An electron is moving in a circle of radius r in a uniform magnetic field B. Suddenly the field is reduced to `B/2`. The radius of the circular path now becomes.

Options

A. `r/2`
B. 2r
C. `r/4`
D. 4r

Answer

B

Explanation

As the electron is moving in the circle of radius r in the uniform magnetic field B,
`:.(mv^2)/r=evB" or "r = (mv)/(eB)`
where m and v is the mass and velocity of the electron respectively.
When the field reduces to `B/2` , the radius of circular path becomes
`r'=(mv)/(e(B/2))=2((mv)/2)=2r`