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Q.

When a proton of charge 'e' mass 'm' is released from rest in a room, it starts to move with initial uniform acceleration a0 toward West. When it is projected towards North with a speed v0, it moves with an initial acceleration 3a0 towards West. The electric and magnetic fields in the room are respectively 

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a

ma0e East, 3ma0ev0upwards

b

ma0e East, 3ma0ev0downwards

c

ma0e West, 2ma0ev0upwards

d

ma0e West, 2ma0ev0downwards

answer is B.

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Detailed Solution

detailed_solution_thumbnail

Given data

Charge=e

Mass=m

Acceleration towards west

Two forces =3ma0

Projected towards north

Force =F

Concepts used moving charges and magnetism

Explanation

Electric force on the proton 

F=qE

Using Newton's second law

F=ma

Both forces are equal

eE=ma

E=ma0e along west

For initial given that

F1+F2=3ma0

ma0+F2=3ma0

F2=2ma0

Maximum magnetic force =ev0B

ev0B=2ma0

B=2ma0ev0 down

as force is along west velocity is along north according to 

F=q(v×B)

if west is along -i^ north is along j^ B must be along -k^

so B is downward.

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