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Negative Energy

  1. May 10, 2010 #1
    Actually i never got this concept clearly in my mind. What is the meaning whern we say that the energy of a body is negative? For example in Bhor's atomic model we have calulated the total energy of an electron to be negative. Please explain me by using Newtonian Mechaincs as i am the undergraduate student of physics.
     
  2. jcsd
  3. May 10, 2010 #2
    This is to do with potential energy.We cannot specify what an absolute potential energy is at any particular particle separation but we can calculate the amount by which potential energy changes with separation.For attracting particles the PE is a maximum at infinite separation and we choose to refer to this separation and to give the PE at infinity a value of zero.At lesser separations the PEs are negative.The choice of infinity and zero are arbitary but generally accepted as being the most useful choices in that they are easiest to work with.When you calculate that the energy of the electron is negative what that really means is that it has less energy than it would have if it were at infinity.
     
  4. May 11, 2010 #3
    Thanks a lot for your wonderful reason. I got it. So you mean PE and KE are frame dependent, right?
     
  5. May 11, 2010 #4
    KE is definitely frame depended. However, potential energy is a little bit more interesting. It's actually what is called gauge invariant. One cannot measure the energy of something at a given point, only the energy between points. So, we can take electric potential energy (that defines the electric field E = grad V) and add a constant to it an nothing changes physically.
     
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