What quantities appear in the first law of thermodynamics?

There are in total 3 quantities appearing in the first law of thermodynamics.

They are;

  • Heat (Q)
  • Work (W) and 
  • Change in internal energy (∆U)

What is Heat in first law of thermodynamics?


Heat is defined as a flow of thermal energy due to differences in temperatures.

Heat is simply thermal energy in transit.

If two bodies are at different temperatures and they are kept in contact with each other, then the heat energy will flow from higher temperature body to lower temperature body.

What is Work in first law of thermodynamics?


When heat is applied or rejected from a thermodynamic system, the volume of the system may increase or decrease. Due to this, the work done by the system is given as;

      W = P (V2 – V1)

           = P ∆V

Where, P is the pressure of the system and V is the volume of the system.

Thus, work is said to be done by the application of heat on the system.

What is Internal energy in first law of thermodynamics?

All the substances are made up of atoms and molecules.

These atoms and molecules perform various motions like linear motion, vibrational motion as well as rotational motion.

Due to these motions, they possess kinetic energy as well as potential energy (and many more…)

The sum of kinetic energy and potential energy is known as internal energy.

Note: We cannot measure the internal energy of any substance. But only the change in internal energy (∆U) is measurable.

I hope you have come to know about the quantities with which the first law of thermodynamics is related to.

For more detailed information on First law of thermodynamics, check the ultimate guide on First law of thermodynamics.

(This guide has all the important knowledge about First law of thermodynamics along with real life examples and lots more. You will love this guide for sure.)

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