• Support PF! Buy your school textbooks, materials and every day products Here!

Units in a quantum barrier problem

  • Thread starter ClaesF
  • Start date
2
0
I have a quesion regarding a quantum physics assignemnt, I wonder what units I should use when calculating the transmission coefficient of a quantum barrier problem.

I have got the following expression:

[tex]
T = \frac{4(E+V_0)}{(2E+V_0)cos^2a\sqrt{\frac{2m}{\hbar^2}(E-V_0)} + (E-V_0+\frac{E(E+V_0)}{E-V_0}+2\sqrt{E(E+V_0)})sin^2a\sqrt{\frac{2m}{\hbar^2}(E-V_0)}}

[/tex]

where

T = the transmission coefficient
E = the energy of an incoming electron = 2.1 eV
[itex]V_0[/itex] = a potential = 1.5 eV
a = a point along the x-axis = 12 angstrom (= [itex]12*10^{-10}[/itex] m)
m = the mass of the electron (= [itex]9.109*10^{-31}[/itex] kg)
[itex]\hbar[/itex] = [itex]1.0546*10^{-34}[/itex] Js or [itex]6.582*10^{-16}[/itex] eVs.

I don't know if I should translate all values in the whole expression into SI units, or if I somehow can use the values given in the assignment in eV and angstrom directly?
If I use the eV- and angstrom values, I guess it is wrong to use the kg-value of the electronmass in the [itex]\sqrt{\frac{2m}{\hbar^2}(E-V_0)}[/itex]-expressions.
 

Answers and Replies

If you use one of the planks constants in one form (Js) and one in the other form (eVs) in the h^2 expression it should work out OK.

If you think about getting the final expression inside the trig functions to be unitless. Remember J=kg m/s
 
2
0
ok, but I must still express a in meters(=[itex]12*10^{-10}[/itex]m) then. (Js=[itex]kgm^2/s[/itex])

thanks for the help.

/Claes
 
Gokul43201
Staff Emeritus
Science Advisor
Gold Member
6,987
14
You want the argument in the trig terms to be unitless. Write "a" in meters and the terms inside the square roots in SI units (so the units of the wave vector will be 1/meter).

You can leave the energies outside the trig terms in eV or SI, since the transmission coefficient is the ratio of these energy terms.
 

Related Threads for: Units in a quantum barrier problem

  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
3
Views
503
Replies
4
Views
1K
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
15
Views
1K
  • Last Post
Replies
1
Views
669
  • Last Post
Replies
1
Views
603
Top