Answer:
- negative z-direction
Explanation:
First of all, we have to determine the direction of the magnetic force acting on the proton.
The direction of the magnetic force acting on a moving charge can be determined by using the right hand rule. For a positively charged particle:
- Index finger: direction of the motion of the particle
- Middle finger: direction of the magnetic field
- Thumb: direction of the force
For a negatively charged particle, the direction of the thumb must be reversed.
In this problem, we have a proton (positive charge), and we have:
- Direction of motion: positive x
- Direction of the field: positive y
Therefore, by using the right-hand rule, we see that the thumb points towards z-positive direction: so, this is the direction of the magnetic force.
Therefore, the proton is deviated towards z-positive direction due to the magnetic force; in order to balance this and make the proton continuing along a straight line, the electric force must be in the z-negative direction.
The electric force experienced by a charged particle has the same direction of the electric field (if the charge is positive) or opposite direction (if the charge is negative): in this problem, the proton is positive, so the direction of the electric field must be the same of the force, therefore
- negative z-direction
