# Notation of physical constants

### 11.2.1 Notation of physical constants

If you want to use a physical constants inside Xcas, put its name between two characters _ ("underscore"). Don’t confuse physical constants with symbolic constants; for example, e,π are symbolic constants and _c_,_NA_ are physical constants. The physical constants are in the Phys menu, Constant sub-menu, and table 11.3 gives the Constants Library:

> _F_ | Faraday constant | _h_ | Planck’s constant
> _G_ | Gravitational constant | _hbar_ | Dirac’s constant
> _I0_ | Reference intensity | _k_ | Boltzmann constant.
> _NA_ | Avogadro’s number | _kq_ | k/q (Boltzmann/charge of the electron)
> _PSun_ | Power at the surface of the Sun | _lambda0_ | Photon wavelength (ch/e)
> _REarth_ | Radius of the Earth | _lambdac_ | Compton wavelength
> _RSun_ | Radius of the Sun | _mEarth_ | Mass of the Earth
> _R_ | Constante universelle des gaz | _me_ | Electron rest mass
> _Rinfinity_ | Rydberg constant | _mp_ | Proton rest mass
> _StdP_ | Standard pressure | _mpme_ | Quotient mp/me (mass of the proton/mass of the electron)
> _StdT_ | Standard temperature | _mu0_ | Permeability of vacuum
> _Vm_ | Molar volume | _muB_ | Bohr magneton
> _a0_ | Bohr radius | _muN_ | Nuclear magneton
> _alpha_ | Fine structure constant | _phi_ | Magnetic flux quantum
> _angl_ | 180 degree angle | _q_ | Charge of an electron
> _c3_ | Wien displacement constant | _qe_ | Electron charge
> _c_ | Speed of light in vacuum | _qepsilon0_ | q*epsilon0 (charge of the electron*permittivity)
> _epsilon0_ | Permittivity of vacuum | _qme_ | Quotient q/me (charge/mass of the electron)
> _epsilon0q_ | epsilon0/q (permittivity/charge of the electron) | _rad_ | 1 radian
> _epsilonox_ | Dielectric constant of Silicon dioxide | _sd_ | Duration of a sidereal day
> _epsilonsi_ | Dielectric constant | _sigma_ | Stefan-Boltzmann constant
> _f0_ | Photon frequency (e/h) | _syr_ | Duration of a sidereal year
> _g_ | Acceleration of gravity | _twopi_ | 2π

> Table 11.3: Physical constants

#### Examples.

- Input:
  _c_
  Output:

  1 c

  which represents the speed of light in vacuum. You can use the mksa command (see Section 11.1.3) to put this in terms of standard units:
  Input:
  mksa(_c_)
  Output:

  299792458.0 m s−1

- Input:
  _NA_
  Output:

  1 NA

  which represents Avogadro’s number:
  Input:
  mksa(_NA_)
  Output:

  6.0221367×1023 mol−1