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Homework Help: Help Quick

  1. Dec 5, 2005 #1
    Help Quick!!

    ok so i made a cheap thermometer this the manual to make the thermometer

    1. Tap water
    2. Rubbing alcohol (do not drink this)
    3. Clear, narrow-necked plastic bottle (11-ounce water bottles work well)
    4. Food coloring
    5. Clear plastic drinking straw
    6. Modeling clay

    and this is my procedure

    Pour equal parts of tap water and rubbing alcohol into the bottle, filling about 1/8 to a 1/4 of the bottle.
    Add a couple of drops of food coloring and mix.
    Put the straw in the bottle, but don't let the straw touch the bottom (DO NOT DRINK THE MIXTURE).
    Use the modeling clay to seal the neck of the bottle, so the straw stays in place.
    Now hold your hands on the bottle and watch what happens to the mixture in the bottle.

    So what i used was food coloring ( for change of color), isopropyl rubbing compound ( for rubbing alcohol), normal modeling clay to seal the top, a normal straw and normal water out of the sink with a normal 11oz pop plastic bottle

    For the 5-7 first days it worked but now it doesnt!! so when i make the ''thermometer'' the food coloring does not go up the straw!!!, i tried it in front of my over, my heat and on the refrigerator nothing happens!! it stays in the same place as the first time the substance is suppose to go up the straw if its hot or what not but nothing happens and i don't know what im doing wrong because i placed the right measurements for alchool and water :S!!

    Help this is due on wednesday and i got a poster i also gotta do for it
    Last edited: Dec 6, 2005
  2. jcsd
  3. Dec 5, 2005 #2


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    I presume you want to take advantage of the linear expansion of liquid (alcohol). The funny thing is, and I laughed for a good 30 seconds, is that you want to push a column of air with a force so pathetically miniscule!

    You need a much smaller diameter straw, and you have to suck the air out of it to create vacuum. The diameter of the wall as well as equally spaced out markings will give you the volume of each division.

    By using the linear expansion formula you can calculate how much volume a given amount of temperature change will create. I dont have a slight clue why you would even use WATER, but I suppose its your call.

    Coefficient of expansion
    grain alcohol 1.12 E-3/° C
    water 2.1 E–5/°C

    Edit: I think you are trying to recreate http://www.energyquest.ca.gov/projects/thermometer.html -- I dont think they ever got it to work.
  4. Dec 5, 2005 #3
    yes im doing that one - but it seems that there's no much math or science involved in that , so i don't understand it now that you explained it to me, i'm really lost - grade 10 and not as smart as 98% of the people in this forums

    P.s you edited your post - are you serious man, i cannot believe this is happening to me all this time wasted for nothing.
    Last edited: Dec 5, 2005
  5. Dec 5, 2005 #4


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    Change in volume = (coefficient) * (Initial Volume) * (change in Temperature)

    So if you want to create a thermometer that will measure the difference when a human touches the jar (for example you are 36.6 deg C) - and you want it to work at room temperature (around 20 deg C), then you have your (change of temperature) = 16.6

    Coefficient of expansion for grain alcohol is 0.00112 per deg C

    Therefore, change in Volume = 0.00112 * 20 * 16.6 = 0.37184 meters^3

    If you have a drinking straw, its diameter is 6 mm. The volume is pi*r^2 * height = 9.424 * height mm^3

    So with volume of 371.84 mm^3 = 9.424 * height mm^3, your height = 39 mm = 3.9 cm

    Just keep in mind to have your bottle sealed on top VERY well as any leaking air will prevent it expanding into the straw. There is also the issue of atmospheric air pressure (14.7 lbs/in^2) so I dont know how much of that 3.9 cm that will cut down

    Edit: dont use water because that will cut down the amount of volume displaced significantly. Especially at the mixture concentrations you want to do.
    Last edited: Dec 5, 2005
  6. Dec 5, 2005 #5
    i'm sorry but i really don't understand what you're telling me, first you said it never worked then don't let air go in :S it's really hard to understand i mean is it gonna work or not :S?
  7. Dec 5, 2005 #6


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    It will work if you seal the bottle very well and make sure there is some glue or bubble gum between straw and hole on top - so that no air can escape. The 3.9 cm number will vary for your case, because there is air to be pushed on top of the straw, so if you want you should suck the air out of the straw and seal it on top. Make sure you stick the straw deep enough into the alcohol-color liquid but let some space on bottom for liquid to flow in.

    Sorry for late reply, I thought I hit reply button an hour or so ago :blushing:
  8. Dec 6, 2005 #7
    np thx for replying back - well i left it well sealed and nothing still happens to it, a bit mad, but im ok they'll take points off the function thats it :S
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