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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##?