TP: Measured Properties

Measured Properties

  • Amount: Volume $\rightarrow$ V [=] LxLxL extensive
    Mass/moles $\rightarrow$ M [=] M or N [=] moles extensive
    Intensive "volume" (often called specific volume,)
    $\hat v = V/M$ or $v = V/N$
  • Temperature: Always intensive. Refers to the degree of hotness. On a molecular scale it is a measure of the average kinetic energy of the molecules/atoms in a system (that is, $E_k$ = (3/2) kT). Obviously, because it is an average, there will be a distribution of kinetic energies of the molecules. The distribution that is observed in gases is called the Maxwell-Boltzmann distribution and is how one determines when there are enough gas molecules in a sample to consider the continuum hypothesis (i.e., a macroscopic description of the material) to be valid.
  • Pressure: Always intensive. Refers to the normal force per unit area exerted on a "surface". Pressure itself has no direction (it is a scalar quantity), but it can obtain a direction from the surface. The pressure in a continuous medium is the force exerted on a hypothetical surface. From a molecular viewpoint, this force is exerted due to the exchange of momentum between molecules and the surface during collisions with that surface (note that the units of force is the rate of change of momentum).

Converting between temperature scales:

  • T(K) = T(C) + 273.15
  • T(R) = T(F) + 459.67
  • T(R) = 1.8*T(K)
  • T(F) = 1.8*T(C) + 32
note:

Converting temperature differences is slightly different (the additive corrections cancel out!).

  • Amount: Volume $\rightarrow$ V [=] LxLxL extensive
    Mass/moles $\rightarrow$ M [=] M or N [=] moles extensive
    Intensive "volume" (often called specific volume)
    $\hat v = V/M$ or $v = V/N$
  • Temperature: Always intensive. Degree of hotness.
    molecular scale $\rightarrow$ average kinetic energy ($E_k$ = (3/2) kT).
  • Pressure: Always intensive. F/A.
    molecular viewpoint $\rightarrow$ momentum exchange during molecular "collisions".
TEST YOURSELF:

Convert 20C to F. How about R? Convert -40C to F :)