Electric field

[Postictal Raiden]

Question #165 page #137 of EK physics is asking about the direction of the electric field.

A charged particle is allowed to fall through the electric field created by the plates as shown (two parallel plates, the right one is negatively charged and the left one is positively charged). In order to give the oil drop a straight trajectory, a magnetic field should be established with field lines pointing:


A. left to right
B. Right to left
C. out of the page
D. Into the page













Answer: C. I thought it should be D. I followed the right hand rule, pointing my thumb down, my palm facing the direction of the magnetic field (right), and my fingers where pointing into the page. I thought since the right plate is negative, the electric field must be pushing toward the positive plate (left). To compensate, the magnetic field must push in the opposite direction of the electric field (right).

I looked at the solution and noticed that my reasoning was right except that the electric field is pushing to the negative plate (right). Why is that? shouldn't opposite attract and like repel?

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[blueshockey]

In my experience the electric field always points from the source of the positive charge to the negative charge. I'm not looking at the diagram but from the first part of the question "the charged particle is allowed to fall through the electric field as shown". Were there arrows?

 

[rjosh33]

Were we to assume the charge on the drop is positive?

 

[Postictal Raiden]

In my experience the electric field always points from the source of the positive charge to the negative charge. I'm not looking at the diagram but from the first part of the question "the charged particle is allowed to fall through the electric field as shown". Were there arrows?

only one arrow, extended from the particle, pointing downward.

 

[pm1]

only one arrow, extended from the particle, pointing downward.

Your thumb goes on the direction of the Electric field. The charge would be accelerating from one plate to the other, but it would be the magnetic force that would drag the particle down. (Please someone correct if I am wrong).

This is how I would do it: Thumb pointing rightward (just because convention is a positive charge) then in order for magnetic field force to be pointing downwards I want my palm to be facing downwards, this way my fingers will be pointing out of the page.

 

[rjosh33]

Were we to assume the charge on the drop is positive?

I was looking around and I found the answer to my own question -- it doesn't matter. Whichever direction works for a positive charge will work for a negative charge too (TBR tests this concept a couple times in Section VIII).

OP, I'm not sure if you've gotten your answer or not, but to begin, electric field lines go from positive charge to negative. Always. This means a positive test charge (or cation) placed in an electric field will move along the field lines, whereas a negative charge (like an electron) will naturally move in the opposite direction of the field lines, because it's attracted to the positive charge.

So, let's say the oil drop has a negative charge on it. It will therefore naturally be attracted to the positive plate, or the left side. So to make it go through undeflected, the magentic force must be to the right. Using the left hand rule (LHR because it's a negative charge, I use RHR for positive particles) where my thumb points in the direction of the magnetic force (right), my fingers pointing straight out in the direction of the drop's velocity (down), and seeing where my palm is pointing that represents the magentic field (out of the page). Therefore, this is the answer -- out of the page.

If you assume the particle is positively charged (thus pulling it to the right side with the negative plate) and use the RHR with the same conventions as above, you'll see you get the same answer -- the field points out of the page.