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:

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