Friday, August 30, 2013

What is a gas?

    Gases are an important state of matter!  After all, we need them to breathe.  Moving air can transfer heat better and more cheaply than most other methods (think of how much cooler a breeze makes you feel).  Stagnant air, on the other hand, can prevent heat transfer better and more cheaply than most other methods (house insulation works simply because it traps air in little stagnant pockets).

    The traditional states of matter are solids, liquids, and gases.  Solids and liquids are different from gases in that their particles (which can be atoms, molecules, polymers, etc.) are basically 'in contact' with each other.  This means that in solids and liquids, a particle can hardly begin to move before it bumps into its neighbors.  A gas is different because its particles can fly around relatively huge distances before they bump into one another.  This allows us to differentiate solids and liquids, which have around 10^22 (10,000,000,000,000,000,000,000) atoms per cubic centimeter, from gases like atmospheric air, which are 1,000 times less dense than that.  It might seem like the air you breathe is mostly empty space, but it is actually full of microscopic particles.  It seems empty because it's not dense, which makes it easy to move around!  Each particle in the air you're breathing right now actually zooms around at about 1,000 miles per hour and has a collision with another particle an average of 10^10 times every single second!  That means it can only make it about ~70 nanometers before it collides again.

    With so many collisions being made on such a small scale, we scientists must look very closely at the details of how these particles actually interact with one another.  A gas that is made up of unbonded atoms, such as argon (Ar, which makes up about ~1% of air), has properties which reveal the nature of atoms themselves.  The nature of atoms is basically that when they are very far apart, they don't influence or affect one another.  However, if they happen to move within a certain range, they spontaneously start to attract one another!  This attraction draws them closer, which causes even more attraction, and they are in turn pulled more strongly together.  They eventually reach a point where if they got any closer, they'd be invading each other's space, and this is highly unfavorable and unlikely.  So the basic behavior of gas particles is the same as atoms in general:  very far away they don't affect each other, nearby they attract one another, and too close and they greatly repel one another.  These simple phenomena are the root causes of many many things throughout the universe.

    The figure below is an example of this.  Specifically, we look at an atom's energy to see how favorable its state is, and whether it is likely to vacate that state or stay there.  The figure looks at a kind of energy (potential), and how it changes when another particle is brought closer and closer.  If the atom is far away then the energy stays at zero (0), represented by a neutral green coloring.  This means the other atom has no affect on ours.  Moving closer makes the energy decrease to more red values--this means the atom is relaxing into a more favorable state and will likely continue to ease into that state.  It eventually reaches the minimum of the curve, where the energy is the lowest (and reddest) it can be.  Imagine rolling a ball into a valley like this.  The ball will stay at the bottom, in exactly the same way the atom is likely to stay at this distance.  If we keep pushing them closer, however, they get too close.  They start bumping up against each other and the energy shoots almost straight up to extremely unfavorable and unmanageable heights.  This corresponds in a color change from red to orange, then yellow, green, blue, and ultimately violet at the very highest energy.  In practice, this means the two atoms would simply bounce off one another, similar to the way a golf ball bounces off pavement.


It's a lot more fun to interact with the ball and make it go faster, but first you'll need to download this free program.  Once you download and install that you should be able to manipulate the demonstration below!
 


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