39. The charge on the metal plate, which is negative, exerts a force of repulsion on the electron and stops it. First find an expression for the acceleration of the electron, then use kinematics to find the stopping distance. We take the initial direction of motion of the electron to be positive. Then, the electric field is given by E= σ/ε0, where is the surface charge density on the plate. The force on the electron is F = –eE = –e/ε0and the acceleration is 0Feammε==−where mis the mass of the electron. The force is constant, so we use constant acceleration
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