Thermodynamics: What is Thermodynamics, Laws of Thermodynamics (For CBSE, ICSE, IAS, NET, NRA 2022)

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What is Thermodynamics?

Laws of Thermodynamics

  • The fundamental principles of thermodynamics were originally expressed in three laws.
  • It was determined that a more fundamental law had been neglected, apparently because it had seemed so obvious that it did not need to be stated explicitly.
  • Laws of thermodynamics define the fundamental physical quantities like energy, temperature, and entropy that characterize thermodynamic systems at thermal equilibrium. The four laws of thermodynamics are:
  • Zeroth Law: It states that when two systems are in thermal equilibrium with the third system which is separate, the whole system together is said to be in equilibrium with one another.
  • If two bodies are in thermal equilibrium with some third body, then they are also in equilibrium with each other.
  • This establishes temperature as a fundamental and measurable property of matter.
  • First Law: It states the law of conservation of energy when applied to a system in which the energy transfer occurs from or towards the system (with the help of heat and work) .
  • This states that heat is a form of energy and is therefore subject to the principle of conservation.
  • It represents the relation between the work done by the system, heat supplied to the system and the change in the internal energy of the system.
  • Second Law: heat energy cannot be transferred from a body at a lower temperature to a body at a higher temperature without the addition of energy.
  • It states that it is not possible to find a system, where the absorption of heat from the reservoir is the complete conversion of heat into work.
  • Third Law: This law states that at an absolute zero temperature, the system posses minimum energy.
  • Entropy is sometimes called “waste energy,” i.e.. , energy that is unable to do work, and since there is no heat energy whatsoever at absolute zero, there can be no waste energy.
  • It is also a measure of the disorder in a system, and while a perfect crystal is by definition perfectly ordered, any positive value of temperature means there is motion within the crystal, which causes disorder.
  • For these reasons, there can be no physical system with lower entropy, so entropy always has a positive value.


  • In a thermodynamic setup, enthalpy is the measurement of energy.
  • Quantity of enthalpy equals the total content of the heat of a system, equivalent to the system՚s internal energy plus the product of volume and pressure.
  • So, H (the enthalpy) , equals the sum of E (the internal energy) , and P (the product of the pressure) , and V (volume) of the system.
  • H = E + PV


  • It՚s value depends on the physical state or condition of a system.
  • It is a thermodynamic function that is utilized to measure the disorder or randomness of a system.
  • For instance, the entropy of a solid, where particle movement is restricted, is less than the entropy of gas, wherein the particles occupy the container.