Electric Force on Charged Particle in Uniform Field

Steps:

• The particle is positively charged and enters a uniform electric field, experiencing only the electric force $\vec{F} = q\vec{E}$, as mass is negligible (no gravity) and it's in vacuum (no other forces).
• The field between parallel plates is uniform and perpendicular to the initial velocity, causing deflection without changing speed.
• Initial constant velocity is horizontal; electric force acts vertically downward (assuming standard setup with field pointing down).
• After a short time, the particle deflects downward, confirming the force direction.

Why D is correct:

• D shows the electric force arrow pointing downward, matching $\vec{F} = q\vec{E}$ for a positive charge in a downward-pointing field.

Why the others are wrong:

• A shows no force, ignoring the electric field interaction.
• B shows upward force, opposite to field direction for positive charge.
• C includes horizontal force, but field is uniform and vertical, so no horizontal component.

Final answer: D