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Physics Question on Magnetic Force

An electron projected in a perpendicular uniform magnetic field of 3×10−3T,3 \times 10^{-3} T, moves in a circle of radius 4 mm. The linear momentum of electron (in kg−ms−1)(in \, kg-ms^{-1}) is

A

1.92×10−211.92 \times 10^{-21}

B

1.2×10−241.2 \times 10^{-24}

C

1.92×10−241.92 \times 10^{-24}

D

1.2×10−211.2 \times 10^{-21}

Answer

1.92×10−241.92 \times 10^{-24}

Explanation

Solution

\hspace15mm \frac {mv^2}{r}=evB
ie, \hspace15mm r= \frac {mv}{eB}
As for a particle in motion p = mv
\therefore \hspace15mm r= \frac {mv}{eB}= \frac {p}{eB}
\Rightarrow \hspace15mm p=eBr
\therefore \hspace5mm p=1.6 \times 10^{-19}\times3\times10^{-3}\times 4 \times 10^{-3}
\hspace5mm =1.92 \times 10^{-24}kg-ms^{-1}