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**__Solution:__
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As the magnetic field only changes the direction of motion and not the
velocity, the speed of the electron when it reaches the top plate will be
determined by the potential difference between the higher and the lower plates.
The speed *v* when the electron is able to reach the top plate will,
therefore, be

We are asked to find the minimum magnetic field *B* such that the
electron will not strike the top plate. If this has to happen then the velocity
vector will be parallel to the plates when it approaches the top plate. In this
situation the Lorentz force on the electron will be zero, and the electron will
graze the top plate and move parallel to it with speed *v*. For the minimum
magnetic field, we have the condition

As