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Robert Krampf's Experiment of the Week |
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This Week's Experiment - #187 Gasping at Straws |
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This week's experiment comes from a question sent in by a list member who |
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wanted to know how a drinking straw works. At first this seems to be a very |
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simple thing, but like most very simple things, the more you try to explain |
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it, the more complicated it gets. To explore this subject, you will need: |
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a glass of water |
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several drinking straws |
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a pin or needle |
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First, lets try using the straw in the usual way. Feel free to substitute |
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other liquids for the water. I used cold orange juice and found it |
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delightful. Place the straw into the liquid and drink a few swallows through |
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the straw. As you drink, pay attention to exactly what is happening. What |
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are you doing to cause the liquid to rise up the straw into your mouth? |
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To help you see what is happening, lets try it again, with one difference. |
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This time, use two straws. Place both straws in the liquid and try drinking |
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through both at once. It still works very well. Two straws work just about |
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as well as one. Now lets make another change. Leave one straw in the liquid |
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and hold the other one beside it, but on the outside of the glass. With one |
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straw in the liquid and the other in the air, try drinking with both straws. |
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Does it work as well this way? |
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No. In fact, it does not work at all. The only thing you get in your mouth |
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is air. Why? To understand the straw, we have to understand air pressure. |
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The air around you is pressing in on you with tremendous force. At sea level |
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(where I live), the air presses in on you with 14.7 pounds of pressure on |
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each square inch of your skin. It also presses on the glass and on the water |
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in the glass. |
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While the straw is just sitting in the glass. The air pressure is the same |
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inside and out. When you drink through the straw, you seal your lips against |
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the straw and then you use your lungs and mouth to reduce the air pressure in |
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your mouth. You do this by expanding your mouth or lungs. There is now more |
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space to contain the same amount of air, so the air pressure is less. |
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At this point, the air pressure in your mouth is lower than the air pressure |
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surrounding you. Your mouth is connected to the straw, so the air pressure |
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inside the straw is lowered as well. There is the same amount of pressure on |
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the outside, but there is less pressure on the inside. The outside pressure |
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is pushing the water up the straw harder than the inside pressure is pushing |
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it down, so the liquid moves upwards. |
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When you used the two straws, one in and one out of the liquid, the straw on |
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the outside of the glass allowed air to enter, equalizing the air pressure. |
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Since inside and outside are pushing equally, the liquid stays where it is. |
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You can see this in a different way by using a single straw and making a tiny |
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hole in it with a pin or needle. This also allows air to enter, preventing |
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the straw from working properly. If you make the hole very small, someone |
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might try using the straw without realizing why it did not work, but of |
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course you would never do that, would you? |
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send $9.95, plus $3.00 postage and handling to: |
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Robert Krampf |
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PO Box 60982 |
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Jacksonville, FL 32236-0982 |
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Include your e-mail address and I will notify you when I get your order. |
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If you would like to be on the list, just send me an e-mail at [email protected], asking to be added to the Experiment of the Week List. |
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Check out our web site at: |
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http://www.krampf.com |
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From Robert Krampf's Science Education Company |
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4850 A1A South |
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St. Augustine, FL 32084 |
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(904) 471-4578 |
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