Four Laws Of Thermodynamic: Zeroth, First, Second, Third

"The three laws of thermodynamics define physical quantities (such as temperature, energy, and entropy) that characterize thermodynamic systems at thermodynamic equilibrium". The laws describe how these quantities behave under various circumstances, and preclude the possibility of certain phenomena (such as perpetual motion).

➛ These laws of thermodynamics use various parameters for thermodynamic processes, such as thermodynamic work and heat, and also establish relationships between them.

➛ Thermodynamics deals with changes in energy, and the laws of thermodynamics describe the limits within which these changes occur.

➛ In thermodynamics, "we usually study three fundamental laws: the first law, the second law, and the third law of thermodynamics", but to this another law has been added, which we call the zeroth law of thermodynamics. The zeroth law of thermodynamic was actually established much later than the original three fundamental laws.

LET US SEE ALL THESE LAWS ONE BY ONE;

Zeroth law of thermodynamic: Thermal Equilibrium/Temperature

This law tells us about the concept of "Temperature". The zeroth law of thermodynamics deals with "Thermal Equilibrium". 

According to zeroth law of thermodynamic, "If two systems A and B are in thermal equilibrium with a third system C, then systems A and B will also be in thermal equilibrium with each other". 

Here,

        T.E. represent thermal equilibrium

      A, B, C are three system which is in

               thermal equilibrium with each

               other according to this law.

The two systems are in thermal equilibrium, which simply means that both systems have the same temperature. Here, the temperature of system A is equal to system C and the temperature of system B is equal to system C and the temperature of system A is equal to system B. This law of thermodynamic is important for the mathematical formulation of thermodynamics.

First Law of Thermodynamics: Energy Conservation

This law tells us about the concept of "Energy". The first law of thermodynamics deals with the "Law of Energy Conservation". 

Therefore, the first law of thermodynamics states that "Energy can be converted from one form to another with the interaction of heat, work and internal energy, but it cannot be created nor destroyed, under any circumstances". 

Generally, the first law of thermodynamic tells that;

      ➩ ∆(System) + ∆(Surrounding) = 0

                  ➩ ∆(Universe) = 0

☛ One formal statement of first law of thermodynamic is as following; 

"Although energy assumes many forms, the total quantity of energy is constant and when energy is disappears in one form, it appears simultaneously in other form".

In a mathematical form, the first law of thermodynamic is a relationship between heat, work and internal energy of a system and represented as;

              ➩ { ∆U = δQ - δW }

Here,

    ∆U represent change in internal energy

    δQ represent heat transfer to the system

    δW represent work-done by the system

Second law of thermodynamic: Entropy Concepts

This law tells us about the concept of "Entropy (entropy is a measure of randomness or dis-orderness of a system)".

The second law of thermodynamics deals with the "the direction of heat flow or direction of changes". 

In a general way, second law of thermodynamic states, "The total entropy of an isolated system (such as the universe) can never decrease over time and is constant if and only if all processes are reversible". Thus,

              ➩ ∆(Entropy)universe = +ve

There are two important statement to describe the old form of second law of thermodynamics. These are the "Clausius Statement" and "Kelvin-Plank Statement".

➛ CLAUSIUS STATEMENT

"It is impossible to a self acting machine working in a cyclic process without any external force, to transfer heat from a cold body to a hot body (or in a simple way, heat can't transfer to a hot body from a cold body without any external force)".

➛ KELVIN-PLANK STATEMENT

"It is impossible to construct an engine which is operating in a cyclic process, produces no other effect except to external heat from a single reservoir and do equivalent amount of work (or in a simple way, heat can't be completely converted into work)".

Third law of thermodynamic: Entropy Limitations

This law tells us about the concept of "Absolute Entropy". 

The third law of thermodynamic states - "The absolute entropy of a pure, perfectly crystalline substance at absolute zero temperature (or 0k temperature) is zero". That means, only a pure and perfectly ordered crystalline substance at absolute zero temperature exhibit no molecular motion and have zero entropy.

As we know that the higher the molecular motion in a system, the higher the number of possible microstates and hence the higher the entropy of the system. And for a perfect crystalline substance at absolute zero temperature, the system has only a single micro-state and hence the entropy of a system become zero.

     ➩ {0 kelvin = -273.15°C = -459.67°F}

Points to be remember

1. Absolute zero temperature is unable to achieve in practice.
2. At absolute zero temperature, all the molecular motion within the system stops.
3. A process can occur only if it obeys the first and second laws of thermodynamics.
4. The second law of thermodynamics tells us that a process occurs in a certain direction and not in any direction.
5. Entropy can tell us about whether a process is spontaneous or not.
6. In general way, second law tells us that heat can't be completely converted into work while work can be completely converted into heat.
7. The first law of thermodynamics does not tell us in which direction the process will take place. For example, how does heat flow between two systems at different temperatures??

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