Thermodynamics: First Law of Thermodynamics

By Akhil Gupta|Updated : November 9th, 2021

  First Law of Thermodynamics

  • The first law of thermodynamics is to relate the various form of energy and energy interaction.
  • Its states that “In a process the creation or destruction of energy is impossible but it can be changed from one form to another form” and it is known as the “Conservation of energy principle”.

                                                                                                                                                                  

First Law of Thermodynamics

  • The first law of thermodynamics is to relate the various form of energy and energy interaction.
  • Its states that “In a process the creation or destruction of energy is impossible but it can be changed from one form to another form” and it is known as the “Conservation of energy principle”.
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    From the above statement, we can say that the energy leaving the system must be equal to the energy supplied to the system.
    Ein = Eout
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FIRST LAW FOR A CLOSED SYSTEM UNDERGOING A CYCLE

As per the first law for a closed system undergoing a cycle, net heat interaction in a cycle is equal to the net Work interaction.

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where J is the Joule’s equivalent. This is also expressed in the form

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Fig.1: Closed System Undergoing A Cycle

1st LAW FOR A CLOSED SYSTEM UNDERGOING A CHANGE OF STATE (for a process):

The expression:

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The above equation is applicable only to systems undergoing cycles, and the algebraic sum of all energy interaction across system boundaries is zero.

If both heat transfer and work transfer are involved in a change of state of the system, then the net energy transfer will be stored or accumulated within the system.
If heat transferred to the system is Q and work transferred from the system is W, during the process

Then, net energy transfer (Q – W) will be stored in the system.

Energy in storage in not in the form of heat and work and it is simply the energy of the system, thus referred as the internal energy of the system.

Therefore: Q – W – ΔE

where ΔE: Increase in the energy of the system.

Q = ΔE + W

Consequences of 1st Law of Thermodynamics

(i) Heat is a path function.

(ii) Energy-a property of the system.

(iii) Energy of isolated system is Constant.

(iv) Perpetual motion machine of first kind-PMM1 is not possible. 

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