What is the expression for the magnetic force on a moving charge in a uniform magnetic field?

• A. F = q E
• B. F = B I L
• C. F = q v B sin theta
• D. F = q v / B

Answer: (C)

The magnetic force on a moving charge comes from the Lorentz force F = q v × B. Because it’s a cross product, the force depends on the angle between the velocity and the magnetic field. Its magnitude is F = q v B sin theta, where theta is that angle. This means the force is zero when the velocity is parallel to the field (theta = 0), and it’s maximal when the velocity is perpendicular to the field (theta = 90°).

This matches the chosen expression because it explicitly includes sin theta and the factors q, v, and B. The other forms describe different situations: F = qE is the electric force, F = I L B (with a sine factor if needed) is the force on a current-carrying wire, and F = q v / B isn’t a valid expression for magnetic force.