A uniform but time varying magnetic field `B=(2t^3+24t)T` is present in a cylindrical region of radius `R =2.5` cm as shown in figure.

The force on an electron at `P` at `t=2.0` s is

Options

A. `96xx10^-21N`
B. `48x10^-21N`
C. `24xx10^-21N`
D. zero

Answer

B

Explanation

`(dB)/(dt)=(6t^2+24)T//s`
At `t=2s, (dB)/(dt)=48T//s`
`El=(dphi)/(dt)=S((dB)/(dt))`
or `E(2pir)=pir^2((dB)/(dt))`
` :. E=r/2.(dB)/(dt)` ...(i)
`F=qE=(qr)/2 (dB)/(dt)`
`=((1.6xx10^-19)(1.25xx10^-2))/2(48)`
`=48xx10^-21N`