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.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook