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| Shared Idea |
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| Pressureh
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| Pressure is defined as a force applied to a unit area (P = F/A). A simple demonstration can make this concept much clearer. Have a volunteer come up and give him or her a normal pen or a sharpened pencil. Hold up a cardboard box and have the student hold the tip of the pen or pencil against the box while also placing a fist formed with the other hand against the same box. Hold the box firmly and direct the student to push against the box with both hands. The pen or pencil will pierce the box while the fist merely presses against it. Have the student release the pen or pencil and display it to the audience. The force applied with each hand was the same, but the areas were very different. This makes it very clear that the smaller area increased the pressure that was produced.
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| Pressure can be expressed in many forms. Standard atmospheric pressure, the normal pressure at sea level can be expressed as 1.00 atmosphere (atm), 14.7 pounds per square inch (psi), 101.3 kilopascals (kPa), 760 millimeters of mercury (mm Hg) or 760 torr, plus a variety of other units. (Torr is short for Torricelli. Evangelista Torricelli invented the barometer, which can measure atmospheric pressure.) To help make this clearer, have each student hold out a flat hand with the palm upward, which you display. Using the value of 14.7 psi, this means that over 150 pounds of air pressure is pushing down on each hand. Why do they not feel it? The answers may vary, but despite what is suggested, the air is pushing down. If a 150-pound block of metal was placed on their hand, they would certainly feel it. Why are their hands not being pushed down? If necessary, point at the region beneath your own hand. When someone finally comes up with the idea that equal air pressure is pushing up on their hands, ask why their hands are not being crushed. With some encouragement, someone might hopefully recognize that equal air pressure within our body is pushing outward!
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| Ask everyone to puff up their cheeks and then suck in their cheeks. Demonstrate to make sure that they understand. Then ask them to repeat that while keeping their mouths open. It cannot actually be done, as the limited muscles in the cheeks are only involved in chewing. To inflate or deflate the cheeks, the mouth has to be closed so the pressure inside the mouth can be changed. To help clarify this, do a simple demonstration on suction cups. Otto von Gueriche, a German scientist, invented the air pump and carried out the first demonstration on air pressure in 1654 in Magdeburg. He created two metal hemispheres that were 14 inches in diameter and used his air pump to extract the air within them, creating a vacuum. Two teams of eight horses, each set up on opposite sides of this sphere, were unable to separate them. To show the students, use two Westward suction cup lifters. Place them against one another and press the moveable arms to create the desired vacuum. Two students should find that they are unable to separate them unless air is readmitted between the cups!
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| Next, give each student a cup of water and two straws. Have them place one straw in the cup and take a sip. Then place the other outside and, using both at the same time, try to draw some water into their mouths. There is only a minimal attraction between the water molecules. To drink water, we need to decrease the pressure inside our mouth so that the atmospheric pressure will push water up the straw and into the mouth. The second straw interferes with that. As an additional demonstration, have a volunteer come up and take off his or her footwear and then sit inside a large plastic garbage bag. Give the student the end of a vacuum hose and hold it with a cupped hand so that it will not be able to suck in the bag or any clothing. With the second hand, hold the edge of the bag together next to his or her neck. Then turn on the vacuum. As the pressure within the bag decreases, the evidence of atmospheric pressure will become very evident as he or she experiences "vacuum packing." (Be sure to turn off the vacuum before the student experiences too much pressure!)
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| Take a hard plastic glass with a flat rim and fill it with water. Take an index card and place it on top of the glass. Place a hand flat on top of the card and, with the other hand, invert the glass over a sink or large container. The hand is holding the card in place, which is keeping the water in the glass. Carefully remove your hand and the students should be surprised to see the water staying in the cup. The atmospheric pressure pushing up on the card is greater than the force of water pushing down on the card. Atmospheric pressure would work if the cup was three, ten or twenty feet tall. Standard atmospheric pressure, at sea level, can actually support a column of water that is 33.9 feet tall!
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| Next, hold two flat sheets of paper at a visible height, parallel to the floor. They should each be a different color. Ask the students what will happen if you release both at the same time from the same height. If they think the first one will hit the floor first, they should hold up one finger. If they think the second should hit the ground first, they should hold up two fingers. If they think both will land at the same time, they should hold up three fingers. (I often ask those who do not respond if they think both will somehow remain in the air.) I then crumple one sheet into a small ball, which usually leads to noisy protests. I point out that all I had said was that I would drop both at the same time from the same height. Promising that I will not further change the form of either sheet, I let them change the number of fingers they are holding up. I then place the flat sheet on a thin hardcover book that is only slightly larger than the flat sheet and release both at the same time from the same height. They hit the floor at the same time. The balled-up sheet faced less air resistance while the book similarly pushed air out of the way. We don’t feel the air pressure, but it is very real.
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| Boyle's Law (P1V1 = P2V2), which is named after Robert Boyle, says that, if the temperature and quantity of a gas are constant, then the pressure and volume of a sample of gas are inversely proportional. When one increases, the other decreases. I learned a simple demonstration from Mr. Wizard (Don Herbert) in his television show back in the 1950s. Select a plastic or metal straw and a wooden rod (such as a chopstick) that fits into the straw. Now cover one end and force the other through a small potato, creating a plug in that end. Reverse the straw and repeat it in the other end. A sample of air is trapped between the two plugs. Place the wooden rod in one end and thrust it into the straw. The pressure of the air trapped in the straw increases as the volume decreases, and the potato plug will be forced out of the opposite end of the straw. This again adds to the conceptualization of pressure.
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| Another demonstration can be used to show how pressure plays a part in the Ideal Gas Law (PV = nRT). Take four plastic film canisters and place a small quantity of water (about one centimeter) in each. Take two Alka Seltzer tables and break each in half. Place a half tablet in each lid and then quickly invert each lid and attach it to the canister and step back. As the tablet reacts, it produces a gas. (In the equation, n is the number of moles of gas, referring to the quantity.) The volume and temperature do not change much, and R is a constant that regulates the units. As the quantity of gas increases, so does the pressure, until the force inside is enough to cause the lid to pop off, flying upward. (I learned to always do four, as one or two may not be a good enough seal to work.)
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| As a closing demonstration, take a Mason jar with a piece of screen held in place by the lid. Do not let them see the screen. Place the jar under a tap and fill it with water. Place an index card over the lid and invert it. I then ask the students what is keeping the water in the jar. The correct answer is my hand. Holding it over a sink or large container, I remove my hand and ask what is now keeping the water in the jar. The correct answer is atmospheric pressure. I then say my magic phrase and peel the card away. The audience is startled when the water still remains in the jar! I can wave my other hand over the jar and some water starts coming out, until I move that hand away, which I can repeat a few times. The audience wants to know how I am doing this magic. I pour the rest of the water out and then show that a piece of screen mesh is held in place by the lid of the Mason jar. Surface tension was now playing a part, as long as the lid was parallel to the floor. This can work as an entry to teaching about surface tension.
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| I like one T-shirt that I have which states: "Science is magic that works!"
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