# Collisions (`plasmapy.formulary.collisions`)¶

Functions to calculate transport coefficients.

This module includes a number of functions for handling Coulomb collisions spanning weakly coupled (low density) to strongly coupled (high density) regimes.

## Coulomb collisions¶

Coulomb collisions are collisions where the interaction force is conveyed via the electric field, instead of any kind of contact force. They usually result in relatively small deflections of particle trajectories. However, given that there are many charged particles in a plasma, one has to take into account the cumulative effects of many such collisions.

## Coulomb logarithms¶

Please read the documentation of `Coulomb_logarithm` below for an explanation of the seven PlasmaPy-supported methods of computing the Coulomb logarithm.

## Collision rates¶

The module gathers a few functions helpful for calculating collision rates between particles. The most general of these is `collision_frequency`, while if you need average values for a Maxwellian distribution, try out `collision_rate_electron_ion` and `collision_rate_ion_ion`. These use `collision_frequency` under the hood.

## Macroscopic properties¶

These include:

### Functions¶

 `collision_frequency`(T, n, species[, z_mean, ...]) Collision frequency of particles in a plasma. `Coulomb_cross_section`(impact_param) Cross section for a large angle Coulomb collision. `Coulomb_logarithm`(T, n_e, species[, z_mean, ...]) Compute the Coulomb logarithm. `coupling_parameter`(T, n_e, species[, ...]) Ratio of the Coulomb energy to the kinetic (usually thermal) energy. Average momentum relaxation rate for a slowly flowing Maxwellian distribution of electrons. `fundamental_ion_collision_freq`(T_i, n_i, ion) Average momentum relaxation rate for a slowly flowing Maxwellian distribution of ions. `impact_parameter`(T, n_e, species[, z_mean, ...]) Impact parameters for classical and quantum Coulomb collision `impact_parameter_perp`(T, species[, V]) Distance of closest approach for a 90° Coulomb collision. `Knudsen_number`(characteristic_length, T, ...) Knudsen number (dimensionless) `mean_free_path`(T, n_e, species[, z_mean, V, ...]) Collisional mean free path (m) `mobility`(T, n_e, species[, z_mean, V, method]) Return the electrical mobility. `Spitzer_resistivity`(T, n, species[, z_mean, ...]) Spitzer resistivity of a plasma