Final answer:
In a vacuum, a proton moving parallel to a uniform electric field experiences a force that causes it to accelerate. The force on the proton is calculated by the product of its charge and the electric field, and the resulting motion is determined by Newton's second law. An electron in the same field experiences an equal but opposite force and a greater acceleration due to its smaller mass.
Step-by-step explanation:
In a vacuum, a proton moving parallel to a uniform electric field experiences a force due to the electric field. The magnitude of the force F on a proton can be calculated using the equation F = qE, where q is the charge of the proton (approximately 1.6 × 10^{-19} C) and E is the strength of the electric field. The direction of the force is along the direction of the electric field if the field is positive, or opposite if the field is negative. This force will cause the proton to accelerate according to Newton's second law, F = ma, where m is the mass of the proton and a is its acceleration. Since the force and, subsequently, the acceleration are constant, the proton's velocity will change linearly over time in the direction of the force.
For an electron, the calculation is similar but note that it has a negative charge. So for the same electric field E, the electron will experience a force of the same magnitude but in the opposite direction to that of the proton. The acceleration of the electron will also be much greater due to its much smaller mass compared to the proton.