A particle of mass m and charge q is fastened to one end of a string of length. The other end of the string is fixed to the point O. The whole sytem liles on as frictionless horizontal plane. Initially, the mass is at rest at A. A uniform electric field in the direction shown in then switfched 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)`