Q.

When a proton is released from rest in a room, it starts with an initial acceleration a0 towards 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

1 ma0e east, 3ma0ev0 down  
2 ma0e west, 2ma0ev0 up  
3 ma0e west, 2ma0ev0 down  
4 ma0e east, 3ma0ev0 up  

Ans.

(3)

Acceleration of charged particle      a→=qm(E→+v→×B→)

Released from rest⇒ a→=qmE→=a0 (west)    ⇒    E→=ma0e (west)

When it is projected towards north, acceleration due to magnetic force=2a0

Therefore magnetic field=2ma0ev0 (down),    

Direction of magnetic field F→m→(v→×B→)-i^→(j^×-k^), 

 The direction of magnetic force is towards west, for that the direction of magnetic field is vertically downward (-k^)