Magnetic Force on Moving Charges

Steps:

• Electrons move horizontally with velocity $\vec{v}$ perpendicular to the magnetic field $\vec{B}$ between poles.
• Force on charged particle: $\vec{F} = q (\vec{v} \times \vec{B})$, where $q$ is negative for electrons.
• Apply right-hand rule to $\vec{v} \times \vec{B}$ for positive charge, then reverse direction for negative $q$.
• In standard setup (beam rightward, $\vec{B}$ into page), force points out of the page.

Why B is correct:

• Negative charge reverses Lorentz force direction, deflecting electrons out of the page per right-hand rule.

Why the others are wrong:

• A: Matches positive charge deflection; electrons go opposite.
• C: Deflection perpendicular to both $\vec{v}$ and $\vec{B}$, not toward poles.
• D: Same as C; force not along field lines.

Final answer: B