Answer
$1.4\times 10^{-7}\ s$
Work Step by Step
Given that:
Radius of circular path $r=15\ cm = 0.15\ m$
We know that the speed of an electron $v=6.7\times 10^{6}\ m/s$.
Also, time period of the motion $T=\frac{2\pi r}{v}$.
Therefore;
$T=\frac{2\pi \times 0.15\ m}{6.7\times 10^{6}\ m/s}$
$T=1.4\times 10^{-7}\ s$
