Thermodynamics and Statistical Mechanics

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Thermodynamics and Statistical Mechanics

• Spring 2006

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Thermodynamics

• Thermodynamics is an old science, formulated
  empirically without reference to atoms or
  molecules. It deals with energy, as does
  mechanics, but adds some new quantities, besides
  work, potential energy and kinetic energy.

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Thermodynamic quantities

• Internal energy (U) the energy of atoms or
  molecules that does not give macroscopic motion.
• Temperature (T) a measure of the internal energy
  of a system.
• Heat (Q) a way to change internal energy,
  besides work. (Energy in transit.)

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Thermodynamic quantities

• Entropy (S) a measure of the disorder of the
  system. The change of entropy is equal to the
  heat added or removed divided by the temperature.

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Laws of Thermodynamics

• First law dQ dW dU
• Energy is conserved

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Laws of Thermodynamics

• Second Law The entropy of an isolated system
  increases in any irreversible process and is
  unaltered in any reversible process. This is the
  principle of increasing entropy.
• DS ³ 0

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Laws of Thermodynamics

• Third Law The entropy of a true equilibrium
  state of a system at a temperature of absolute
  zero is zero.
• Equivalent to It is impossible to reduce the
  temperature of a system to absolute zero using a
  finite number of processes.

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History

• Much of early thermodynamics development was
  driven by practical considerations. For example,
  building heat engines and refrigerators. The
  original statements of the second law were
  different.

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Second Law Variations

• No series of processes is possible whose sole
  result is the absorption of heat from a thermal
  reservoir and the complete conversion of this
  energy to work.
• There are no perfect engines!

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Second Law Variations

• No series of processes is possible whose sole
  result is the transfer of heat from a reservoir
  at a given temperature to a reservoir at a higher
  temperature.
• There are no perfect refrigerators!

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Third Law

• It is impossible to reach a temperature of
  absolute zero.

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Popular versions

• 1st Law You cant win.
• 2nd Law You cant break even.
• 3rd Law Theres no point in trying.

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Statistical Mechanics

• In the late 19th and early 20th century, with the
  discovery of atoms, thermodynamics was treated in
  terms of atoms and molecules, on a statistical
  basis, and the subject of statistical mechanics
  came about.

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Zeroth Law

• If two systems are separately in thermal
  equilibrium with a third system, they are in
  thermal equilibrium with each other.

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Zeroth Law
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Constant Volume Gas Thermometer
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Constant Volume Gas Thermometer

• It is calibrated at the triple point of water,
  which is 273.16 K on the Kelvin scale.

Even better
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Boiling Point of Water