- This is a cross section
of a cubical box.
- The number of molecules is N = 32
and say the length of a box side is L = 3. So the
number density
      n = N/V = N/L**3 = 32/27 ≅ 1.2 .
-
The macroscopic effect of the collisions
of the molecules
is
pressure---which is
force per unit area.
Pressure
in solids and
liquids is different.
It originates in the resistance to compression of closely packed
atoms
and molecules.
- Temperature, among other things,
is a measure of the kinetic energy
of the molecules.
The lower temperature,
the lower the kinetic energy
of the molecules.
- The
classical physics
formula for
kinetic energy is
    KE = (1/2)mv**2     .
The classical physics
absolute zero is when
all the kinetic energies
of the microscopic
particles
are zero: i.e., all the
velocities
are zero.
The classical physics
absolute zero
(temperature) is
the coldest since you CANNOT have less motion than none.
- The above statement about absolute zero
has to be modified for
quantum mechanics.
In quantum mechanics,
there is a irremovable amount of
kinetic energy
called the zero-point energy.
In quantum mechanics,
absolute zero
(i.e., the coldest) is when
you reach the zero-point energy.
- The Kelvin scale
for temperature
has its zero point
at absolute zero
which makes the Kelvin scale
the most useful temperature scale
for many formulae and calculations.
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