## How Do I Use Standard Physical Constants in *Mathematica*?
In order to make use of standard physical constants in *Mathematica*,
it is useful to load the `Miscellaneous`PhysicalConstants`` package.
**Needs["Miscellaneous`PhysicalConstants`"]**
The constants in this package are defined using updated values from the following source:
Peter J. Mohr and Barry N. Taylor, "CODATA Recommended Values of the
Fundamental Physical Constants: 1998" (CODATA 1998),
http://physics.nist.gov/cuu/Constants.
Also published in *Journal of Physical and Chemical Reference
Data* 28, no. 6 (1999) 1713-1852; *Reviews of Modern Physics* 72,
no. 2 (2000) 351-495; and *CRC Handbook of Chemistry and Physics*, 80th ed.
(HCAP 80), edited by David R. Lide (1999-2000).
As of CODATA 1998, some conventions used for the electron and muon
*g*-factors and for the electron, muon, and neutron magnetic moments are
different than before; they are all expressed as a negative number in
CODATA 1998, and a factor of two that was previously divided out of the
electron *g*-factor is present. For `QuantizedHallConductance`, HCAP 80
gives a value for *e*^{2/h}, while
CODATA 1998 gives a value for 2*e*^{2/h}.
The CODATA value was taken, and the factor of 2 was divided out to match the
HCAP value and the previous use in this package.
When this package is loaded, a combination of 74 physical constants and SI
units of measure are defined. All of the constants defined in this package
are expressed in SI units. A complete list of symbols can be found below.
As with any symbol in *Mathematica*, you can obtain a description of the symbol
by evaluating an expression such as the following.
**?AccelerationDueToGravity**
`AccelerationDueToGravity is the acceleration of a
body freely falling in a vacuum.`
To see the value of any constant, simply evaluate the symbol name of the desired constant
as follows.
**CosmicBackgroundTemperature**
`2.726 Kelvin`
Calculations can be carried out using any of the defined constants. In
this example, the energy of a photon is calculated based on a specified wavelength.
**energy[lambda_] := (PlanckConstant*SpeedOfLight)/lambda**
**energy[400*10^-9 Meter]**
Here is a list of all of the symbols defined when the
`Miscellaneous`PhysicalConstants`` package is loaded.
**Flatten[Names/@{"Miscellaneous`PhysicalConstants`*",**
"Miscellaneous`SIUnits`*"}]
`{AccelerationDueToGravity, AgeOfUniverse,
AvogadroConstant, BohrRadius,
BoltzmannConstant, ClassicalElectronRadius, CosmicBackgroundTemperature, DeuteronMagneticMoment, DeuteronMass,
EarthMass, EarthRadius, ElectronCharge, ElectronComptonWavelength, ElectronGFactor, ElectronMagneticMoment,
ElectronMass, FaradayConstant, FineStructureConstant, GalacticUnit, GravitationalConstant, HubbleConstant,
IcePoint, MagneticFluxQuantum, MolarGasConstant, MolarVolume, MuonGFactor, MuonMagneticMoment, MuonMass,
NeutronComptonWavelength, NeutronMagneticMoment, NeutronMass, PlanckConstant, PlanckConstantReduced, PlanckMass,
ProtonComptonWavelength, ProtonMagneticMoment, ProtonMass, QuantizedHallConductance, RydbergConstant,
SackurTetrodeConstant, SolarConstant, SolarLuminosity, SolarRadius, SolarSchwarzschildRadius, SpeedOfLight,
SpeedOfSound, StefanConstant, ThomsonCrossSection, VacuumPermeability, VacuumPermittivity, WeakMixingAngle, Amp,
Ampere, Becquerel, Candela, Coulomb, Farad, Gray, Henry, Hertz, Joule, Kelvin, Kilogram, Lumen, Lux, Meter, Mole,
Ohm, Pascal, Siemens, Tesla, Volt, Watt, Weber}`
For more information on physical
constants and their use in *Mathematica*, refer to the Documentation
Center.
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