 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
pressurewhich 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 can't 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 zeropoint energy.
In quantum mechanics,
absolute zero
(i.e., the coldest) is when
you reach the zeropoint 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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