# 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.
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