1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Confused to the max! Re-aranging equations.

  1. Jun 9, 2010 #1
    Okay so today i sat a AS level physics paper and after finishing the exam i looked threw the equation booklet and decided to do some manipulation and i came up with this and was wondering what you guys think of it. Im confused!! :)

    E=hc/λ AND λ=h/mv
    subbing in gives E=hcmv/h which is E=cmv
    conservation of energy...

    how can velocity = 2*speed of light?

  2. jcsd
  3. Jun 9, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Just because something has the same name or the same symbol, does not mean they are representing the SAME thing.

    [itex]E=hc/\lambda[/itex] is the energy per photon with that particular wavelength, whereas [itex]\lambda=h/mv[/itex] is the deBroglie wavelength for a non-relativistic particle with mass m at velocity v!

    You can't simply substitute apples into oranges. If you do that, you get ridiculous answer like what you got.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook