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!

How are units combined/cancelled in this simple equation?

  1. Aug 19, 2012 #1
    The numerical answer is easy to find, but I'm wondering which units are in the answer and how those units are determined. Thanks in advance.

    (6.63E-34 J×s)(7.09E14 s) - 2.17E-19 J =
  2. jcsd
  3. Aug 19, 2012 #2
    The unit of a frequency is s-1, not s.
  4. Aug 19, 2012 #3
    That must be a mistake on my teacher's part, then. Can you explain further? I'm very new to physics.
  5. Aug 19, 2012 #4
    The second from the units of Planck's constant cancels with the inverse second from the unit of frequency. The product is left with a unit of energy, namely the energy of the photon with the given frequency.

    You can only add and subtract quantities with the same units, namely joules. Here, you subtract the work function of a metal. Your result, the kinetic energy of the photoelectron, is also in joules.
  6. Aug 19, 2012 #5
    If I have some kind of periodic phenomenon, the frequency is defined as the inverse of the period, the period is the time it takes for the system to complete one cycle
  7. Aug 19, 2012 #6
    I was just given the equation alone and told to simplify it. Is (7.09E14 s) the frequency in this equation? Can I convert it to the period so that the seconds cancel or is it simply a typing error that it was not measured in inverse seconds in the first place?
  8. Aug 19, 2012 #7


    User Avatar
    Homework Helper

    it looks almost certainly like it is a typing error. it should be 7.09E14 s-1 Which is the frequency of the photon I presume.
  9. Aug 19, 2012 #8
    Looks like your teacher was trying to find real equations for your exercise, but made a typo with the units.

    P.s. the equation at hand is the relation between a photons energy and its frequency
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook