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Laws of Thermodynamics

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The branch of physics which deals with the relation between the heat and mechanical work is defined as thermodynamics. It is concerned with the properties of systems composed of many particle such as a gas, liquid or solid. The thermodynamics here is concerned with the mutual conversion of heat and also the mechanical work. It also deals with the interactions between the objects. The subject matter of thermodynamics is based on three experimental laws, namely
These laws are based on experimental facts but not on theoretical facts. The laws provide criteria for predicting the feasibility or otherwise of a process and the energy transformations that occur in the process. In this section we will learn more about laws of thermodynamics.

There are four laws of thermodynamics, beginning with the “zeroth” law. The need for the zeroth law become apparent after the others were developed and the law is a statement about the concept of temperature.
When two bodies A and B are separately in thermal equilibrium with a third body C, then A and B are in thermal equilibrium mutually, i.e., If $T_{A} = T_{C}$ and $T_{B} = T_{C}$ then $T_{A} = T_{B}$
→ Read More The first law of thermodynamics is based on the principle of conservation of energy, which states that the energy cannot be created or cannot be destroyed but it can be transformed from one form to another. It state that the change in the internal energy of a thermodynamic system is equal to the amount of energy added by heating the system excluding the amount lost due to the work done by the system on its surroundings. It also state that the energy is conserved in any process in the thermodynamic system and its surroundings.Let us take $dU$ is the change in internal energy, $dQ$ be the heat added to the thermodynamic system and $dW$ be the work done, then the first law of thermodynamics written as

$dQ = dU + dW$
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Qualitative aspect of second law of thermodynamics:
It is not possible to extract a definite amount of heat (Q) from a hot reservoir and use it wholly to do a work (W). Some amount of heat has to be lost to a cold reservoir. This is called the Kelvin-Planck statement. Heat will not flow spontaneously from a body at a low temperature to a body at lighter temperature. This is called Clausius statement.
Examples
  • The green plants capture the light of the sun and condense the molecules of carbondioxide and water with the help of enzymes and chlorophyll along with sun's energy to form a more orderly sugar molecule. Thus all of the sun's energy cannot be extracted by green plants without converting a part of it into usable form .
  • When we mix oil and water, the two layers always appear immiscible. But if we stir this mixture rigorously we find small droplets of oil thoroughly suspended in the water solution. Some amount of energy has been expended from the internal energy of the molecules of oil and these macro molecules have been made into micro-molecules well separated by water molecules. The total energy of the oil has not been lost but only diffused over a larger space that is the entropy of the system has increased. After some time when the two layers of oil and water are allowed to stand. there are formed two distinct layers of oil and water.
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The third law of thermodynamics stated that it is impossible by any procedure, no matter how idealized, to reduce any system to the absolute zero of temperature in a finite number of operations. This is what is called the Fowler-Guggenheim statement of the third law.
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