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: Stopping Potential

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data

    I'm doing some practice problems for an introductory quantum mechanics course and am unsure whether or not I'm solving this problem properly - I need confirmation if I'm doing it right and help if I'm doing it wrong! :)

    "What is the stopping potential when 250 nm light strikes a zinc plate?" (Chapter 3, #18 in Modern Physics 2nd ed. by Randy Harris)

    2. Relevant equations

    K = E - [itex]\varphi[/itex] (where K is the kinetic energy, E is the energy of the incident light and [itex]\varphi[/itex] is the work function)

    This can be written as:

    [itex]\frac{mv^2}{2}[/itex] = [itex]\frac{hc}{\lambda}[/itex] - [itex]\varphi[/itex] (where m is the mass of a scattered electron, v is the speed of this electron, h is Planck's constant and c is the speed of light)

    [itex]\frac{mv^2}{2}[/itex] = qV (where q is the electron charge and V is the stopping potential)

    h = 6.626 x 10^(-34) Js
    c = 3 x 10^8 m/s
    [itex]\varphi[/itex] = 4.3 eV

    3. The attempt at a solution

    If my equations above are correct, I can write:

    [itex]\frac{mv^2}{2}[/itex] = [itex]\frac{hc}{\lambda}[/itex] - [itex]\varphi[/itex] = qV
    [itex]\frac{hc}{\lambda}[/itex] - [itex]\varphi[/itex] = qV
    V = [itex]\frac{\frac{hc}{\lambda} - \varphi}{q}[/itex]

    I can then simply plug in my values (remembering to either convert h in eV*s or [itex]\varphi[/itex] into J) and this should give me the stopping potential, correct?
    Last edited: Feb 5, 2012
  2. jcsd
  3. Feb 5, 2012 #2


    User Avatar
    Homework Helper

    Yes, that all looks good. No quantum mechanics in there, though; its all high school physics.
  4. Feb 5, 2012 #3
    Thanks! And you're right, there isn't really any quantum mechanics here; it's just included in part of the course and its part of the introduction leading into the actual quantum mechanics.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook