Lecture 10/10/14

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Lecture 10/10/14
2014-12-12 01:02:26
BME 221

BME 221
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  1. What is Adiabatic expansion?
    Adiabatic expansion is a process that does not allow transfer of heat. The gas cools down whengas expands.
  2. To solve questions involving the ∆U in adiabatic expansions, the key thing is to approach the process in two steps. What are these steps
    first, an isothermal expansion (∆U = ∆H = 0); two, a constant volume cooling. This two-step only holds when the gas is ideal.
  3. How is adiabatic expansion affected by non-ideal gas?
    If this gas is non-ideal, the change in P is greater than if we assume the gas is ideal with the same initial and final states due to the interactions (attractions) between the gases.
  4. With the same initial states and final volume, the work done by the system is always less in adiabatic expansion than in isothermal expansion. Explain this.
    If a gas expands, its initial pressure must behigher than external pressure. With thermal isolation, all the energy for the gas to expand comesfrom the internal energy of gas. The gas will cool down and the pressure will decrease and eventuallyhinder its ability to expand. Under isothermal conditions, the works comes from a reservoirthat has infinite amount of thermal energy: every time that the gas expands a bit, the reservoirwill restore the energy of the system by transferring heat.
  5. What kinds of energy do real gases have in addition to the ones ideal gases have?
    Real gases have intermolecular potential energy in addition to the kinetic energy that ideal gases have.
  6. Modern windows have two layers of glass with Kr filled in, why?
    The main reason is that Kr has a higher molecular mass and the speed of the molecule is lower. Therefore, it is less likely that the kryptonmolecules will collide less often with the two sides of the window. Assuming the heat transfer for each collision is the same, the thermal insulation is better.
  7. The first kind of perpetual motion machine that rotates constantly while allows work to be drawn from it without any energy input, why is this not possible
    cannot exist due to the first law of thermodynamics (conservation of energy)