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Thermodynamics Qn! Please help

  1. Nov 12, 2007 #1
    Thermodynamics Qn! Please help!!!!

    An ideal gas at 300K and 1.5X10^5Pa is contained in a piston cylinder device. The piston has a diameter of 0.15m and is allowed to move freely. Heat is transferred to the cylinder causing the piston to move a distance of 0.05m. The gas specific heat at constant volume is 718 J/kg K and the gas constant is 287 J/kg K.
    (a) Calculate the work done by the gas and the gas pressure at the end of the process?
    (b) Calculate the final gas temp. and the amount of heat transferred to the gas during the whole process.

    I am having problems with this question. Please help me out with. Thanks a lot!
     
  2. jcsd
  3. Nov 12, 2007 #2
    where were you stuck at?
     
  4. Nov 12, 2007 #3
    I finished ques a but am stuck for ques b, there is no information for the mass of the gas and I do not know how to find the mass of the gas also. Thanks.
     
  5. Nov 14, 2007 #4
    I still need help! Please! Thanks.
     
  6. Nov 14, 2007 #5

    Andrew Mason

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    You can determine the change in volume. You can determine the heat flow. You know the initial pressure. You know that the piston is stationary to begin with, ie at that pressure. So you can determine the external pressure. Does the external pressure change? So what must the internal pressure be when the piston stops moving?

    You can calculate the work done by the piston in expanding that amount against that pressure.

    What is the change in internal energy of the gas? How is that related to temperature? What is the relationship between volume, pressure and temperature?

    AM
     
  7. Oct 9, 2011 #6
    Re: Thermodynamics Qn! Please help!!!!

    I have the exact same question.

    Anyone could help me tackle this question.
    I know that the

    heat supplied to system=work done by gas+ increase in internal energy of the gas molecules.

    But how do i find out the internal energy of the molecules when given specific heat only?

    May i also know what does the gas constant mentioned in the question refer to? does it refer to the value of R?
     
  8. Oct 9, 2011 #7

    Andrew Mason

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    Re: Thermodynamics Qn! Please help!!!!

    What is the relationship between internal energy and temperature? Can you determine the final temperature (hint: how is T related to P and V?).

    The gas constant here is the constant for this particular gas when using PV = mRT where m is the mass of the gas. It depends on the molecular weight of the gas.

    R, the universal gas constant, is in units of J/mol K. How would you convert to J/Kg K?

    AM
     
  9. Oct 9, 2011 #8
    Re: Thermodynamics Qn! Please help!!!!

    I got an incredbily large answer.

    I tried using gas constant/R to get kg/mol.

    then i found n using pV=nRT

    after that i used mc(change in T) to find Q.
     
  10. Oct 9, 2011 #9
    Re: Thermodynamics Qn! Please help!!!!

    My bad. A calculation error on my side. Thanks for the help!!
     
  11. Nov 19, 2011 #10
    Re: Thermodynamics Qn! Please help!!!!

    for the last part, by conservation of energy, we know that the heat transferred to the gas will be used to increase the temperature of the particles which is calculated by mcT or it will be used to do work.
    We've gotten the first part which is work done -132.53 and we need to add the mcT to get the final answer.
     
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