24) For this question we have a proton moving at a constant velocity up the plane of the page. Additionally, there is a magnetic field going into the plane of the page. Our job is to find the direction of the electric field acting on the proton.
Firstly, we should notice that problem states that the proton moves at a constant velocity.
This tells us that the magnetic force produced by the magnetic field must cancel out with the electric force.
Let's find the direction of the magnetic force using the right-hand rule. If you are unfamiliar with it, you can check this link out to learn more. We will be working in the 3d coordinate system, and we want to make sure we set a coordinate reference frame.
This is a great step, but not our final answer. This is the direction of the magnetic force. Coming back to the constant velocity of the proton, we know that in order to have a zero net acceleration the electric force must point in the opposite direction as the magnetic force. The electric force will point in the +y direction(to the right).
Our question asks about the electric field. For positive charges, the electric field points in the same direction as the electric force. However, for negative charges, the electric field will point in the opposite direction as the electric force.
Since we have a proton, our electric field and force point in the same direction(right).
Answer: B
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