### 24.2.1 Notation of physical constants

If you want to use a physical constants inside Xcas, put its name between two characters _ (“underscore”). Do not 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 24.3 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/electron charge)
> _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_ | Universal gas constant | _me_ | Electron rest mass
> _Rinfinity_ | Rydberg constant | _mp_ | Proton rest mass
> _StdP_ | Standard pressure | _mpme_ | Quotient mp/me (proton mass/electron mass)
> _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· ε0 (electron charge*permittivity)
> _epsilon0_ | Permittivity of vacuum | _qme_ | Quotient q/me (charge/electron mass)
> _epsilon0q_ | ε0/q (permittivity/electron charge) | _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_ | 2 π

> Table 24.3: Physical constants in Xcas

#### Example

To get the object representing the speed of light in vacuum, enter:

_c_

1 c |  |  |  |  |  |  |  |  |  |

Use the mksa command (see Section 24.1.3) to put this in terms of standard units:

mksa(_c_)

299792458.0 m s−1 |  |  |  |  |  |  |  |  |  |