Answer:
Negative z-direction
Explanation:
First of all, we need to understand the direction of the magnetic force on the proton. This can be determined by using the right hand rule. So we have:
- index finger: direction of the proton, positive x-direction
- middle finger: direction of magnetic field, positive y-direction
- thumb: direction of the force, positive z-direction
In order to balance this magnetic force, the electric force must act in the opposite direction (negative z direction). Since for a proton (positive charge) the force and the electric field have same direction, it means that the electric field must also be in the negative z direction.
