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Thermal & Statistical Physics
-Thermodynamics: is the study of the macroscopic
behaviour of physical systems under the influence of
exchange of work and heat with other systems or their
Thermodynamics can be defined as the science of energy.
Energy can be viewed as the ability to cause changes.
-thermodynamic states and properties
-energy, work, and heat
-with the laws of thermodynamics
- PV = k, a constant --- R Boyle
The 1st and 2nd laws of thermodynamics
-simultaneously in the 1850s
-W Rankine, R Clausius, and W Thomson (Lord Kelvin).
-Late 19th century -- molecular interpretation of thermodynamics
-bridge between macroscopic and microscopic properties of
-The statistical approach is to derive all macroscopic properties
(T, V, P, E, S, etc.) from the properties of moving constituent
particles and the interactions between them (including quantum
-The number of the elements can be very large --- impossible to
keep track of the behaviour of each element.
-A statistical property is a single measurement which gives a
physical picture of what is occurring in all the individual parts.
-Is the study of the interrelation of heat with chemical reactions or
with a physical change of state within the confines of the laws of
QUANTUM MECHANICS OF
ATOMS & MOLECULES
Large number of molecules
-System is the region of the universe under study.
-Surroundings - everything in the universe except the system.
-Boundary -separates the system with the remainder of the
-fixed, moveable, real, and imaginary
There are five dominant classes of systems:
1. Isolated Systems matter and energy may not cross the boundary.
2. Adiabatic Systems heat must not cross the boundary. 3. Diathermic Systems - heat may cross boundary. 4. Closed Systems matter may not cross the boundary. 5. Open Systems heat, work, and matter may cross the
Energy transfers between thermodynamic systems as the result of
a generalized force causing a generalized displacement conjugate
The most common are
Pressure-volume (mechanical parameters)
Temperature-entropy (thermal parameters)
Chemical potential-particle number (material parameters)
Two types: the meter and the reservoir.
A thermodynamic meter is any device which measures any
parameter of a thermodynamic system.
-The zeroth law --it is possible to measure temperature.
-An idealized thermometer--ideal gas at constant pressure.
-a barometer-constructed from a sample of an ideal gas held
at a constant temperature.
-a calorimeter is a device which is used to measure and
define the internal energy of a system.
A thermodynamic reservoir is a very large system does not alter
its state parameters when brought into contact with the test
The earth's atmosphere is an example of heat reservoir.
State - a system is at equilibrium under a given set of conditions
The state of the system can be described by a number of intensive
and extensive variables.
The properties of the system can be described by an equation of
state which specifies the relationship between these variables.
-the energetic evolution of a thermodynamic system proceeding
from an initial state to a final state.
The seven most common thermodynamic processes are;
1. An isobaris process occurs at constant pressure. 2. An isochoric process, or isometric/isovolumetric process,
occurs at constant volume.
3. An isothermal process occurs at a constant temperature. 4. An adiabatic process occurs without loss or gain of heat. 5. An isentropic process (reversible adiabatic process) occurs at
6. An isenthalpic process occurs at a constant enthalpy. Also known as a throttling process.
7. A steady state process occurs without a change in the internal energy of a system.
The laws of thermodynamics
Classical thermodynamics is based on the four laws of
Zeroth law of thermodynamics, stating that thermodynamic
equilibrium is an equivalence relation about temperature
and temperature scale.
First law of thermodynamics is about the conservation of
energy -- deals with macroscopic properties, work, energy,
Second law of thermodynamics is about entropy
Third law of thermodynamics is about absolute zero
temperature --the determination of entropy.
-the quantitative measure of the stored energy in the system.
The five most well known potentials are:
Internal energy U
Helmholtz free energy A + U TS
Enthalpy H = U + PV
Gibbs free energy G = U + PV TS
Potentials are used to measure energy changes in systems as they
evolve from an initial state to a final state.
Note: the term thermodynamic free energy is a measure of the
amount of mechanical (or other) work that can be extracted from
The Kinetic Theory of Gases
Kinetic theory or kinetic theory of gases -- explain macroscopic
properties of gases, such as P, T, or V, by considering their
molecular composition and motion.
Ideal gases kinetic theory of -modeling the gases as molecules (or atoms) in constant motion in
-A mathematical explanation of the behaviour of gases
-the KE depending on the temperature of the gas
The kinetc theory makes seven assumptions:
The volume occupied by the
gas molecules themselves is
negligible compared with the
volume of space between them
All the particles that
make up the gas are
The distribution of
particles is random
There are sufficent
numbers of molecules for
the statistical average to
travel in straight
mechanics can be
applied to molecule interactions
The kinetic theory of gases -- deduced equations that related the
easily observable properties such as P, , V and T to properties not easily or directly observable -such as the sizes and speeds of
The pressure of a gas is caused by collisions of the molecules
of the gas with the walls of the container.
The magnitude of the pressure is related to how hard and how
often the molecules strike the wall
The absolute temperature of a gas is a measure of the average
kinetic energy of its' molecules
If two different gases are at the same temperature, their
molecules have the same average kinetic energy
If the temperature of a gas is doubled, the average kinetic
energy of its molecules is doubled
All the molecules average kinetic energy (and therefore an average speed)
the individual molecules move at various speeds, exhibit a DISTRIBUTION of speeds
Collisions can change individual molecular speeds but the
distribution of speeds remains the same.
At the same temperature, lighter gases move on average
faster than heavier gases.
The average kinetic energy, , is related to the root mean square (rms) speed u
Statistical mechanics is the application of probability theory to the
field of mechanics, which is concerned with the motion of
particles or objects when subjected to a force.
-relating the microscopic properties of individual atoms and
molecules to the macroscopic or bulk properties of materials that
can be observed in everyday life
-it can be used to calculate the thermodynamic properties of bulk
materials from the spectroscopic data of individual molecules.
The fundamental postulate in statistical mechanics:
Given an isolated system in equilibrium, it is found with
equal probability in each of its accessiblemicrostates.
This postulate is necessary because it allows one to conclude that
for a system at equilibrium, the thermodynamic state (macrostate)