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Concept in thermodynamic

  1. Nov 7, 2007 #1
    1.
    One question ask
    "Which statement below regarding the First Law of Thermodynamics is best? Select the correct answer."
    One of the wrong answer is
    "A system can do work externally only if its internal energy decreases. "
    What does it mean do work externally?

    2.
    Another question ask
    "Which of the following statements is true? Select the correct answer."
    One of the wrong answer is
    "You only need to know the amount of thermal energy a body contains to calculate its temperature. "
    How to correct this sentence?

    3.
    For definition STP,
    the tempature is 273.15k,
    but what is the pressure in the unit of pa and atm?
     
  2. jcsd
  3. Nov 7, 2007 #2
    If you have a gas in a cylinder with a piston, and you compress that gas wit the piston, you are doing work on the gas.

    If you attempt to hold that piston down and ignite the gas (assume it's a mixture of air a fuel), then the gas is doing work on the piston (externally).

    If the gas, despite being ignited, is not able to push the piston up, then all the work done is actually 0, yet the energy has to go somewhere. The energy remains in the gas as heat. The work done by the gas is internal and it heated up.

    What are the other answers?

    1 atm = 101325 Pa.
     
  4. Nov 7, 2007 #3
    Thanks,
    a. is correct, other is wrong

    a.The quantity of thermal energy exchanged by two bodies in contact is directly proportional to the difference in their temperatures.
    b.You only need to know the amount of thermal energy a body contains to calculate its temperature.
    c.The temperature of a body is directly proportional to the amount of work the body has performed.
    d.The quantity of thermal energy exchanged by two bodies in contact is inversely proportional to the difference in their temperatures.
    e.Different amounts of thermal energy are transferred between two bodies in contact if different temperature scales are used to measure the temperature difference between the bodies.
     
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