Alpha Particles, Each Having a Charge Of $+ 2 e$ And a Mass Of

Multiple Choice

Alpha particles, each having a charge of $+ 2 e$ and a mass of $6.64 \times 10 ^ { - 27 } \mathrm {~kg}$ , are accelerated in a uniform $0.80$ -T magnetic field to a final orbit radius of $0.30 \mathrm {~m}$ . The field is perpendicular to the velocity of the particles. How long does it take an alpha particle to make one complete circle in the final orbit? $\left( e = 1.60 \times 10 ^ { - 19 } \mathrm { C } \right)$

A) $0.49 \mu \mathrm { s }$
B) $0.40 \mu \mathrm { s }$
C) $0.15 \mu \mathrm { s }$
D) $0.33 \mu \mathrm { s }$
E) $0.25 \mu \mathrm { s }$