Question

A proton is released from rest in a region of space with a non-zero electric field.

(a) As the proton moves, does the electric potential energy of the system increase, decrease, or stay the same.
(b) Choose the best explanation from among the following.
(I) As the proton begins to move, its kinetic energy increases. The increase in kinetic energy is equal to the decrease in the electric potential energy of the system.
(II) Because the proton has a positive charge. Its electric potential energy will always increase.
(III) The proton will move perpendicular to the electric field and hence its electric potential energy will remain the same.

Answer 1

a) As the proton moves, its electric potential energy will decrease.(b) Explanation (I) is the best option. As the proton begins to move, it gains kinetic energy. This kinetic energy comes at the expense of the electric potential energy of the system. The electric potential energy of the system is highest when the proton is at rest and decreases as the proton moves in the direction of the electric field. This is because work is done on the proton by the electric field, which transfers energy to the proton and increases its kinetic energy. The decrease in electric potential energy is equal to the increase in kinetic energy, as per the law of conservation of energy. Option (II) is incorrect because the electric potential energy can either increase or decrease, depending on the direction of motion of the charged particle. Option (III) is also incorrect because the motion of the proton will not necessarily be perpendicular to the electric field.: