An electron (mass `=9xx10^(-31)kg`, charge= `1.6xx10^(-19)C)` enters a magnetic field with velocity `10^(6)m*s^(-1)` and starts rotating in a circular path of radius 10 cm. What is the magnetic field ?

Explanation

The electron is rotating in a circular path i.e. the direction of velocity of the electron is perpendicular to the direction of magnetic field. So that magnetic force `qvB` , which provides the necessary centripetal force for the circular motion.
`:.qvB=(mv^(2))/(r)` [ r= radius of the circle path 10cm =0.1m]
or `,B=(mv)/(qr)((9xx10^(-31))xx10^(6))/((1.6xx10^(-19))xx0.1)=5.6xx10^(-5)T`.