Air Pressure Demo
Two of the labs exploring air pressure were performed as a demonstration in front of the class. In the first demonstration, we placed a peeled, hard-boiled egg on the opening of a bottle in which there was a lit scrap of paper. I hypothesized that the egg would be sucked into the bottle, because the flame would consume the air inside the bottle. We observed the flame inside the bottle flicker and go out, and the egg squeeze through the neck of the bottle before falling with a pop and breaking on impact. My hypothesis was mostly correct, but incomplete — while I’d guessed what would happen, my ‘why’ wasn’t quite right. The egg wasn’t being sucked into the bottle, exactly, rather it was being pushed in by the higher air pressure outside the bottle. The reason that that higher air pressure existed because the flame had consumed the oxygen in the bottle, lowering the air pressure inside.
The second experiment was the same, only varying in that this time, the bottle was upside-down. I hypothesized that the same thing would happen as with the previous experiment, but that it would be slower this time because now air pressure would be fighting against gravity. This did end up happening — after a much longer period of time, the egg was pushed into the bottle with a slurp, because the flame inside had created an area of low air pressure.
These demonstrations served as proof for a number of properties of gases. First, we know that gases take up space, because before the flame consumed oxygen and the air pressure dropped, air was creating an equalized area of air pressure in the bottle, and was taking up space in a way that created the amusing reaction when absent. Secondly, we know that gases are compressible, because the compression is usually necessary to create the force exerted by the high pressure air on the egg in the two egg and bottle cases. Thirdly, we know that gases exert pressure, because it was air, a mix of gases, causing the pressure that pushed the egg into the bottle in both demonstrations.
I’ve been talking a lot about air pressure, but haven’t defined it — it’s the force exerted on a certain area by the mix of gases that make up air.
Air Pressure Labs
In class this week, we conducted a series of experiments exploring air pressure. Two experiments I found particularly interesting were “Ping Pong Funnel” and “Kissing Balloons.” Both of these were very simple, compared to the others, but still displayed the results of air pressure in an easily understandable manner.
In “Ping Pong Funnel,” our task was simple — we were to place a ping pong ball in a funnel, then blow up the tube and try to push the ball into the air. I hypothesized that we would be able to blow the ball into the air, because we would create an area of pressure on the ball and cause it to hover in the air. However, my hypothesis was incorrect. We found no matter how hard we blew or how we held the funnel, the ball remained at the bottom of funnel. In reality, blowing up the tube caused the air touching the bottom of the ball to move faster, creating an area of lower pressure. This meant that the air around all the other parts of the ball was now by comparison higher pressure, exerting downward force on the ball and keeping it in the funnel.“Kissing Balloons” was also about the connection between the movement of air and air pressure. In this experiment, we had two balloons attached to strings taped to opposite ends of the table. We then held them six inches apart, and I blew between them. I had hypothesized that my breath would, in my words, “push” the air out from between the balloons and they would come together to fill the space. As with my hypothesis regarding the first demonstration, I was right in my prediction but less so in my reasoning. The two balloons, after we blew hard between them, did sort of drift closer together, touch, then drift away. However, I once again thought more about the low pressure area “pulling” rather than the high pressure area “pushing.” Actually, since blowing between the two balloons caused an area of low pressure, the relatively high pressure around them pushed them together to create the “kissing” of the balloons.
I know air pressure exists because, even if it's unseen, it exerts force on many things around us. In both of the above experiments, I saw how areas of lower air pressure make us realize the impact of the higher, normal air pressure that is all around of us.
Air Pressure Reading
In the article we read this week, yet more evidence was presented that proves that gases exert pressure. First, there was the example of the tires that hold up our cars. As the article stated, “the pressure exerted by the air inside the tires is enough to hold up a car weighing 2000 pounds.” It’s not the rubber of the tires holding up those 2000 pounds - if that was the case, the tires would collapse and the car would be stuck a few inches off the ground. Rather it is the constant pushing of the air inside the tire outwards that provides the force necessary to hold the car. The second piece of evidence is the fact that you can’t blow up a balloon inside a bottle. If gasses couldn’t exert pressure, you would be able to blow it up, because the air trapped inside would offer no resistance. However, you can’t blow it up, so the air must exert pressure. Finally, when you place a crumpled piece of paper in a plastic cup and turn it upside down, submerging it in the water, the paper stays dry. This is because the air trapped inside the cup exerts pressure on the water, keeping it from touching the paper.
These examples - air pushing the walls of a tire, a paper cup, and bottle - all additionally show that gas molecules exert pressure on the walls of whatever container they are in. As well, in this week’s article, the author wrote that “gases exert pressure on every surface they come in contact with.” This is especially true when gases are trapped in containers, like tires, cups, or bottles.
Air pressure is caused by the weight of the air around us. While individual air molecules are very light, the collective mass of all the air molecules in our atmosphere pressing down becomes a force to be reckoned with. It doesn’t always feel like pressure to us, because we’re used to constantly feeling its weight, but it does exist, and as we saw with our experiments, can definitely affect our everyday life.
