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100 keV electron energy

  1. Jun 25, 2015 #1

    blue_leaf77

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    This may be a naive question but this has been bothering me. I read that electrons having energy above 100 keV are considered as relativistic, on the other hand electron's rest mass is 511 keV. How can a free electron have energy less than its rest mass? What kind of energy implied in saying that 100 keV? Is it ##\sqrt{p^2c^2+m_e^2c^4}-m_ec^2##?
     
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  3. Jun 25, 2015 #2

    jtbell

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    Yes, they are surely referring to the kinetic energy of the electron, not the total energy. I think this is common practice, because the kinetic energy relates directly to how you produce the electrons in the first place, i.e. effectively accelerate them through a potential difference of 100 kV.
     
  4. Jun 25, 2015 #3

    blue_leaf77

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    Thanks!
     
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