# Numerical value of physical constants Mathematica 10

Is there a way to use the physical constants in calculations and have Mathematica 10 figure out the final unit and numerical magnitude? When I try

pcM = N[Quantity[1, "PlanckConstant"]]


I simply get 1. h without the numerical value of Planck constant. Subsequently, using this in all further steps, keeps the answer in h and does not work out units.

However, if I define the planck constant by hand and use it in a calculation, everything works as expected. I am curious as to why the internally defined constants do not show up with numerical values.

Thanks,

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what would you like the result to be, i.e. which units should it be with respect to? –  chuy Aug 27 '14 at 15:19
I learned just a few days ago that one can (often) use UnitConvert for this sort of thing. In[199]:= UnitConvert[Quantity[1, "PlanckConstant"]] Out[199]= Quantity[6.626070*10^-34, ( "Kilograms" ("Meters")^2)/("Seconds")] –  Daniel Lichtblau Aug 27 '14 at 15:20
or In[647]:= UnitConvert[Quantity[1, "PlanckConstant"], "eV*s"] Out[647]= Quantity[4.135668*10^-15, "Electronvolts" "Seconds"] –  chuy Aug 27 '14 at 15:22

In physics Planck constant may be used as a natural unit.

If you want to switch to another unit system, use UnitConvert[].

For example, you can switch to standard SI units this way:

UnitConvert[Quantity[1, "PlanckConstant"], "SIBase"]


Which will give you:

Quantity[6.626070*10^-34, ("Kilograms" ("Meters")^2)/("Seconds")]


This can be done at the end of calculation.

If you like to get rid of Quantity head, just do:

QuantityMagnitude[%]


Which outputs:

6.626070*10^-34

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you can also use QuantityMagnitude to get the value out. –  chuy Aug 27 '14 at 15:29
@chuy, indeed, thank you! –  Alexey Bobrick Aug 27 '14 at 15:31
Or just UnitConvert without the second argument, which assumes SI units. –  Mark Adler Aug 28 '14 at 3:40
@MarkAdler: definitely right! Though, of course, using the second argument is a bit more general. For example, in engeneering applications one unit measure can be used and in spectroscopy - something totally different. –  Alexey Bobrick Aug 28 '14 at 8:04

You should use the Physical Constants Package by using

<< PhysicalConstants


When you enter now

PlanckConstant


you directly get the Planck Constant. With

PlanckConstant/(Joule Second)
`

you get the Planck Constant without units.

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True, but note that PhysicalConstants package is obsolete and has been biult into Mathematica since version 9. –  Alexey Bobrick Aug 27 '14 at 15:49