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The Chemistry of Physics

  1. Nov 24, 2004 #1
    Hello Everyone. Happy Turkey Day! Even though its a holiday the studies continue. I have moved on to my next on-line module that discusses the physics behind some chemistry concepts. I have posted and answered my self-assessment questions (answer marked with astericks). Please review and let me know if I have missed any. Remember, I do not expect the correct answer but instead just a hint on a different way to approach the problem. I will provide my thought process on those that were incorrect. Thanks.

    1) If 1 gram of hydrogen combines completely with 8 grams of oxygen to form water, how many grams of hydrogen does it take to combine completely with 32 grams of oxygen?
    a) 2
    b) 3
    c) 4*
    d) 5

    2) Avogadro suggested that each liter of gas under identical conditions has the same
    a) mass
    b) number of atoms
    c) number of molecules*
    d) density

    3) Two students are sick in bed with fevers. Bill has a temperature of 2° C above normal, while Sally's is 2° F above normal. Which student has the higher fever?
    a) Bill*
    b) Sally
    c) They have the same fevers

    4) The pressure in a container filled with gas increases when it is heated because
    a) The walls do work on the gas
    b) The average momentum of each gas particle increases in size*
    c) The number of gas particles increases
    d) The volume of the gas decreases

    5) If you hold the temperature of an ideal gas constant, what happens to its volume if you double its pressure? The volume
    a) Quadruples
    b) Doubles
    c) Is cut in half
    d) Is cut one-fourth*

    6) A kilogram of metal A has a volume 20% larger than a kilogram of metal B. Which metal has the smaller density?
    a) Metal A*
    b) Metal B
    c) They have the same density

    7) Which of the following effects is NOT a result of surface tension?
    a) A glass can be filled with milk beyond its brim
    b) Liquids floating in the Space Shuttle form into spheres
    c) The pressure increase with the depth in a fluid*
    d) Soap films on wire frames have minimal surface areas

    8) Two barometers are made with water and oil. The density of oil is 900 kg/m3 and that of water is 1000 kg/m3. If the oil column is 10 m tall, how tall is the water column?
    a) 11.1 m
    b) 10.0 m
    c) 9.0 m*
    d) 8.1 m

    9) You have two cubes of the same size, one made of aluminum and the other of lead. Both cubes are allowed to sink to the bottom of a water-filled aquarium. Which cube, if either, experiences the greater buoyant force?
    a) The aluminum cube*
    b) The lead cube
    c) Both cubes experience the same buoyant force

    10) Which of the following is NOT due to the Bernoulli effect?
    a) Raised tarpaulins on moving trucks
    b) The curve of a baseball
    c) The lift produced on airplane wings
    d) The floating of icebergs
     
  2. jcsd
  3. Nov 24, 2004 #2
    Use the ideal gas equation, PV = nRT. You can see that V = nRT/P and that if P is doubled, V will halve.
     
  4. Nov 24, 2004 #3

    Doc Al

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    Staff: Mentor

    Your answers to #5 and 9 are incorrect. #10 is missing an answer. (The others are correct.)
     
    Last edited: Nov 24, 2004
  5. Nov 24, 2004 #4
    9: Find the equation for a bouyant force, does it speak of the object's density?
    10: What does bernoulli's effect/equation say? And in which element is it most often applied?
     
  6. Nov 24, 2004 #5
    I think 4 is alright, cos as you increase the temperature of the gas, the average KE (and hence momentum) increases, force is dp/dt and pressure = force/area, so an increase in momentum causes an increase in pressure for the same area (I guess the area of the wall of the container). At least, I think this is the right way to think about it :/.
     
  7. Nov 24, 2004 #6
    Hey Doc Al. Nylex gave me the ideal gas equation and I completely see that by doubling the pressure you half the volume. What do the variables P,V R and T as well as the constant n represent in the ideal gas equation? As for question #9. The tutorial did not give any equations for a bouyancy force. When I read the question I assumed that the bouyant force was a force associated with floating objects and also related to density. If the density of the object is less than the medium that it is placed in it will float. Aluminum is less dense than lead so it had the greater possibility to float so that is why I selected that answer. I guess I am not ready to work for CERN yet am I? As for question #10 - Bernoulli effect has to do with internal pressure as they relate to stream velocity in a fluid. Not quite sure what that means and not sure which response in #10 is NOT due to this effect. The icebergs floating are in a fluid but the baseball, the tarpaulins and the airplane wing seem to be more related to an pressure differences and object movements. I think that (d) the floating if icebergs is the correct answer. Icebergs floating are due to the expansion of the water (larger volume) with the same mass thus less density than liquid water. That is what causes the icebergs to float. I so not see any tie into the Bernoulli effect. Thanks to everyone for your help.

    PS - I just realized that my Happy Turkey Day does not apply to all. So just have a fantastic day everyone.
     
  8. Nov 24, 2004 #7
    Doc Al. One more comment. I read a little bit about bouyancy and fluid displacement. Does this tie into question #9?
     
  9. Nov 24, 2004 #8
    P is pressure (Pa), V is volume (L), T is temperature (must be in Kelvin), n is the amount of gas in moles, and R is the universal gas constant, 8.314 (figure out the units from the rest).

    http://en.wikipedia.org/wiki/Universal_gas_equation
     
  10. Nov 24, 2004 #9

    Doc Al

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    Staff: Mentor

    Don't you have a textbook? Look here: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html#c1
    If your tutorial doesn't discuss buoyant force, why is there a question on it? The buoyant force is the net upward force that a fluid exerts on a submerged object (due to pressure differences in the fluid); Archimedes's principle says that the buoyant force equals the weight of the displaced fluid. Since both cubes displace the same volume of fluid, they experience the same buoyant force.
    Right. All the other answers have to do with pressure differences in moving fluids, which is what the "Bernoulli effect" deals with. The floating iceberg is an example of buoyancy due to the density of the ice being less than that of water.
    Absolutely.
     
  11. Nov 24, 2004 #10
    Thanks again for all of your assistance. I think that I understand most of these concepts. I will continue to review before proceeding further. Also, there is no text with this on-line course. However, there is a workbook to write notes and solve some problems. My statement to you was that the module did not give an equation for the bouyant force that ponjavic mentioned in thread #4. It certainly discussed buoyancy, displacement and Arcamede's Principle. Have a good day and talk with you again soon. Tks.
     
  12. Nov 24, 2004 #11
    I apologize a bit physicsnovice, I was just blasting out ideas, in Sweden we also call it archimedes principle but since you mentioned bouyancy I just guessed that there was an equation for bouyancy, and when it comes to question 10 you solved it quite neatly, bernoulli's equation speaks of exactly what you mentioned, you can think that all of the; plane, baseball, tarpaulins are cutting through the air while the iceberg is floating. I would recommend just taking a peek at how planes fly at www.howstuffworks.com if you are interested in fluids, it's a quite fundamental thing to know and can be understood without dwelving too deeply into bernoulli's equation.
    As I am still in high school maybe I shouldn't even be trying to help as I might only confuse but I hope you got it together ;)
     
  13. Nov 30, 2004 #12
    hey, i'm fairly new to the board and came across Novice's questions and realized he is pretty much up to the same part of his physics course as i am, so we have similar HW questions...
    i have a few similar questions, could u help me out in the same way, like w/ a lil explanation of why its wrong/right, etc? I'd appreciate it since i'm barely getting some concepts :rolleyes:

    1.) Imagine your in a fluid at rest, in which direction can you move without the pressure changing?
    a.- horizontal
    b.- down
    c.- up
    d.- pressure doesnt change as you move in any of these directions*

    2.) Water barometer reads 10m today, whats the pressure at the bottom of a 20m deep pool?
    a.- 4atm
    b.- 3atm
    c.- 2atm
    d.- 1atm

    ^this one i have right at the tip of my tongue, but i'm not sure about this one part. is there a specific formula you would use for this? i think i'm just skipping right past it without realizing while i read my textbook...
    this is probyably my easiest question in a while and i dunno why its bothering me so much... :redface:
     
    Last edited: Nov 30, 2004
  14. Nov 30, 2004 #13
    oh sorry, just wanted to say thanks to any help you guys got...
    i'm a physics amateur so i appreciate it
     
  15. Nov 30, 2004 #14

    Doc Al

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    Incorrect. Is the pressure 1 foot below the ocean surface equal to the pressure 1000 feet below?

    What does a barometer measure?
     
  16. Nov 30, 2004 #15
    ooo, i'm an idiot, the answers horizontal, because pressure doesnt change left to right, but changes vertically...wow, i think THAT was the easiest one...

    anyways, for the second one, a barometer measures pressure in the atmosphere or air pressure
     
  17. Nov 30, 2004 #16

    Doc Al

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    Staff: Mentor

    Right.
    Right! So now you know that atmospheric pressure equals the pressure of 10 m of water. So... what's the total pressure at the bottom of that pool?
     
  18. Nov 30, 2004 #17
    oooo, so thats like 1atmophere = 10m of water?
    so at 20m deep, the answer should b 2atm
     
  19. Nov 30, 2004 #18

    Doc Al

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    Staff: Mentor

    Exactly.
    The pressure from 20m of water equals 2 atm. But don't forget to add the pressure of the atmosphere itself: Total pressure at a depth of 20m = 3 atm.
     
  20. Nov 30, 2004 #19
    oooo, i got it... :rolleyes: its always so annoying how easy the answers are sometimes, just need to use a little more logic.
    THANKS ALOT,Doc! I'm glad I joined when i did. Its helped
     
    Last edited: Nov 30, 2004
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