1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Energy, time and length corresponding to mass of electron.

  1. Mar 10, 2012 #1
    I am starting to learn the basics of QED and I am trying to get acquainted with the units and scales that are used.

    2. Relevant equations
    The exercise is to estimate the energy, time and length that correspond to the mass of an electron.

    3. The attempt at a solution

    That’s what I did:
    The mass of an electron is 9.11E-31 kg.

    E=mc^2 gives me an energy of 0.511MeV.

    Here I used the equation: c * hbar = 197 MeV * fm
    -> 386fm = c * hbar / (0.511MeV)
    -> In natural units: 386fm = 1 / (0.511MeV)

    Here I used the equation: hbar = 6.58E-22 MeV * s
    -> 1.288E-21 s = hbar / (0.511MeV)
    -> In natural units: 1.288E-21 s = 1 / (0.511MeV)

    Is this calculation correct? Is that the way one can calculate the length and time corresponding to a mass?
  2. jcsd
  3. Mar 12, 2012 #2
    Makes sense, but I'd take a look at the equation you used to find the length. Do you really want to be using the reduced planck constant (hbar) there, or simply the planck constant (2*pi*hbar)? You can look up the de Broglie wavelength of an electron, and you'll see your value is a bit small.
  4. Mar 12, 2012 #3


    User Avatar
    Homework Helper

    That depends on the convention, right? For instance, in Peskin, the convention is to use hbar as the base unit (of action). I don't recall seeing anyone use just plane h as the base unit, but it is just a convention.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook