## A lot of what we learned about fluids and liquids works equally well for gases as well as liquids. When it comes to Archimedes principle, we can examine the buoyant force on objects.  This is generally only noticeable when the buoyant force exceeds the force of gravity which you know form playing with Helium balloons.  Generally, you probably do not play with Hydrogen balloons as they tend to be slightly flammable.  The buoyant calculations are simple for airships as they are always totally submerged by the atmosphere. ## As air pressure varies with altitude, this makes a useful tool for pilots for determining the altitude of an airplane.  In fact, one of our Vernier sensors has a built in air pressure altitude sensor!  This could be a fun experiment to test in the John Hancock Center!  As you get farther from Earth, there is less air over your head, and less air pushing down on you. ## As mentioned before, Galileo made a crude barometer using a vacuum pump made from a leather bag and water as the fluid.  Torricelli, a student of Galileo’s, instead used mercury, which is thus the convention we will use today to measure air pressure.  Essentially, all a Torricelli Tube is merely a flask where the air has been evacuated connected to an upsidedown tube in a dish full of Mercury.  Air pressure pushes down on the Mercury and then pushes it up into the tube which has been evacuated of air.  The level of the Mercury depends on the air pressure, which is normally around 29” of Hg.  P/T = constant

PV/T = constant

## PV = nRT

Where they realized that this depends on the number of molecules in the gas.

n = number of moles of gas

R=8.31 J/K/mol

The above equation is what often appears in chemistry books.

In a physics book you will often see this equation expressed as

PV = NkT

Boltmann k = R/ NA = 1.38 x 10 –23 J/K

NA = 6.022 x 10 -23 per  (g mol)

N = number of molecules or particles

What is important to note here is what we actually mean by temperature, heat, and internal energy, all concepts we will explore in the coming week.

We can think of temperature as proportional to the average kinetic energy per molecule. T = avg KE * 2/(3k)

Heat is actually the flow of energy from one object to another (either by conduction, convection, or radiation).

The internal energy is the sum of the molecular energies of a substance (due to both the kinetic energies and the electrostatic potential interaction energies).

The internal energy is usually expressed as 3/2 NkT