A uniform magnetic field acts at right angles to the direction of motion of electrons. As a result, the electron moves in a circular path of radius $2\, cm$. If the speed of the electrons is doubled, then the radius of the circular path will be.....$cm$
AIPMT 1991, Easy
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(c) $r = \frac{{mv}}{{qB}} \Rightarrow r \propto v,\;\,\; \Rightarrow {r_2} = 2{r_1} = 2 \times 2 = 4\,cm$
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