Notice that the proton (actually the proton-field system) lost potential energy

Question

Notice that the proton (actually the proton-field system) lost potential energy when it moved in the positive x-direction, whereas the electron gained potential energy when it moved in the same direction. Finding changes in potential energy with the field reversed was only a matter of supplying a minus sign, bringing the total number in this case to three! It's important not to drop any of the signs.

Question When an electron is released from rest in a constant electric field, how does the electric potential energy associated with the electron, and the kinetic energy of the electron, change with time? (Select all that apply.)

A The electric potential energy becomes more negative.
B The kinetic energy becomes more positive.
C The electric potential energy becomes more positive.
D The kinetic energy stays the same.
E The electric potential energy stays the same.
F The kinetic energy becomes more negative.

Explanation / Answer

Releasing an electron in a constant electric field is no different than releasing a rock in a constant gravitational field. It accelerates at a constant rate, so:

kinetic energy increases (becomes more positive), and;

potential energy decreases (becomes more negative), so

(A) and (B) are correct, the other four are not.

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