Physics Lesson 13.5.2 - Internal Energy of a Gas

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Welcome to our Physics lesson on Internal Energy of a Gas, this is the second lesson of our suite of physics lessons covering the topic of The First Law of Thermodynamics, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Internal Energy of a Gas

We can consider gases as representatives of matter when dealing with thermodynamic quantities as they move freely in space and therefore, their thermodynamic parameters are easily identifiable.

As stated earlier, a gas possesses only kinetic energy because the potential energy between their molecules (binding energy) is negligible since gases move freely in space.

The average kinetic energy of all particles of a thermodynamic system is

< KE_tot> = N × < KE₁>

where N is the number of system's particles (molecules) and < KE1 > is the average kinetic energy of a single particle.

Since the total average kinetic energy of gas molecules is equal to the internal energy U, and also it is proportional to the temperature T, we can write

U = constant × N × T

The last expression means internal energy of a thermodynamic system is a function of temperature. In other words, we cannot change the internal energy of a gas without changing its temperature.

There are three possible cases in this regard:

∆U > 0 ⟺ ∆T > 0
∆U = 0 ⟺ ∆T = 0
∆U < 0 ⟺ ∆T < 0

This is not always true for solids and liquids as during the phase change, their internal energy changes without any change in temperature.

You have reached the end of Physics lesson 13.5.2 Internal Energy of a Gas. There are 6 lessons in this physics tutorial covering The First Law of Thermodynamics, you can access all the lessons from this tutorial below.

More The First Law of Thermodynamics Lessons and Learning Resources

Thermodynamics Learning Material
Tutorial ID	Physics Tutorial Title	Revision Notes	Revision Questions
13.5	The First Law of Thermodynamics
Lesson ID	Physics Lesson Title	Lesson	Video Lesson
13.5.1	Useful Definitions in Thermodynamics
13.5.2	Internal Energy of a Gas
13.5.3	Giving Energy to a Thermodynamic System
13.5.4	Work Done on a System and Work Done by a System
13.5.5	The First Law of Thermodynamics
13.5.6	Special Cases of the First Law of Thermodynamics

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Continuing learning thermodynamics - read our next physics tutorial: The Kinetic Theory of Gases. Ideal Gases

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