Thermal Physics
• Too many particles… can’t keep track!
• Use pressure (p) and volume (V) instead.
• Temperature (T) measures the tendency of an object to
spontaneously give up/absorb energy to/from its
surroundings. (p and T will turn out to be related to the
too many particles mentioned above)
• p, V, and T are related by the equation of state: f(p,V,T) = 0
e.g. pV = NkBT
• Heat is energy in transit and it is somehow related to
temperature
Zeroth law of thermodynamics
A
C
If two systems are separately
in thermal equilibrium with a
third system, they are in
thermal equilibrium with each
other.
Diathermal
wall
B
C
C can be considered the
thermometer. If C is at a
certain temperature then A
and B are also at the same
temperature.
Temperature scales
• Assign arbitrary numbers to two convenient temperatures such as
melting and boiling points of water. 0 and 100 for the celsius scale.
• Take a certain property of a material and say that it varies linearly
with temperature.
X = aT + b
• For a gas thermometer:
P = aT + b
• X can also be resistance, capacitance, etc.
• Temperature is related to heat and somehow
related to the motion of particles
• Need an absolute definition of temperature based
on fundamental physics
• A purely thermal physics definition is based on the
Carnot engine
• Can also be defined by statistical arguments
Most likely macrostate the system will find
itself in is the one with the maximum
number of microstates.
E1
E2
1(E1)
(E)
2(E2)
1
d ln 1 d ln  2


dE1
dE2
k BT