An electron having kinetic energy $T$ is moving in a circular orbit of radius $R$ perpendicular to a uniform magnetic induction $\vec B$ . If kinetic energy is doubled and magnetic induction tripled, the radius will become 
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$r=\frac{m v}{q B}=\sqrt{\frac{2 m T}{q B}}$

$r^{\prime}=\frac{\sqrt{2 m(2 T)}}{q(3 B)}=\frac{\sqrt{2}}{3} r \Rightarrow \sqrt{\frac{2}{9}} R$

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