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!
 

When Cornstarch Attacks

I was thinking back to some fun experiments that I did when I was a kid and remembered this little trick with cornstarch. It also amuses college students and people of all ages. Be sure to ask your parent or guardian before you make the glop! The only ingredients are cornstarch and water. Pour the water into the cornstarch slowly while mixing until it flows. If you put it on an old speaker (BE SURE TO ASK YOUR PARENTS, FIRST!), the glop dances like this.

This mixture is a non-Newtonian fluid. Contrary to the video, this just means that the stresses that are making the material flow are not proportional to the actual flow (the shear strain rate) at a given point. As the particles of corn starch are moved past each other, they catch on each other. The faster you try to move them against each other, the more they catch on each other, making it more difficult for the cornstarch to flow. The property that is used to describe something's resistance to flow is called viscosity.

Do you have any recommendations for science experiments that I can do at home?

Be sure to ask your parents before doing any science experiment at home (or taking anything apart). On our Science Experiments page, you can find some links to some pretty cool science experiments. Some other cool experiment recommendations can be found in this Youtube video.

Some of my favorite engineering exercises are to see, using only straws and pins, how tall a tower you can build. Compete with friends! Another fun exercise is to build a bridge out of only toothpicks and glue. Try to build the bridge that can hold the most weight without collapsing.

How does my computer work?

Well, this question could be answered on many levels. Your computer is a bunch of switches that are organized in such a way that you get meaningful signals out.

Computer programs organize how these switches work together to get the meaningful information. They make use of the existing arrangement of switches to do this. A compiler takes the computer code that you write and puts it in a usable form that can be eventually acted on by the switches. This "usable form" consists of specific voltages. You can either have no voltage (grounded) or have a finite voltage (above some pre-defined level). These are called 1s and 0s. These voltages are what gives you answers to your calculations and makes things show up on your screen. They are the lifeblood of computing.

Now, you may be thinking, what are these switches and how do they work. They can't be the same as a light switch. The type switches used in computers are called transistors. Typically, the ones that are used are MOSFET transistors. I have talked about current and voltage before. Resistance relates these two quantities by (note that V signifies voltage, I signifies current, and R signifies resistance):

V=IR.

By changing the voltage at a gate, the resistance between two other contacts is changed, either allowing or not allowing charge to move between the two contacts. Watch this video if you are interested in a more detailed explanation (or ask in the comments section).

So, who cares about vacuums?

You have probably seen your parents vacuuming around the house. You may have used a vacuum, yourself, to clean up messes. While they may seem like mundane systems, they are actually pretty cool. Check out this vacuum chamber video!

Pierre wasn't so lucky. What happened? When the vacuum pump started to pump, the much of the air in the vacuum chamber was removed. By removing the air in a constant volume at a constant temperature, the pressure in the chamber decreases. Pierre the Peep is full of air. The marshmallow has pores that trap the air, but allow the air to escape from the Peep slowly. The air that is trapped in the peep has a higher pressure than the air in the rest of the chamber, making the peep expand. The air in the peep slowly escapes. Once air is again allowed in the chamber (the vacuum is broken), Pierre shrivels, since the air that made him nice and fluffy was pumped away.

You now may be thinking, "Who cares?" Scientists who use vacuum systems care. Certain experiments and processes can only be done at extremely low pressures (high or medium vacuum conditions). It takes time for gas trapped in materials to escape from them and, if the gas escapes to quickly, the materials can crack or break. Also, if materials keep on letting out air (or other materials), it can take impractical amounts of time to reach the low pressures required for the experiment.

I keep on talking about processes. What am I talking about? Watch this video demo showing of sputter deposition. We do processes like this in our laboratory. Sputtering is a process that can be used to lay down thin layers of material. We use sputtering to deposit some of the materials used in our solar cells.

What is a solar cell?

A solar cell is a device that is used to convert the sun's energy into electricity. We work with a device called a photovoltaic. In these types of devices, energy from the sun is absorbed. When this energy is absorbed, the energy is transferred to charge carriers in the material. Because of the structure of the device, an electrical current develops. If the current is not allowed to leave the cell, a voltage develops. If you are interested in a more detailed description, watch this video. Most of the terms and concepts described in this video are described on the glossary page.

Comet of the century?

Out beyond Mars, a faint speck of light is speeding through the black of space towards our sun. With just out eyes, it does not look like much more than a faint star, but through a big telescope, we could make out a comet. It has been named ISON after the Russian telescope that discovered it.

Track ISON's progress with this site's java applet: http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=ISON;orb=1;cov=0;log=0;cad=0#orb

So what is a comet? A comet is a space rock that, when it passes by the sun, sometimes grows a colorful tail. This tail come from the ice and dust the comet managed to pick up when it was forming, making it different than the other common space rock, asteroids. When the comet gets close to the sun, the ice and dust melt making a tiny atmosphere around the comet. Comets are commonly nicknamed “dirty snowballs”.

This particular comet is special because of its size and the direction or orbit it is traveling. Around Nov. 28^th this year, it will fly through our sun’s atmosphere, a little more than one million kilometers from the star’s surface. If it survives, which is a big IF, it could emerge glowing as brightly as the Moon, briefly visible near the sun in broad daylight. The comet's dusty tail stretching into the night sky could create a worldwide sensation. A dirty snowball named Lovejoy, which was half as big as ISON, managed to make it through the sun’s atmosphere in 2011 so there is hope.

Multiple comet discoverer David Levy, who was on the team to spot Shoemaker-Levy 9 (a comet that collided with Jupiter in the 90s), offered up this bit of advice concerning comets: "Comets are like cats; they have tails, and they do precisely what they want."

 Comet C/2006 P1(McNaught) taken from Victoria, Australia 2007

There are a lot of other facts about comets that are incredibly interesting. If you would like to find out more about comets, I suggest starting with this site: http://science.nationalgeographic.com/science/space/solar-system/asteroids-comets-article/

Don't be afraid to ask questions!

How do you learn best? By doing? By hearing? By seeing? We don't always need to reinvent the wheel (although that can be a good learning/teaching exercise). Albert Einstein was not born with a full knowledge of physics. He was also sometimes wrong. All people have had to learn what they know at some point. Sometimes people learn by experience (a sometimes painful strategy). Other times, people read books. Still more times, people ask questions. Books are not always clear. There is no such thing as a bad question, although some questions can inform more than others. Hey, questions are what motivate science.

Sometimes, however, people become afraid of questions or afraid of getting wrong answers. This fear doesn't help anyone, since questions drive discovery and understanding. Putting questions aside does not make the questions go away. Often, the answer to a question is not obvious and sometimes there can be multiple correct answers to a question. Sometimes the real answer also may be different from the possible answer that immediately comes to mind. For example, if you asked me what color the sun looked (do not look directly at the sun), I might say white, if you were in outer space, yellow, if you were on Earth's surface during the day, or orange, if you were on Earth's surface at dusk or dawn. These would all be correct answers. This question immediately led to the followup question: where are you?

Kopp-Etchells Effect

Source: U.S. Army/ Sgt. Machael J. MacLeod

Do you see the halos around the helicopter? I saw this and thought it looked cool. What is actually happening is a matter of debate. What is known is that these halos tend to form around the helicopter blades when helicopters kick up dust in the desert and they are visible at night. The light intensity is greatest at the blade edges.

Some say that these halos are the result of discharges of static electricity. When the dust hits the nickel-titanium alloy that is used to protect the helicopter propellers, static electricity develops. This electricity discharges, causing the glow. The problem with this theory is that, if this were happening, the halos would have a blue tint, when they really have a red tint.

Some say that the dust particles are being heated by the collisions to glow red-hot. Similar to a meteor, the particles would burn up. The color of the light seems reasonable with this explanation (I have not seen a measurement of the light spectrum). The problem with this theory is that, if this were the case, the propellers would heat up, as well, which clearly is not happening since the propeller would have serious problems at the temperatures at which that might happen.

Another theory (put forward by physics blogger Kyle Hill; I find it the most convincing of those that I have come across) is that the dust hits the propellers and knocks off tiny pieces of the nickel-titanium alloy. This alloy is what is called pyrophoric. This means that it can randomly ignite in air. Another pyrophoric material is steel, which, when hit with flint, generates a spark. The friction between the dust particles and the blades would ignite the particle. You may be concerned that we use steel everywhere. The steel in your parent's cars does not randomly catch fire. There is no need to worry. Steel is most likely to ignite in a powdered form.

What do you think?
References: NPR, The Daily Mail, Kyle Hill