State and explain the first law of thermodynamics?


The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.

The mathematical representation of the first law of thermodynamics is:

ΔU = Q + W


  • ΔU is the total change in the internal energy of the system
  • Q is the heat exchanged between the system and its surroundings
  • W is the work done either by the system or on the system

Below is the table explaining the sign conventions for heat, internal energy, and work done:

It will be positive if the temperature increases It will be positive if heat enters the gas It will be positive if the gas is compressed
It will be negative if the temperature decreases It will be negative if heat leaves the gas It will be negative if the gas expands
It will be zero if the temperature is constant It will be zero if no heat is exchanged It will be zero if the volume is constant

Why is the first law of thermodynamics important to the environment?

The importance of first law of thermodynamics to the environment are,

  • All living organisms on Earth depend on the sun for their life.
  • Photosynthesis is the mechanism by which plants transform solar energy into chemical energy.
  • These energies are not returned to the solar system by the plants; instead, they are passed on to herbivores that eat green plants.
  • Some part of the energy obtained by the herbivores is utilized by carnivores or transferred to the decomposers when the herbivores die.

Limitations of first law of thermodynamics

The law states that whenever a system undergoes any thermodynamic process it always holds a certain energy balance. However, the first law fails to give the feasibility of the process or change of state that the system undergoes.

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