Magnetic force on beta particles via right-hand rule for negative charges

Steps:

• Beta particles are electrons with negative charge, moving vertically downward (velocity vector points down).
• Assume magnetic field between poles is horizontal, directed rightward (standard setup, perpendicular to velocity).
• Compute v × B: downward v crossed with rightward B yields force direction out of page for positive charge.
• Reverse for negative charge: force points into the page.

Why C is correct:

• The Lorentz force law F = q(v × B) dictates that for q < 0, the force opposes the positive-charge direction, resulting in "into the page" perpendicular to both v and B.

Why the others are wrong:

• A: Vertical upward force would require B aligned with v, violating perpendicularity.
• B: Vertical downward force would align with v, but magnetic force is always perpendicular to v.
• D: Out-of-page force applies to positive charges, not negative beta particles.

Final answer: C