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• Complete Physics Course - Class 11
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```THIRD LAW OF THERMODYNAMICS:-

In all heat engines, there is always loss of heat in the form of conduction, radiation and friction. Therefore, in actual heat engines Q1/T1 is not equal to Q2/T2.

Thus, Q1/T1 – Q2/T2 is not zero but it is a positive quantity.

When cycle after cycle is repeated, the en

tropy of the system increases and tends to a maximum value. When the system has attained the maximum value, a stage of stagnancy is reached and no work can be done by the engine at this stage. In this universe the entropy is increasing and ultimately the universe will also reach a maximum value of entropy when no work will possible. With the increase in entropy, the disorder of the molecules of a substance increases. The entropy is also a measure of the disorder of the system. With decrease in entropy, the disorder decreases. At absolute zero temperature, the entropy tends to zero and the molecules of a substance or a system are in perfect order (well arranged). This is the third law of thermodynamics.

Example:-

The molecules are more free to move in the gaseous state than in the liquid state. The entropy is more in the gaseous state than in the liquid state. The molecules are more free to move in the liquid state than in the solid state. The entropy is more in the liquid state than in the solid.  Thus when a substance is converted from a solid to a liquid and then from the liquid to the solid state, the entropy increases and vice versa. When ice is converted into water and then into steam, the entropy and disorder of the molecules increase. When steam is converted into water and then into ice, the entropy and disorder of the molecules decrease. Hence entropy is a measure of the disorder of the molecules of the system.

By any ideal procedure, it is impossible to bring any system to absolute zero temperature performing a finite number of operations. This is called the principle of unattainability of absolute zero. Thus, the unattainability principle is called the third law of thermodynamics.

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