# Gases: Basic Theory

by Jared Rovny

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00:01 We've discussed fluids, so now we're going to move to gases.

00:04 And see the way that the gases or the air is a little bit different in its behavior from the liquid fluids.

00:11 As a reminder, what we've done is started with the mechanics in this course, we give a big overview of many different kinds of variables to describe systems.

00:18 Now, we're getting into of what I've listed on the bottom here, many different practical ways to implement these mechanics.

00:25 We started with fluids and now we're going to go to the mechanics of and the properties of gases.

00:31 What we'll discuss is first starting with the basic theory of gases and how to describe gases and some basic laws and then we'll get to some more gas properties and more applied systems.

00:42 But starting with the basic theory, we'll cover a few things.

00:45 We'll start with how we measure gases especially historically and some of the ways that we came up with the laws governing gases that we have.

00:56 We'll then move to the kinetic theory of gases and try to develop a way of speaking about gases that is very systematic.

01:04 We'll cover Boyle's, Charles' and Avogadro's law and the way that they came up with these laws.

01:10 And then finally the ideal gas law, which is a combination of these and how to use it as well.

01:15 But starting with the basic measurable, the measurable properties of gases.

01:20 First, when we're trying to measure a gas, we can asks ourselves as we did when we're talking about simple objects like apples or anything that can move.

01:29 What are the basic properties of this object? How can I measure? How can I discuss it quantitatively and scientifically? So some of these properties are the pressure of the gas, the volume, how much space the gas takes up.

01:44 The temperature that the gas is at and finally just how much is there.

01:49 The numbers, specifically even just the number of molecules of the gas.

01:53 And as we'll talk about what measure this number, the number of molecules and the unit called moles.

02:01 Historically, the way we came up with the way to measure the pressure of a system, any system for example using just the atmosphere pressure here is with the mercury barometer.

02:14 And what this is, is very simple it's actually a dish, literally a dish of a mercury which is a liquid at room temperature.

02:21 So you have this very heavy liquid mercury in a dish with a cylinder as a tube place over the mercury.

02:28 In a tube and this is the very important part about the way this works.

02:32 There's a vacuum above the mercury in the top of the tube, so there's no air in the top of the tube, that's very important.

02:38 And also something to know about mercury as we go forward, is that its chemical name is Hg, so if you see Hg or mmHg, which we'll discuss in a moment, that means mercury or millimeters of mercury.

02:52 In a barometer like this, what happens is we have the atmosphere pressure, pushing down on the surface of all the mercury that's in the dish and that force that's acting downward on the mercury will cause the mercury in a tube to be lifted up like almost like a lever, pushes down, causing the mercury in the tube to go up.

03:09 And the reason it goes up it's that again, there's no air in the top of the tube, on top of the mercury.

03:14 So there's nothing else to react or to push the mercury down.

03:17 What this mean is that the only thing fighting the pressure that's pushing the mercury down and trying to cause it to rise, is simply the weight of the mercury and the tube itself, so all we have to do is understand how heavy that mercury is, know how heavy it is, what the weight is and we can directly measure and compare the weight of the mercury and the tube to the pressure that's trying to lift it.

03:38 In this case from the outside of the atmosphere.

03:41 It turns out that at atmosphere pressure, as we've listed here, the height of the mercury rises to is 760 millimeters.

03:49 So we often called this a simply a unit of pressure, 760 millimeters of mercury.

03:55 It's important to get that number right.

03:56 And what we do like I said, is we compare the amount of pressure coming from the column of the mercury pushing down simply by its weight to the atmosphere pressure.

04:05 We've already discussed exactly how to find the pressure from a column of any fluid and in this case it's a mercury fluid.

04:12 So we know that in our equation for pressure, which is again rho the density times g times the height that the mercury has risen to.

04:19 We know that the pressure is simply going to be this equation with the density of mercury being used for that variable rho.

04:27 The way to measure pressure as I discussed, is literally to measure just the height, how high does the mercury go and we can in fact use this height as an actual measure as what were unit of the pressure and so we could say that one atmosphere of pressure is equal to 760 millimeters, the height of mercury which again is measured as or written as Hg.

The lecture Gases: Basic Theory by Jared Rovny is from the course Gas Phase.

### Included Quiz Questions

1. 10⁵ kg/s²
2. 0.76 mm Hg
3. 10⁵ kg/(m.s²)
4. 100 kPa
5. 760 mm Hg
1. 10.3 m
2. 10 m
3. 0.76 m
4. 103 m
5. 7.6 m
1. Pressure, volume, temperature, number of particles of the gas (measured in mols)
2. Pressure, volume, mass, number of particles of the gas (measured in mols)
3. Energy, mass, temperature, number of particles of the gas (measured in mols)
4. Energy, pressure, mass, volume
5. Volume, pressure, mass, momentum

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