# Constants (`plasmapy.constants`

)¶

The `constants`

module contains many physical and mathematical
constants that are commonly used in the plasma sciences.

Mathematical constants (such as `pi`

) are provided as `float`

objects.

Physical constants are imported directly from Astropy’s
`constants`

subpackage as instances of the
`Constant`

class (which is a subclass of
`Quantity`

). We recommend reviewing the
documentation for Astropy’s constants subpackage for more
details (including Astropy’s strategy for versioning constants).
Physical constants by default are provided in SI units.

## Getting Started¶

Constants in SI units may be imported directly from PlasmaPy’s
`constants`

subpackage. These objects may be treated very
much like `Quantity`

instances.

Suppose we want to calculate the fine structure constant,

using `constants`

.

```
>>> from plasmapy.constants import eps0, e, hbar, c, pi as π
>>> α = (e ** 2 / (4 * π * eps0 * hbar * c)).si
>>> α ** -1
<Quantity 137.03599911>
```

Warning

The values of constants that are not defined exactly will change slightly as improved measurements become available.

## Reference/API¶

### plasmapy.constants Package¶

Contains physical and mathematical constants commonly used in plasma physics.

The following constants are available:

Name | Value | Units | Description |
---|---|---|---|

pi | 3.141592653589793 | Ratio of circumference to diameter of circle | |

G | 6.67408e-11 | m3 / (kg s2) | Gravitational constant |

L_sun | 3.828e+26 | W | Nominal solar luminosity |

M_earth | 5.97236473042e+24 | kg | Earth mass |

M_sun | 1.98847541534e+30 | kg | Solar mass |

N_A | 6.022140857e+23 | 1 / (mol) | Avogadro’s number |

R | 8.3144598 | J / (K mol) | Gas constant |

R_earth | 6378100 | m | Nominal Earth equatorial radius |

R_sun | 695700000 | m | Nominal solar radius |

Ryd | 10973731.5685 | 1 / (m) | Rydberg constant |

a0 | 5.2917721067e-11 | m | Bohr radius |

atm | 101325 | Pa | Standard atmosphere |

au | 149597870700 | m | Astronomical Unit |

c | 299792458 | m / (s) | Speed of light in vacuum |

e | 1.6021766208e-19 | C | Electron charge |

eps0 | 8.854187817e-12 | F/m | Electric constant |

g0 | 9.80665 | m / s2 | Standard acceleration of gravity |

h | 6.62607004e-34 | J s | Planck constant |

hbar | 1.0545718e-34 | J s | Reduced Planck constant |

k_B | 1.38064852e-23 | J / (K) | Boltzmann constant |

kpc | 3.08567758147e+19 | m | Kiloparsec |

m_e | 9.10938356e-31 | kg | Electron mass |

m_n | 1.674927471e-27 | kg | Neutron mass |

m_p | 1.672621898e-27 | kg | Proton mass |

mu0 | 1.25663706144e-06 | N/A2 | Magnetic constant |

muB | 9.274009994e-24 | J/T | Bohr magneton |

pc | 3.08567758147e+16 | m | Parsec |

sigma_T | 6.6524587158e-29 | m2 | Thomson scattering cross-section |

sigma_sb | 5.670367e-08 | W / (K4 m2) | Stefan-Boltzmann constant |

u | 1.66053904e-27 | kg | Atomic mass |