# Calculations with final result in electronvolts

1. Sep 20, 2016

### barbapapa

1. The problem statement, all variables and given/known data
I have to do a calculation in electronvolt, using physical constants. The final result must be in electronvolts. I can grab the physical constant from scipy (python library). How do I know which constants I can keep in SI units, and which constants I have to transform?

2. Relevant equations
The relevant physical constants are:
• elementary charge
• hbar
• electron mass
• bohr magneton

3. The attempt at a solution

Use the following physical constants:
• 'elementary charge' (in coulombs)
• 'Planck constant over 2 pi in eV s'
• 'electron mass energy equivalent in MeV' * 10^6 / c^2, where c is in m/s
• 'Bohr magneton in eV/T'

I am not sure if I have to scale c, and the elementary charge.

Many thanks!

2. Sep 20, 2016

### BvU

Hello bbpp,

You might enlighten us by stating the full problem statement, the relevant equations you plan to use and your attempt at solution. Just like the template suggests you to do ...

Alternatively do it all in SI and use e (= 1.60217662 × 10-19 coulombs) to convert joules to eV.

3. Sep 20, 2016

### barbapapa

Thank you for your response :)

The question literally is: If we want electronvolt as a unit for energy in the result of a calculation of energy, what must be the units of the physical constants that are defined (instead of SI units).

The answer should be independent of the equations, like SI units can be used for the physical constants to get an answer in joules with any calculation of energy.

So the question is very general and I defined it as much as I can I think :). That is also why I used the physical constants as 'Relevant equations' and my proposed units as 'The attempt at a solution'.