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An electron and a proton of equal linear momentum enter in a direction perpendicular to a uniform magnetic field. If the radii of their circular paths are rₑ and rₚ respectively, then (rₑ)/(rₚ) is equal to ______. (mₑ= mass of the electron, mₚ= mass of the proton)

Asked in GUJCET 2009 · Circular motion: radius, period and energy

Answer: (2) 1

Step-by-step solution

Given: an electron and a proton with the same linear momentum p, each entering perpendicular to the same field B.

Idea: the magnetic force supplies the centripetal force, qvB=(mv²)/r, so r=(mv)/(qB)=p/(qB).

Written this way the radius depends only on the momentum, the charge and the field — not on the mass separately.

The electron and the proton carry charge of the same magnitude e, and p and B are the same for both.

So rₑ=p/(eB)=rₚ and the ratio is 1.

Why the other options are wrong

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