A particle of mass `m` and charge `q` is fastened to one end of a string fixed at point O. The whole system lies on a frictionless horizontal plane. Initially, the mass is at rest at A. A uniform electric field in the direction shown is then switched on. Then

Options

A. the speed o fthe particle when it reaches B is `sqrt((2qEl)/m)`
B. the speed of the particle when it reaches B is `sqrt((qEl)/m)`
C. the tension in the string when the particle reaches at B is `qE`
D. the tension in the string when the particle reaches at B is zero

Answer

B

Explanation

`W_T=0`
`W_(F_e)=(F_e)` (displacement in the direction of force)
=Kinetic energy of the particle
`:. 1/2 mv^2=qE[l/2-l/2cos60^@]`
`:. v=sqrt((qEl)/m)`