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A particle of charge ‘q’ and mass ‘m’ starts moving from the origin under the action of an electric field  E=2E0i^ and B=B0i^ with a velocity v=v0j^ . The speed of the particle will becomes 2v0 after a time.

a
t=2mv0qEo
b
t=2Bqmv0
c
t=3Bqmv0
d
t=3mv02qEo

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 given by V=Vx2+Vy2+Vz2here,  Vy2+V22=V02   at all times because the yz components will remain same.Only the x component will increase due to electric field.After some time, it is given that   V=2VoUsing,  V=Vx2+Vy2+Vz2 ⇒2Vo=Vx2+Vo2 ⇒Vx = 3Vo Equation of kinematics,  Vx=Ux+axt ⇒3V0=q2EoMt⇒t=3MV02qEo .

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