A proton moving in a region of a space does not experience any acceleration in the direction of motion. Which of the following statements is correct with respect to this situation? A) The electric and magnetic fields must be perpendicular to each other in that region. B) The electric and magnetic fields must be parallel to each other in that region. C) Both electric and magnetic fields must be zero in that regionD) The electric field must be zero but not the magnetic field in that regionE) The magnetic field must be zero but not the electric field in that region

A) The electric and magnetic fields must be perpendicular to each other in that region

Explanation:

The proton does not experience any acceleration: this means that the net force acting on it is zero.

Therefore, this also means that the electric force and the magnetic force acting on the proton are balanced.

Let's remind that:

- The electric force acting on a positive charged particle has the same direction as the electric field:

F=qE

where F is the force, q is the charge, E is the electric field

- The magnetic force acting on a positive charged particle in motion is perpendicular to the direction of the magnetic field:

F=qv ∧ B

where q is the charge, v is the velocity of the particle, B is the magnetic field

Therefore, for the two forces to be along the same line (but in opposite directions), we must have that the electric field E and the magnetic field B are perpendicular to each other.