Fluids: Liquids and Gases




PV = constant





V = constant * T


Pressure in Gases

Boyles Law / Charles and Gay-Lussac's Law


The molecules that make up gases aren’t bound together at all so each molecule moves on its own random path. Their only interaction with each other or other things is when they collide. As they hit the walls of a container they exert pressure on it.

Boyles Law

The 17th century chemist Robert Boyle studied the relationship between the pressure P and the volume V of a gas held at a constant temperature. Boyle observed that the product of the pressure and volume are nearly constant. The product of pressure and volume was always the same number for an ideal gas, and in fact, most normal gases act this way:


 Pressure x volume = a constant PV = constant


This relationship between pressure and volume is called Boyle's Law in honor of his work. See the relationship in action at the NASA Glenn Research Center – The Beginners Guide to Aeronautics.  Yes, these are concepts you need to know to understand the science of flight.


Charles and Gay-Lussac's Law

The relationship between temperature and volume (holding constant the number of molecules and pressure) was named for the two French scientists who first investigated it. They found that if the pressure were kept constant, the volume V of a gas was equal to the product of a constant and the temperature T:


 Gas volume = a constant x gas temperature V = constant * T

This equation is also explorable at NASA Glenn Research Center – The Beginners Guide to Aeronautics.

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