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Q: 11.20  (b) Use the same formula you employ in (a) to obtain electron speed for an collector potential  of 10 MV. Do you see what is wrong? In what way is the formula to be modified?

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Using the same formula we get the speed of electrons to be 1.88\times109 m/s. This is wrong because the speed of the electron is coming out to be more than the speed of light. This discrepancy is occurring because the electron will be travelling at very large speed and in such cases(relativistic) the mass of the object cannot be taken to be the same as the rest mass.

In such a case

m=\frac{m_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}}

where m is the relativistic mass, m0 is the rest mass of the body, v is the very high speed at which the body is travelling and c is the speed of light.

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Sayak

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