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Why don't gases get pulled to the ground by gravity?

  1. Oct 28, 2012 #1
    I'm wondering why the gases don't stick to the ground like liquids and solids do. I read that gravity is pulling on the molecules but because of the electrons, the molecules repel each other. There's also something called "Brownian motion", what's that?

    And if they repel each other, why don't the gases escape the atmosphere altogether?
  2. jcsd
  3. Oct 28, 2012 #2
    gases *are* held close to the surface of the earth because of gravity. Gases have mass and therefore there is a gravitational force acting on them.

    The reason why they don't "stick to the ground" is because gas is not very dense. The atmosphere of the Earth is incredibly thin compared to the size of the Earth itself.

    according to this: http://en.wikipedia.org/wiki/Atmosphere_of_Earth#Pressure_and_thickness

    90% of the mass of the atmosphere is 16km above the ground. The Earth has a radius of ~6300km. This means that the atmosphere is like 0.25% of the Earth's thickness. It's not very thick.

    So, really, it is pretty much sticking to the ground, it's just that it's so much less dense than solids and liquids, so it takes up a lot more space.

    Like pretend you're a fish at the bottom of the ocean "why doesn't the ocean stick to the ground like solids do". See what I mean?

    Brownian motion is a result of the random motions of the gas molecules in a gas hitting tiny little things. It happens to tiny little things because they are small enough to be influenced by the even tinier gas molecules.

    Gas generally doesn't escape the atmosphere because it is being held down to the ground by gravity.
  4. Oct 28, 2012 #3


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    The repulsion between molecules is only at very close quarters. The tendency of gases to occupy a large volume comes from the way they bounce around off each other. Yes, the bounce itself is down to repulsion, but that they bounce so far is down to their kinetic energy (temperature).
    A key difference between gas molecules and more sedentary phases is that they don't attract each other much. Water is only a liquid at everyday temperatures and pressures because water molecules tend to stick together.
    A gas molecule will only leak into space if it reaches escape velocity. Only the lightest gases, hydrogen and (I think) helium achieve that readily.
  5. Oct 28, 2012 #4
    So, just because gas isn't very dense, it isn't held to the ground? I still don't understand. And the escape velocity is 11km/s right? And why does hydrogen and helium only achieve that?
  6. Oct 28, 2012 #5
    it *is* held to the ground, there's just a lot of it so it looks like it's not

    it's like if you're a fish and you ask "why isn't the water held close to the ground like the sand is" and the answer is that the water *is* held close to the ground, there's just a lot
  7. Oct 28, 2012 #6
    Okay, so it's like water occupying the ocean, there's a lot of it so it doesn't look like it is attached to the ground. And correct me if I'm wrong. The atoms aren't attached together because the energy is agitating them and causing them to move a lot?

    Ugh, can someone briefly explain it in quick notes?
  8. Oct 28, 2012 #7
    how much education have you got on this subject. Do you know about the concept of average kinetic energy of a gas and velocity distribution?
  9. Oct 28, 2012 #8
    I'm umm... 13. So not a lot. And what is average kinetic energy of gas and velocity distribution?
  10. Oct 28, 2012 #9
    okay so basically gases are made up of a bunch of tiny particles called molecules. These molecules are not just sitting around not moving, they are indeed moving around quite a bit. The speed at which they are moving around is directly related to the *temperature* of the gas. If the gas is higher, then there is more energy in the gas, and so the particles are going to moving around faster; however, all of the particles are not moving around at the same speed. Some are moving faster than others, some slower, and there is an average speed. It looks like a Bell Curve *sort of*.

    So basically, there is some small fraction of the molecules that is moving *way faster* than the rest of the molecules. Since the velocity of the molecule is influenced by mass, this means the lightest molecules will have a better chance of getting *really really* fast. And so, there are some molecules way up at the top of the atmosphere that are going fast enough to escape the Earth's gravitational pull. These molecules would be Hydrogen and Helium most likely because they weigh the least.

    And so yeah, the gas in the atmosphere acts very much like the water in the ocean. Except there's no "surface" to the atmosphere because gas molecules are not attracted to one another as much as liquid molecules are. In fact, I'm pretty sure that the molecules in a gas are not connected whatsoever. While the molecules in a liquid have a little bit of a bond. I don't know very much about the specifics of that, though.

    Gas molecules zip around totally free and collide with one another. The energy giving the molecules their motion is related to the temperature of the gas.

    hopefully some of that helps
    Last edited: Oct 28, 2012
  11. Oct 28, 2012 #10
    Wow! Thank you so much! I love you in a heterosexual way!
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