An electron moving with a speed of is shot parallel to an electric field of strength arranged so as to retard its motion. We have to find (a) the distance the electrons will travel in the field before coming (momentarily) to rest; (b) the time that will have lapsed. (c) If the electric field ends abruptly after , we have to calculate the fraction of initial kinetic energy that the electron will lose in traversing it.

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We recall the following data:

(a)

Electrons will be decelerating in the retarding electric field with the acceleration

The distance d the electrons entering the electric field with initial speed will travel before momentarily coming to rest can be calculated using the kinematics. We have

(b)

The time that will have lapsed between when the electrons enter the field and when they momentarily come to rest can be calculated using the relation

.

We find

(c)

Fraction of kinetic energy that electrons will lose in traversing in the field can be calculated using the kinematical relation

.

We find