# 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,

$\alpha = \frac{1}{4 \pi \epsilon_0} \frac{e^2}{\hbar c},$

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