Q 12-04-021NEETJEE MainMedium
A charged particle enters a region of uniform magnetic field at right angles to the flat boundary of the region and to the field. It moves in a semicircle and leaves the region. If its speed is doubled, the time it spends in the field
Answer: (B) remains the same
The time for a semicircle is half the period: $t = \dfrac{\pi m}{qB}$.
This does not depend on speed. A faster particle moves on a larger semicircle, and the longer path exactly balances its higher speed.
Solution by Sreeraj P, M.Sc Physics