# TBR physics electrostatics passage 2 question

#### traitorman

7+ Year Member
The question asks what direction an electric field must point in order to keep a drop of oil suspended in between two parallel plates.

Relevant info:
1) oil drop is negatively charged
2) oil drop is suspended between two parallel plates that are powered by a series of batteries to form a capacitor
3) plates are positioned parallel to one another and parallel to the x axis. top plate is positively charged, bottom plate is negatively charged.

Based on this set up, I concluded that because the oil drop is negative, it will be attracted to the top plate and thus the electric field must point upwards in order to suspend the oil drop.

Electric field direction is taken to go from positive to negative correct? So if the electric field points up, the negatively charged oil drop will be attracted downwards thereby suspending the drop in between both plates.

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#### traitorman

7+ Year Member
but according to the answer:

"To decide direction, remember that gravity points downwards, which requires the electric force to point upwards. Since the charges being supported are electrons, and since electrons are always forced in the direction opposite that of the electric field, the electric field must point down."

#### UCB05

Electric fields point away from positive, toward negative. #3 of your "Relevant info" already gives you your answer. Your top plate is positive, your bottom plate is negative. The field directionality is straight down from positive to negative.

If you didn't know #3, you should know that to suspend the negatively-charged droplet against gravity you need an upward force, so you would conclude the top plate had to be positively charged and the bottom plate negatively charged. This would lead you to the same answer above.

An electric field pointing up would have a negative charge on top, positive charge on the bottom, which would pull the negatively charged droplet down.

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#### traitorman

7+ Year Member
I think I see what you are saying. So there are only 2 forces at work right?

1) Gravity pulling the charge down.
2) Electric field pushing the charge up.

So am I wrong to believe there are 3 forces?
1) Gravity pulling the charge down.
2) Electric field pushing the charge up.
3) Negative charge's attraction to positively charged plate. <--- ?

I did not even consider gravity. My logic was that the only force acting was the negative charge's attraction to the positively charged plate. So to counter act this, I thought an external electric field had to applied pointing upwards to make the charge feel attraction downwards.

Do I not even consider the negative charge's attraction to the positively charged plate?

#### UCB05

2 and 3 represent the same property leading to the same single force calculation. An electrical field will help give you the force of charge attraction/repulsion through space. It accounts for both directionality and magnitude of force at a point d distance away from another charge. How do you calculate the force derived from "3) Negative charge's attraction to positively charged plate." ? by using 2). "Do I not even consider the negative charge's attraction to the positively charged plate? " Of course. This is what is represented by the electric field.

Analogous to this EM question, What direction is the gravitational field needed to suspend a droplet experiencing an upward draft? The gravitational field of course points downward because the droplet is gravitationally attracted to the earth. There's no imaginary force needed to counteract gravity. The gravitational field IS what causes gravitational force.

The problem is, forces of electrical attraction are so variable in space compared to the gravitational force of attraction between the hugely massive Earth and a tiny object, we know gravitational acceleration as 9.8m/s^2, but we have to represent electrical acceleration as a field of many values depending on the exact points in space.

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#### traitorman

7+ Year Member
thanks, you definitely cleared that up for me. appreciate the help. as I was writing that post, I was even thinking if 2 and 3 are really that different and how would i measure 3 compared to how I would measure 2. definitely confused myself on this one.

#### BerkReviewTeach

##### Company Rep & Bad Singer
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10+ Year Member
2 and 3 represent the same property leading to the same single force calculation. An electrical field will help give you the force of charge attraction/repulsion through space. It accounts for both directionality and magnitude of force at a point d distance away from another charge. How do you calculate the force derived from "3) Negative charge's attraction to positively charged plate." ? by using 2). "Do I not even consider the negative charge's attraction to the positively charged plate? " Of course. This is what is represented by the electric field.

Analogous to this EM question, What direction is the gravitational field needed to suspend a droplet experiencing an upward draft? The gravitational field of course points downward because the droplet is gravitationally attracted to the earth. There's no imaginary force needed to counteract gravity. The gravitational field IS what causes gravitational force.

The problem is, forces of electrical attraction are so variable in space compared to the gravitational force of attraction between the hugely massive Earth and a tiny object, we know gravitational acceleration as 9.8m/s^2, but we have to represent electrical acceleration as a field of many values depending on the exact points in space.
Thanks UCB05- that was an excellent explanation.