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
A) ma₀/e west, 2ma₀/(ev₀) up
B) ma₀/e west, 2ma₀/(ev₀) down
C) ma₀/e east, 3ma₀/(ev₀) up
D) ma₀/e east, 3ma₀/(ev₀) down
Explanation
[Solution from source] At rest only the electric force acts: eE = ma₀ westward, so E = ma₀/e towards west. Moving north, the extra 2ma₀ westward comes from the magnetic force: ev₀B = 2ma₀, B = 2ma₀/(ev₀). For a positive charge, v (north) × B must point west, which happens when B points down. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.