Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: What is the ground state energy of an electron? I thought i did this right

  1. Apr 20, 2006 #1
    What is the ground state energy of the following.
    (a) an electron

    Well the formula is:
    E_n = (h^2/(8*m*L^2))*n^2;
    The ground state means, n = 1, its the lowest enegery level possible.
    So i plugged in n = 1,
    mass of e = 9.11E-31;
    h = 4.136E-15 eVs;
    L = 100 pm, because the book says thats tabout the size of a typical atom.
    I ended up getting:
    E_1 = 2.347E20 eV which was wrong, any ideas why? They really don't tell me what value i should give L, so i'm guessing thats where i screwed up maybe? In the book it says the electron is in an infintie well. thanks!
    Last edited: Apr 20, 2006
  2. jcsd
  3. Apr 20, 2006 #2


    User Avatar
    Gold Member

    You can't use the eV*s version of planck's constant if you are using the mass in kg. You must use h=6.6261*10^-34 J*s or switch the mass to the MeV/c^2 unit and use hc = 1239.8ev*nm
  4. Apr 20, 2006 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    because you are mixing units... If you use eV for energy and kg for mass, you are in trouble. If you write the unit sout, you will see that tey don't simplify. My suggestion is, when you are not sure, to put everything in SI units and convert back to eV at the very end
  5. Apr 21, 2006 #4
    Thanks for the responce guys, when i just use the units to see what i end up with, i'm getting (J*s)^2/(kg*m^2), now if i want this in eV, 1 eV = 1.602E-19J;
    but it still doesn't work out to the right units... anyone see what i'm not getting?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook