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Electrons- conservation of energy - electric potential

  1. Feb 6, 2008 #1
    An Electron is Projected An electron is projected with an initial speed of 2.4 x10^5 m/s directly toward a proton that is fixed in place. If the electron is initially a great distance from the proton, at what distance from the proton is the speed of the electron instantaneously equal to twice the initial value?

    im having alot of trouble with this problem i dont even know how to aproach it can anyone pls help me to get started u..u
  2. jcsd
  3. Feb 6, 2008 #2


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    The sum of the kinetic energy (1/2)mv^2 of the electron plus it's potential energy in the field of the proton k*(-e)*(e)/r is a constant (note potential energy is negative!). At large r essentially all of the energy is kinetic. Find that. Now find the energy when v is doubled. At what value of r does the total energy balance?
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