermodynamics that tells us how the heat transfer takes place between system and surroundings.
  1. Zeroth law of thermodynamics
  2. First law of thermodynamics
  3. Second law of thermodynamics
  4. Third law of thermodynamics.

The zeroth law says when two systems are sitting in equilibrium with a third system they are also in thermal equilibrium with each other.

The first law of thermodynamics states when heat is added to a system, some of that energy stays in the system and some leaves the system. Energy that stays in the system increases in the internal energy of the system.

The second law of thermodynamics tells about the enthalpy. It is impossible to have a cyclic or repeating process that converts heat completely into work.

The third law of thermodynamics tells us that all molecular movement stops at a temperature we call absolute zero, or 0 Kelvin

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Thermodynamics Equations

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The basic equation of thermodynamics is
Work Power Formula
Here P = power and W = work and it tells that power is equal to work done for a given time.

The power is also given as
Power Formula
Here m = mass, g = gravity and h = height

The four thermodynamic equations are
Thermodynamic Equations
Here dU, dH, dF and dG gives internal energy, Helmholtz free energy, enthalpy and Gibbs free energy respectively.
Enthalpy is an energy-alike property or function—it has the dimensions of energy, and its value is determined entirely by the temperature, pressure, and composition of the system and not by its history.
Enthalpy is a measure of heat in the system. Enthalpy is a thermal change that tells about the heat inside the system. It is the heat absorbed or released in a system at a constant pressure given as $\Delta$ H.
Helmoltz Free Energy Formula
H is the enthalpy value or Helmholtz free energy, 
U is the amount of internal energy or pressure,
P and V are pressure and volume of the system

This system works really well for gases. The enthalpy change gives the internal energy change in the system. The positive enthalpy change is endothermic reaction and negative enthalpy change is exothermic reaction.
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Entropy is a measure of the random activity in a system. When we say random, we mean energy that can't be used for any work.
Entropy is a measure of disorder in an isolated system. It tells about the energy not available for work and is denoted as $\Delta$ S. A system having temperature T and amount of heat given to it is $\Delta$ Q. The change in entropy is given by
If the process is reversible the change in entropy is
Change in Enthalpy
Where "S" is the entropy value, 
"Q" is the measure of heat, 
"T" is the temperature of the system measured in Kelvin degrees. 
When we use the symbol delta, it stands for the change. 
Delta T would be the change in temperature wherein the original temperature subtracted from the final.
Si and Sf are the initial entropy and final entropy respectively.

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Thermodynamic Equilibrium

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Equilibrium is the state of balance. The two systems are said to be in thermodynamic equilibrium with each other when they are in mechanical, chemical and thermal equilibrium with each other. If two bodies at different temperature are brought near each other. The heat transfer takes place from hot body to a colder one till both attain same temperature. This state is called thermodynamic equilibrium. Whenever the system is in thermodynamic equilibrium, it tends to remain in this state infinitely and will not change spontaneously.
Here temperature of a system is equal to that of the surroundings. The zeroth law of thermodynamics tells a lot about the thermal equilibrium. It two systems are maintaining the thermal equilibrium with a third system; it means that those two are in thermal equilibrium with each other. The state of the system which is in thermodynamic equilibrium is determined by intensive properties such as temperature, pressure, volume etc. For a thermodynamic equilibrium system with given energy, the entropy is greater than that of any other state with the same energy.

Non Equilibrium Thermodynamics

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Non equilibrium thermodynamics is a that which deals with the systems that are not in thermal equilibrium with each other. Most of systems are in thermal equilibrium with the surroundings. It tells about systems where the chemical reactions and processes based on it.

Thermodynamics systems

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There are three thermodynamic systems existing in nature:

Open system: The system across the boundary of which transfer of both mass as well as energy can take place across the boundary is called as open system. An example is an air compressor.

Closed system: It is a system that remains closed. The system of fixed mass across the boundary of which no mass transfer can take place is called as closed system. Here no interaction will be there with surroundings and hence they exchange energy but not the matter. An example is fluid being compressed by the piston in cylinder.

Isolated system:  A system in which both the mass as well as energy content remains constant is called an isolated system. In this system no mass or energy transfer takes place across the boundary.

Thermodynamic Process

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A thermodynamic process is that where all thermodynamic variables like pressure, temperature, volume (P, V, T) vary with time. There are four thermodynamic processes
  1. Isothermal process
  2. Adiabatic process
  3. Isobaric process
  4. Isochoric process.

Thermodynamics table

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Thermodynamic table
Thermodynamics Table

Applications of Thermodynamics

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Any situation where energy transfer is desirable, thermodynamic principles are applied and these applications are all around us. Some applications of thermodynamics are steam generators, refrigeration coolers, power plants, vehicles, air conditioning, internal-combustion engines, steam and gas turbines, and steam power plants.

Human bodies also apply the principles of thermodynamics. Human respiratory systems, skin, mouths, tongues, and other specialized organs and systems of the body apply thermodynamics to regulate and maintain internal temperature in a wide range of ambient conditions.

Thermodynamics Example Problems

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Lets go through some solved problems on thermodynamics:

Solved Examples

Question 1: Calculate the internal energy if work done by a system is 500 J and heat absorbed is 1000 J.
Given: Heat absorbed dQ = 1000 J
          Work done dW = 500 J
The internal energy is given by dU = dQ - dW
                                                 = 1000 J - 500 J
                                                 = 500 J.

Question 2: You could observe a figure given below. Calculate the work done, change in enthalpy and specific heat for it using the following data:
m = 5 kg, v1 = 2.0 m3, v2 = 0.3 m3, P1 = P2 = 10 bar, T1 = 20o C, T2 = 120o C, $\Delta$ U = 2500 kJ.
Thermodynamic Problem
In the fig the Pressure P1 = P2 = P
Work done is W1-2 = P $\times$ [v2 - v1]
                            = 10 $\times$ 100 $\times$ [0.3 - 2.0]
                            = - 1700 kJ

Change in enthalpy ($\Delta$ H)
$\Delta$ H = Q1-2 = $\Delta$ U - W1-2
                           = 2500 - 1700
                           = 800 J.

Specific heat at constant pressure (Cp)

Cp = $\frac{\Delta H}{m \times (120 - 20)}$

     = $\frac{1700}{5 \times 100}$

     = 3.4 kJ/kg K.