A charged particle enters a non-uniform uni-directional field such that initial velocity is parallel to magnetic field, then the radius of curvature of its path is (in standard notation):
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A charged particle enters a non-uniform uni-directional field such that initial velocity is parallel to magnetic field, then the radius of curvature of its path is (in standard notation):

G1 Physics · Accepted Answer

Magnetic force is qv⃗×B⃗. If v⃗ is parallel to B⃗ initially, the force is zero, so the particle continues along a straight line. The radius of curvature is therefore infinite. Hence C.