Ideal Gas Law: Pressure, Density, Temperature, etc.
Discuss the structural differences between a solid and a liquid.
o A solid is structural and arranged in a periodic manner, such that if we know the location of one
molecule, we can infer the location of all other molecules in that structure
o Conversely, the molecules in gas are much farther apart from each other, and there is no defined
What happened when we ran an air hose through liquid nitrogen? Why?
o We caused some of the gases in the air to condense into liquids, while some others remained as gas
o The result was that a combination of gas and liquid came out the other end
o This is a property of gases - at certain temperatures, the molecules come together close enough that the
substance takes on the consistency of a liquid
Explain how the concept of pressure applies to a gas.
o Recall that pressure = force / area
o And the individual molecules in some sample of gas push against the walls of their container, and create
Explain how a steam engine works.
o It's all about this pressure principle - you evaporate water and now we have lots of steam in a confined
o Thus it will create pressure against the walls, and this pressure can be used to crank an engine
Why does a balloon make a loud sound when it pops?
o It is because there is a sudden release of pressure from the gas molecules which were previously inside,
pushing out against the fabric of the balloon
When this pressure is suddenly released, a loud sound is made
o Note that the pressure inside is (definitely) greater than the pressure outside because the pressure
inside needs to keep the balloon stretched out
Explain gaseous temperature on a molecular level.
o Temperature of some sample of gas is related to how quickly the molecules of gas are moving around
o Note that if they move around faster, they will bump into the walls with more force and thus create
greater pressure as well - that's why if a gas is in a container than can expand, heating the gas will cause
the container to expand
How do we apply the molecular explanation of temperature to making bombs and popcorn?
o Both things involve the principle of heating something up so that it expands so much and so rapidly that
an "explosion" is caused
Either the bursting of the kernel into a piece of popcorn
Or the bomb exploding
Explain gaseous density on a molecular level.
o Density is just how many gas molecules there are in a given area
o Again, when the density is higher, the pressure will be higher because more gas molecules will be hitting
the sides of the container
How does this apply to using a straw?
o We are basically sucking the air out of the straw, meaning that there is less pressure available to push
the liquid down
o Thus liquid is able to come up the straw and into our mouth :) Pressure 2
What does the Ideal Gas Law tell us?
o PV = nRT, or P = number density x R x T
o Pressure is proportional to density of particles times temperature
Atmospheric Pressure and Buoyancy
Explain the "box" model of atmospheric pressure.
o If we are talking about the atmospheric pressure acting on a single person, imagine that the column of
air above him is divided into boxes
o Gravity acts (as it always does) on these boxes, and so there is a downward force on the person created
by all this air
However, there are also air boxes which push up to counteract this (that is why we are not
o As we get higher on this "pile" of boxes, there is more air pushing up and less air pushing down, so the
air pressure we experience is less
Why was it hard to pull those rubber disks apart when the air was removed from inside them?
o Because removing the air meant that the air pressure in the "container" was reduced to zero, all the air
from outside wanted to get in so that pressure could equilibrate
o This air created a force which opposed the efforts to pull the discs apart
Why does it not hurt to lie on a bed of nails?
o Because the pain which we traditionally think nails cause is due to pressure, which (recall) is related to
force and area
o In this case, we are spreading out the area of the pressure source by putting many nails, and thus the
total pressure is not too bad
What would happen if the cabin of an airplane became de-pressurized, and I failed to put on my oxygen
o All the intestinal gas we have (which is now kept at a certain density because of the atmospheric
pressure from outside) would now be subjected to less atmospheric pressure, and so it would expand
What is Archimedes' Principle of Buoyancy?