A crossed-field velocity selector has a magnetic field of magnitude 0.03 T. The mass of the electron is 9.10939 × 10−31 kg. What electric field strength is required if 6.5 keV electrons are to pass through undeflected?

E = 4.78*10^6 N/C

Explanation:

In this case you have to take into account that both magnetic force and electric force must be equal

However, you have to know first what is the speed of the electron by using the expression for the kinetic energy (1keV=1000*1.6*10^{-19}J):

by replacing in the expression for the forces you have

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because if the tool that you are using is not balanced it there would be false mass.

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