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 
6.67408e11 
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 
5.2917721067e11 
m 
Bohr radius 
atm 
101325 
Pa 
Standard atmosphere 
au 
149597870700 
m 
Astronomical Unit 
c 
299792458 
m / (s) 
Speed of light in vacuum 
e 
1.6021766208e19 
C 
Electron charge 
eps0 
8.854187817e12 
F/m 
Electric constant 
g0 
9.80665 
m / s2 
Standard acceleration of gravity 
h 
6.62607004e34 
J s 
Planck constant 
hbar 
1.0545718e34 
J s 
Reduced Planck constant 
k_B 
1.38064852e23 
J / (K) 
Boltzmann constant 
kpc 
3.08567758147e+19 
m 
Kiloparsec 
m_e 
9.10938356e31 
kg 
Electron mass 
m_n 
1.674927471e27 
kg 
Neutron mass 
m_p 
1.672621898e27 
kg 
Proton mass 
mu0 
1.25663706144e06 
N/A2 
Magnetic constant 
muB 
9.274009994e24 
J/T 
Bohr magneton 
pc 
3.08567758147e+16 
m 
Parsec 
sigma_T 
6.6524587158e29 
m2 
Thomson scattering crosssection 
sigma_sb 
5.670367e08 
W / (K4 m2) 
StefanBoltzmann constant 
u 
1.66053904e27 
kg 
Atomic mass 