Ideal gas law
The ideal gas law is a fundamental law to physics, and it provides the basis for heat engines, as well as explains how airbags work, and why an empty water bottle can sometimes pop when you take it on an airplane. The principle equation for the ideal gas law is . In this equation
is pressure,
is volume,
is the number of moles of gas,
is temperature, and
is the ideal gas constant. The value of R depends on the units used. It can also be written as
where N is the number of molecules and
is Boltzmann's constant, with the rest of the variables being the same.
The ideal gas law allows for us to determine what will happen to a contained system with gas inside, based on these different variables. For example, if the volume of the system is increased, and all other variables are left alone, the pressure will automatically decrease to compensate for the increase in volume. Conversely, if heat is added to a system, both its volume and pressure will increase to compensate. This latter relationship is the basis for heat engines.
For a deeper treatment of the ideal gas law please see hyperphysics, for an extensive treatment please see the UC Davis's chem wiki.
How an airbag works
Oftentimes, physics and chemistry's applications seem quite distant. Here's an example of how these sciences can save a life during a car crash.
Vehicle airbags work using the ideal gas law. By reacting Sodium Azide (NaN3) with excess heat, a large amount of Nitrogen gas (N2) is created. The balanced chemical formula for this is 2NaN3 (solid) --> 2Na (solid) + 3N2 (gas). How does this pertain to the ideal gas law? If you recall, in the ideal gas equation, is equal to the number of moles (a unit of amount) of gas in the system. Before the reaction, the sodium azide is a solid, so there is no gas in the system. By reacting the sodium azide to create nitrogen gas, we add several moles of gas to the system. As we remember with the ideal gas law, the two sides of the
equation have to balance, so by adding a bunch of moles of nitrogen gas, we force the volume of the system to increase dramatically. This inflates the airbag in between 20-40 milliseconds, giving it time to begin deflating before your head hits it. This disperses the force, dramatically improving chances of avoiding serious injury.[1]