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Lecture 3

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University of Waterloo
SCI 206
Stefan Idziak

Pressure 1 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 structure  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 pressure  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 space 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 crushed) 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 mask? 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 (very uncomfortable!)  What is Archimedes' Principle of Buoyancy?
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