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Questions  

A particle of charge q and mass m starts moving from the origin under the action of an electric field E=E0i^ and B=B0i^ with a velocity  v=v0 j^. The speed of the particle will be become 3v0 after a time

a
t=2mv0qE
b
t=2Bqmv0
c
t=3Bqmv0
d
t=(22)mv0qE

detailed solution

Correct option is D

E→ is parallel to B→ and v→ is perpendicular to both. Therefore, path of the particle is a helix with increasing pitch. Speed of particle at any time t is v=vx2+vy2+vz2Here,  in yz plane speed will remain same. Therefore,  vy2+vz2=v02Given :  v=3v0v=vx2+vy2+vz2 ⇒v2 = vx2+vy2+vz2 ⇒(3vo)2 = vx2 + vo2 ⇒vx2 = 8vo2 ⇒vx = 22vo Equation of Kinematics, along x axis vx=ux+axt ⇒22v0=0+qEmt ⇒t=(22)mv0qE.

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