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Does Helium Expands Faster than Air When Heated?

  1. Jul 17, 2010 #1
    This is a really confusing issue .. even when asked my professor in thermodynamics he tried to derive many theromdynamic equations and could not come to a conclusion about it

    if we have two cylinder piston systems ,, they both have the exact same conditions except the kind of gas in each one ,,, one has Helium as the working gas ,, the other one has air as the working gas

    both cylinder piston systems have the same initial vloume V1 and the same initial Pressure= 1 atm .. and the same initial Temperature= 25 C

    then we expose both systems to exactly the same source of heat with exact condition, and both systems starts to have volume expansion that drive the piston up ,, while keeping constant pressure.

    its known that Cp for Helium is about 5 times greater than Cp for air ,, and also the thermal conductivity of Helium is much greater than that of air.

    my question is : is the piston cylinder system that contains Helium going to expand FASTER than that which contains air? or they both are going to expand at the same speed?

    Thank You
  2. jcsd
  3. Jul 18, 2010 #2
    When you say "faster" I'm guessing that you mean change in volume per unit time.

    I don't think Cp for helium is 5 times greater than for air (probably 1.2 times or so) but Helium will probably expand faster because it will absorb energy faster from the reservoir. Rate of heat is quicker into the cylinder with helium, thus it supplies more energy per unit time.
  4. Jul 18, 2010 #3
    thank you for your reply ,, but based on what you have concluded that Helium will absorb energy faster than air?

    Note: yes, faster means: higher chanage in volume per unit time

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