sem5 thermal nota v1 dr wan

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  • WEEK 1


    WEEK 1

    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.

    Classical thermodynamics

    -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).

    Statistical thermodynamics

    -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

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    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.

    Chemical thermodynamics

    -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


    Thermodynamic systems






    Large number of molecules









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    -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


    Thermodynamic parameters

    Energy transfers between thermodynamic systems as the result of

    a generalized force causing a generalized displacement conjugate





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    The most common are

    Pressure-volume (mechanical parameters)

    Temperature-entropy (thermal parameters)

    Chemical potential-particle number (material parameters)

    Thermodynamic instruments

    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.

    Thermodynamic states

    State - a system is at equilibrium under a given set of conditions

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    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.

    Thermodynamic processes

    -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

    constant entropy.

    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


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    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,

    enthalpy, etc

    Second law of thermodynamics is about entropy

    Third law of thermodynamics is about absolute zero

    temperature --the determination of entropy.

    Thermodynamic potentials

    -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

    Grand potential

    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

    a system.

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    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

    energy amoung

    particles is random

    There are sufficent

    numbers of molecules for

    the statistical average to

    be meaningful.

    Collisions are

    all perfectly


    The molecules

    travel in straight

    lines between



    mechanics can be

    applied to molecule interactions


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    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

    Absolute Temperature

    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


    m vx

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    Molecular Speed

    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


    1 =

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    Statistical mechanics

    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)



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