Kinetic Theory of Gases

by Jared Rovny

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    00:01 We're now going to go over the kinetic theory of gases and the way this works is that in physical systems we have some very complicated things that we're trying to measure directly.

    00:11 It's very messy, we don't actually know what's going on.

    00:14 So what we often try to do is take a step back and come up with some mental way of thinking about this some human created system.

    00:23 Some idea of the way this might work and then ask ourselves how are this human created system that we've come up with our own heads compare with the actual physical system.

    00:31 So this kinetic theory is exactly one of these human models.

    00:34 A model that we come up with and see how well it matches with reality.

    00:37 Our model is very simple and it's what we call an ideal gas.

    00:41 A model of what would be an ideal gas.

    00:43 And an ideal gas has a few properties.

    00:46 First of all, we assume that the particles in our gas are small point particles.

    00:51 And by small we just mean the small relative to the distance between them and so in a picture like this one, you can see, that the distance between each of these particles is a lot greater than the small size of the particles themselves.

    01:02 We also assume that the only interaction that these particles undergo is just pure collisions, the just bounce off each other.

    01:08 So we don't allow there to be any interaction or attractions or repulsions from any electrical force or anything else.

    01:14 When this particles collide we also assume that those collisions are purely elastic and as we saw with elastic collisions, the energy is completely conserve.

    01:24 They just bounce off each other and go their way without energy being lost to any sources.

    01:30 And then finally, we assume that the internal energy that this gas has is proportional to the temperature.

    01:37 So if we increased the temperature, we have also increased the internal energy of our system.

    01:43 This is usually an okay approximation but it's important to understand the regimes and which this breaks down.

    01:48 One time it would break down as if you have very high pressures, you might reasonably assume that at high pressures, the particles would be much closer to each other and you get all this extra stickier interactions that were not assuming in our ideal gas model.

    02:03 It also doesn't work at pressures that are too low.

    02:06 In this case, you can have this particle sort of condensing and slowing down and having other effects, maybe the stickiness, electrical attractions and repulsions start taking over rather than just this pure energy conserving collisions that we're assuming here.

    02:22 It also doesn't work so well when you have strong intermolecular interactions.

    02:26 So for particular types of particles you might have interaction between those particles that are too strong and that would also cause the model that we've created here to breakdown.

    About the Lecture

    The lecture Kinetic Theory of Gases by Jared Rovny is from the course Gas Phase.

    Included Quiz Questions

    1. Molecular interactions between particles in the gas
    2. Elastic collisions between particles in the gas
    3. Gas at room temperature
    4. Gas at atmospheric pressure
    5. No interactions aside from collisions
    1. They are assumed to be very small relative to the distance between the particles
    2. They are assumed to be very small relative to the volume of the gas
    3. They are assumed to be very small relative to their molecular diameter
    4. There is no such assumption for an ideal gas
    5. They are assumed to be very small such that the gas has a low density

    Author of lecture Kinetic Theory of Gases

     Jared Rovny

    Jared Rovny

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