Let's say that an electron is having the speed of X m/s.(adsbygoogle = window.adsbygoogle || []).push({});

So the energy is sometimes calculated like this:

(1)[tex]E= \frac{mv^2}{2}[/tex]

and sometimes like this

(2)[tex]E = mc^2 + \frac{mv^2}{2}[/tex]

When should I use1and when should I use2?

Thank you for any help.

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# Calculating the energy of an electron

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