Dec 2016

Temperature and Ideal Gases

December 23, 2016


Ideal gasses is an area of physics that can seem difficult to grasp- but like many physics concepts, it’s actually easy when you break it down in our physics tuition. Below we look at some of the top things to note about temperature and ideal gasses.

What is an ideal gas?

An ideal gas is more of a theoretical concept than is a reality. Ideal gasses consist of point particles moving with no interaction other than when they collide in an elastic manner. This theoretical gas would occupy 22.4 liters at standard temperature and pressure, to make one mole.

Since it’s a theoretical state, you may be wondering why ideal gasses matter at all. Simplicity is the key- ideal gasses are easy to analyze and also obey the ideal gas law to make calculations simpler. Also, many physical gasses do behave qualitatively like an ideal gas under conditions of standard temperature and pressure. These can include noble gasses, oxygen, nitrogen, hydrogen and even some denser gasses like carbon dioxide. However most dense gasses do not behave in any way similar to ideal even at STP(standard temperature and pressure), and in that case, the ideal gas model is not as useful. This concept also becomes less reliable at lower temperatures.

How does Temperature and Pressure Affects a Gas?

As a rule of thumb, the lower the pressure and higher the temperature, the more likely a gas will behave like an ideal gas. These conditions reduce potential energy [in relation to kinetic energy] as well as making the molecule size less significant [in relation to the distance between them]. The ideal gas model remains one of the most important models for statistical mechanics.

What is the Ideal Gas Law?

If you’ve joined us for JC physics tuition before, you may already be familiar with the ideal gas law:


With P being pressure, V volume, n the moles of the gas, R the gas constant and T the temperature in Kelvin. You will add 273 if given a temperature in Celsius. The gas constant is 8.314 J.K^-1mol^-1. This law was arrived at by combined use of Boyle’s law, Charles’s law and Avogadro’s law. Remember that an ideal gas must also meet the conditions of Joule’s law [that only temperature effects the internal mass of an ideal gas]. Also bear in mind that the lower the temperature, the less like an ideal gas a real gas will perform.

What does the ‘Ideal Gas’ Conditions Assume?

Ideal gas assumes several things:

  • The distances between molecules are far larger than the molecules themselves
  • There is no friction in play
  • Collisions are elastic
  • Newton’s Laws are in effect
  • There are no significant forces between the gas molecules and the environment other than the walls of the container [and no long distance forces]
  • Molecules are in constant, random motion
  • Attraction and repulsion are not in play other than in the point force collisions
  • Particles are rigid and spherical, to avoid rotational modes

Ideal gasses may not be ‘real’ in normal world circumstances, but the degree of error for many gasses is so low that the findings of ideal gas law are of great use to understanding the world around us. Mastering ideal gas laws and concepts is a critical part of mastering physics itself, so be sure to memorize these concepts well.

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