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Find the tension in the wire - Heat and thermodynamics

  1. Apr 8, 2013 #1
    consider a horizontal cylindrical tube of cross sectional area A fitted with two frictionless pistons. The piston are connected to each other by an inextensible wire. Initially, the temperature of the gas is To and its pressure is Po which equals the atmospheric pressure. Find the tension in the wire if the temperature is raised to 3To.

    my attempt..
    since the pressure is constant..
    what i tried to do was assume the volume to be constant..
    so if the volume was constant, pressure developed would be..
    (P1)V = nRTo
    (P2)(V) = nR(3To)
    or, P2 = 3P1
    so, the force on each cross section = (3Po.A)
    so this must be the tension in the wire..
    but the answer says its (2Po.A).. which definitely needs to be correct..
    so where m i wrong??? please suggest.
    Thanks in advance..!!
     
  2. jcsd
  3. Apr 8, 2013 #2

    DrClaude

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    Staff: Mentor

    What about the atmosphere?
     
  4. Apr 8, 2013 #3
    The standard atmosphere (symbol: atm) is a unit of pressure equal to 101.325 kPa
     
  5. Apr 9, 2013 #4

    DrClaude

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    Staff: Mentor

    I mean: how does the atmosphere affect the tension?
     
  6. Apr 9, 2013 #5
    Do a force balance on one of the pistons and see what you get. The forces acting on the piston are the pressure force on the inside face of the piston, the pressure force of the atmosphere on the outside face of the piston, and the wire tension.
     
  7. Apr 9, 2013 #6
    P'A = PoA + T

    Now when the temperature of the system is increased to 3To and if we assume the volume to be constant..
    pressure developed,
    PoV = nRTo and P'V = nR(3To)
    => P' = 3PoA

    thus, T = 3PoA - PoA = 2PoA

    @Chestermiller - Thanks a lot sir..!!
     
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