# Statistical Thermodynamics and Rate Theories/Definition of variables

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This section will contain a list of definitions of physical variables used in this book.

Variable | Name | Definition |
---|---|---|

U | Internal Energy | Internal energy is the total energy needed to create a system. It can be divided into the sum of two other energies (internal kinetic energy and internal potential energy). |

H | Enthalpy | Enthalpy is a thermodynamic variable which corresponds to the total energy of the system and the energy required to create a volume. It has units of energy and can be calculated using the equation H = U +pV |

q | Heat | The thermal energy transferred from the surroundings to the system. |

w | Work | The energy corresponding to the expansion of the system against the surroundings. |

S | Entropy | Changes in the state of a system can also involve a change in the degree of disorder in the system and surroundings. The thermodynamic function entropy corresponds to this disorder. |

A | Helmholtz Energy | The change in energy found within a canonical ensemble. This can be calculated by the following equation: A = U - T x S |

G | Gibb's Energy | The change in energy found within an isothermal-isobaric ensemble. This can be calculated by the following equation: G = H - T x S |

I | Inertia | The moment of inertia is what the rotation of an object depends on. |

Q | Partition Function | The sum over all states of a system. It is used in the denominator of the Boltzmann distribution. |

μ | Chemical Potential | For the canonical ensemble, the chemical potential, is the Helmholtz free energy change when a particle is added to the system. |

F | Fraction of Molecules at a Given Speed | The fraction of molecules within a range of speed (F) is an integral over the probability distribution. |

ℓ | Mean Free Path | The mean free path is the average distance that a molecule can travel before it will undergo a collision. |

τ | Shear Stress | The drag force per unit area. |

J | Momentum Flux | The rate that momentum passes through a plane per unit area, per unit second |