Teaching Physics with the Physics Suite

Edward F. Redish

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Changes in gas molecules

An ideal gas is contained in an airtight box. Complete each of the following five statements below to show the quantitative change that will occur. For example, if you want to say that the volume, initially equal to V, quadruples, complete the statement with "4V".

  1. If the absolute temperature of the gas is halved, the average speed of a gas molecule, v0, becomes _____.
  2. If the average speed of a gas molecule doubles, the pressure, p, on a the wall of the box becomes _____.
  3. If the absolute temperature of the gas is halved, the pressure, p, on a wall of the box becomes _____.
  4. If the absolute temperature of the gas is increased by 25%, the total internal energy of the gas, U, becomes _____.
  5. If the number of gas molecules inside the box is doubled, but the temperature is kept the same, the pressure, p, on the wall of the box becomes _____.

Note to the instructor: I have been very surprised by how difficult many students find this problem, even when presented with the kinetic theory equation relating the pressure, density, and average velocity of gas molecules. As pointed out by Arons, functional dependence -- when it's not linear -- is a non-trivial concept for many students [Arons 1990].

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Page last modified October 30, 2002: M14