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Homework Help: Activated speed

  1. Oct 6, 2005 #1
    Hi to you all.i have an exercise i cannot solve.i think something is missing.

    Cylindrical tube V=10L opened at the top has air with a temperature of T1=27 oC.We provide heat to the tube until the temperature reaches the value of T2=327 oC.A)we have to find the amount of air that leaves from the tube .
    b)if we close at the top the tube and decrease the temperature to T3=127 oCwe have to find the activated speed.
    c)what is the new number of particles.

    i guess at the first question it requires the V of the air that leaves?I think that the pressure is steady as it is opened at the top the tube.so the pressure equals the pressure of the surrounding air.

    How can i find the activated speed without knowing the consistency?i will assume it is one as we are talking about air?

  2. jcsd
  3. Oct 6, 2005 #2
    i was thinking about the above problem for the first question to simply calculate the new V of the air assuming that the pressure is constant before and after the heating and that the pressure equals i atm same as the one existing in the atmosphere.then if i use that the consistency of air is one i can calculate the mass of the air that kleaves the tube.

    For the second question i assume that the air of 10L left in the tube in temperature 600K and with pressure of 1 atm , after the cooling of the tube we have again the same V as the tube is closed the pressure changes because of the coolingand also the temperature is now 400K.Assuming that the V is constant throught the change i can calculate with the use of PV=nRT the new pressure.And from there assuming that the consistency of the air is 1 kgr/m3 and with the use of equation p= 1/3*d*(activated speed )*(activated speed) i can calculate the activated speed.

    For the third i use the PV=nRT to find the number of moles .and then with the use of the number of avogadro i find the number of molecules.

    am i right?
  4. Oct 7, 2005 #3
    please give me a hint!!!!!!!!!i need to have it solved until tomorrow morning.please help
  5. Oct 7, 2005 #4


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    PV= NRT- the ideal gas law.

    Since the tube is open, the pressure remains the same. Since you are dealing with a rigid tube, the volume remains the same. The temperature is changing a known amount and the only other thing that can change is N (NOT V!). Dividing PV= N1RT1 by PV= N2RT2 you get 1= N1T2/N2T1. Be sure that you put T in degrees KELVIN!

    For (b), you close the tube so that both N and V remain the same while T changes. Now the only thing that can change is P.
  6. Oct 7, 2005 #5
    thanks for your help.one question.at the first question when you say that the N changes what do you mean?how can the amount of moles inside the tube change because of the heating of the tube?
  7. Oct 7, 2005 #6
    in adition for the activated speed i have to know the pressure and the concentration of the air in order to calculate it?
  8. Oct 7, 2005 #7


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    Because some of the molecules go out of the tube! That was the question you were asked wasn't it?
  9. Oct 7, 2005 #8


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    Sorry, I have no idea what "activated speed" is!
  10. Oct 7, 2005 #9
    sorry but my english are not so good.does the term effective speed helps?or else i want to find the square root of the average values of speed square.i am not sure how is ti called in english.
  11. Oct 7, 2005 #10
    my problem is that for the second question i calculate the new pressure of the air inside the tube but since i do not know the consistency of the air at that temperature how can i find the effective speed.?(hope it is the correct term)

    And also at the first question i do not know the starting amount of moles of the air inside the tube.so i guess i cannot find the number of moles that have left the tube just an expression that includes the N1
  12. Oct 7, 2005 #11


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    You have the volume of the tube, and (although it doesn't say) I think we can assume standard pressure. The Ideal Gas Law will let you calculate the number of moles from that. The same calculation with the increased temperature will give you the new N, and you can get the change from that.
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