An alpha particle and a proton move with the same speed perpendicular to the same uniform magnetic field. Take the alpha particle’s mass as 4 times and its charge as 2 times that of the proton. What is the ratio of the radius of the alpha particle’s path to that of the proton?
Correct answer D. 2
Explanation
r = mv/(qB), so at equal v and B, r ∝ m/q. For the alpha particle m/q = 4/2 = 2 times that of the proton, so r_α/rₚ = 2.
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