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Homework Help: [thermo] pressure & steam tables related question

  1. May 9, 2010 #1
    you want to boil a pot of water at 110 C
    diameter of pot = 15cm
    atmospheric pressure 101 kPa

    what weight lid should you use?

    i looked up the steam tables and water boils at 110 C at 143.27 kPa

    so do i use that pressure or 143.27-101? (gauge pressure)

    i used gauge and got something like ~76.14 kg.. which seems ridiculous for a lid weight

    formula i used:

    force = mass x 9.81
    force = pressure * area
    mass x 9.81 = pressure * area

    similar question but area = 5mm^2, atmospheric pressure = 101.3 kPa, and you want it to boil at 120 C

    i got an answer of 0.101kg using absolute pressure and 0.049kg using gauge pressure

    could you verify my answer and tell me which pressure i should be using.. absolute pressure or gauge pressure
  2. jcsd
  3. May 9, 2010 #2

    jack action

    User Avatar
    Science Advisor
    Gold Member

    if you think about it, the force equilibrium is:

    PinA = Wlid + PatmA


    (Pin-Patm)A = Wlid
    PgaugeA = Wlid
  4. May 10, 2010 #3
    oh thanks you.. force equlibrium is a better way of looking at it :)
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