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Laws of Thermodynamics

Thermodynamics is a macroscopic science which studies various energy inter-actions, notably heat and work transfer, with matter, that brings about significant changes in the macroscopic properties of a substance that are measurable. It is basically a phenomenological science based on certain laws of nature which are always obeyed and never seem to be violated. These are the basic laws of thermodynamics.

1. Zeroth law of thermodynamics
2. First law of thermodynamics
3. Second law of thermodynamics
4. Third law of thermodynamics

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Zeroth Law of Thermodynamics

According to the zeroth law of thermodynamics given by I. Muller, "At an ideal interface temperature is continuous between the two bodies; typically between a body whose temperature is measured and the thermometer.”

"When a given object or body A is in thermal equilibrium with a body or object B, and separately with a body or object C also, then it is necessary that B and C would also be in thermal equilibrium with each other." This statement tells us that thermal equilibrium is a Euclidean relation between thermodynamic systems. Thermal equilibrium is also a proven reflexive relation. Equivalence relations are those which are both reflexive and Euclidean. Hence thermal equilibrium is a transitive relationship. Another result is that the equilibrium relationship is always symmetric: If the body A is in thermal equilibrium with another body or object B, then B is in thermal equilibrium with A. Hence it can be said that two systems are in thermal equilibrium with each other.

First Law of Thermodynamics

The first law states that the energy of the system remains constant given the system is isolated. It is a special statement for the law of conservation of energy. It was stated by Clausius who had used 2 ways for stating and expressing the law.
• In one cyclic process input – output of the system are considered.
• In another way the increment in the internal energy of the system is considered. Here the process is not considered to be a cyclic one. (A process that can be repeated many times still leaving the system in the original state).
The law states that the algebraic summation of all energy transfer across system boundaries is zero. Let Q be the amount of heat transferred to a system and W is the amount of work transferred from the system during the process, the net energy transfer (Q – W) would be stored in the system. Hence according to the first law:

$Q–W = \Delta E$

$Q = \Delta E+W$

Second Law of Thermodynamics

The first law states that a certain energy balance will hold on even when a system undergoes a state of change. But it does not enlighten us on feasibility of the change of state or the process. All that the first law tells is that if the process occurs then the energy gained by one end would be equal to the energy lost by other end. The criterion to the probability of various processes is given by the second law of thermodynamics.

Kelvin Planck statement says, "It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature."

Clausius statement says, "We cannot construct a device which operating on a cycle produces no effect other than the transfer of heat from a cooler to a hotter body. This implies that heat energy cannot flow from one body to another because of the temperature change, some work definitely has to be done to achieve this."