Particle motion in electric field reveals charge sign

Steps:

• Electric field points from positive plate to negative plate.
• Charged particles accelerate opposite to field if negative, along if positive.
• Particle moves toward plate X, so X is positive plate.
• Negatively charged particle attracted to positive plate X.

Why B is correct:

• Electron has negative charge (-e), so force F = qE directs it toward positive plate per Coulomb's law.

Why the others are wrong:

• A. Alpha-particle: Positive charge (+2e), moves toward negative plate.
• C. Neutron: Neutral charge, no force in electric field, remains at rest.
• D. Proton: Positive charge (+e), moves toward negative plate.

Final answer: B