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E=mc^2 units

  1. Mar 17, 2012 #1
    When solving with E=mc^2 what units should I use in the answer??
    For example: (if I have an object with a mass of 43kg)
    E=3.86464727 × 10^18
    Now here's my problem, do I include units of J/kg (joules per kilogram) or eV (electron volts)??
    If someone could help me it would be very helpful.

  2. jcsd
  3. Mar 17, 2012 #2


    Staff: Mentor

    What is the SI unit of energy?
  4. Mar 17, 2012 #3
    Joules are kg*m2/s2. Since the units you chose for mass and c are kg, m and s, Joules are what you get.
  5. Mar 17, 2012 #4


    User Avatar
    Gold Member

    Electron volts would require the mass to be given in [itex]{{eV}\over{c^2}}[/itex], which is common in high energy physics or when you're talking about atomic scale stuff. For example, the rest mass of a proton is [itex]938 \times 10^8 {{eV}\over{c^2}}[/itex]. By itself, eV is not an SI unit.
  6. Mar 18, 2012 #5
    Oh ok so the unit would be J/kg, no?
    Or would it just be J, because I read that it might be J/kg, so which one??
  7. Mar 18, 2012 #6


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    Staff: Mentor

    1. What do you get with you multiply/divide the units together as indicated by your calculation? (Don't do any conversions!)

    2. What is a joule (J) in terms of kg, m, and s?
  8. Mar 18, 2012 #7
    The SI unit of energy is Joule.
    eV is a convenient unit of energy in some areas of physics. The relationship between eV and Joule is
    1eV = 1.6 x 10^-19 J
  9. Mar 18, 2012 #8
    Oh alright so the unit would be Joule.

    Thanks all,
    Last edited: Mar 18, 2012
  10. Mar 19, 2012 #9


    User Avatar
    Science Advisor

    Either is correct, since joules and electron volts are both units of energy. If you are working with a macroscopic sized system, you probably want to use joules. If you are working with a microscopic system (i.e. single particles) then electron volts are probably more convenient.
  11. Mar 19, 2012 #10
    Ah ok now that explanation makes a lot of sense thanks.

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