Thermal Physics
• Too many particles… can’t keep track!
• Use pressure (p) and volume (V) instead.
Pressure, Volume, Temperature
P, V, T
F/A
L3
Something to
do with heat
Before figuring out what is temperature… What is heat?
Heat is energy in transit
Surroundings
System
Universe
(system +
surroundings)
Empirical relation between temperature and heat
Heat capacity
dQ
C
dT
 Q 
CV  

 T V
 Q 
CP  

 T  P
Specific heat capacity (c) is the
heat capacity per unit mass.
Equations of state
•An equation of state is a mathematical relation between state
variables, e.g. p, V & T.
•This reduces the number of independent variables to two.
General form: f (p,V,T) = 0
Example:
pV – nRT = 0
(ideal gas law)
•Defines a 2D surface in p-V-T state space.
•Each point on this surface represents an unique state of the system.
f (p,V,T) = 0
Equilibrium state
State variables
•Extensive property:  mass
(e.g. V)
•Intensive property: not a function of mass (e.g. T, p, )
•Specific value:
Extensive property
Mass
Heat is NOT a state variable
Ideal gas equation of state
Boyle’s law
p  1/V
Robert Boyle (1627 – 1691)
Charles’ law
pV = NkB T
VT
Jacques Charles (1746 – 1823)
Gay-Lussac’ law
pT
Joseph Louis Gay-Lussac (1778 - 1850)
kB = 1.38  10-23 J/K
What is temperature?
Temperature is what you measure with a thermometer
Temperature is the thing that’s the same for two
objects, after they’ve been in contact long enough.
Long enough so that the two objects are in thermal
equilibrium.
Time required to reach thermal equilibrium is the
relaxation time.
Temperature is a measure of the
tendency of an object to spontaneously
give up/absorb energy to/from its
surroundings.
Diathermal wall is a boundary that freely allows
heat to be exchanged i.e. very short relaxation
time for systems separated by a diathermal wall.
T 1- T 2
T 1- T 2
Adiabatic wall
time
time
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
Platinum resistance thermometer
150
R ()
100
50
0
0
50
100
150
200
T (K)
250
300
350
400
CERNOX thermometer
R ( )
10000
1000
100
0
100
200
T (K)
300
400
Thermometer needs to have a much lower heat capacity
than the sample
T
T0 + T
T0 + T
T0
Heater
T0
Sample
Thermometer
Relaxation calorimetry
t
Heat capacity
dQ
C
dT
 Q 
CV  

 T V
Specific heat capacity (c) is the
heat capacity per unit mass.
 Q 
CP  

 T  P
What is heat doing at the microscopic level to change the temperature?