Solving Electric Potential Problem

[verd]

Hi,

So. I'm preparing for a test, and a problem I was given in order to prepare for this test is confusing me to no end. It's very unlike the rest of the problems I was given, and I'm not quite sure how to solve it.

http://www.synthdriven.com/images/deletable/23-4.jpg

I'm on a chapter dealing with what exactly electric potential is, and given this information, I'm not quite sure what to do. I'm to find the electric potential, V, across those plates that'll hold the droplet of oil motionless. However, I have no charge or value for capacitance. ...It tells me that there are "5 electronic charges". However, I don't have a value for a single one.

Does the horizontal component of electric force have to be equivalent to mg or something?

Any tips or ideas?

 

[Kurdt]

The term electronic charge usually means the charge on an electron which is -1.6x10^-19 coulombs. I hope this helps in your solution and understanding of the problem.

 

[Kurdt]

Oh and the potential should create an electric field that will support the weight of the droplet.

 

[verd]

Oh and the potential should create an electric field that will support the weight of the droplet.

Right, I understand that the problem states that, but I don't really know what I'm lookign for. Am I looking for the horizontal component of the electric force to be equivalent to mg??

 

[Kurdt]

The weight of the droplet acts vertically and it is suspended between two horizontal plates. The electric field between the two plates will be 'vertical'. Remember potential difference is given by V=E*d where E is the electric field and d is the distance.

 

[verd]

i thought that the electric field between two horizontal plates was horizontal... not vertical. it goes from the negative plate to the postive plate as in a capacitor...

Besides, I'm not given anything I can find the E field with. Electron=charge on an electron. "electronic charges" just mean charges to me.

 

[Kurdt]

The weight of the oil is the clue for the strength the electric field has to be. Horizontal plates mean that they are parallel with the horizon so the field between them will be perpendicular to horizontal and thus vertical but that is a little besides the point.

What you want to do is find the force acting on the particle of oil. As you correctly indicated this is given by F=mg. Now the definition of electric field is Force/coulomb charge. Now you know that the electric field has to equal the gravitational force to suspend the oil. you also know the oil has the charge of 5*1.6x10^-19. With this information you should be able to plug it into V=E*d to find the voltage.

 

[SGT]

i thought that the electric field between two horizontal plates was horizontal... not vertical. it goes from the negative plate to the postive plate as in a capacitor...

Besides, I'm not given anything I can find the E field with. Electron=charge on an electron. "electronic charges" just mean charges to me.

The equipotential surfaces will be horizontal and parallel to the plates. The electric field is orthogonal to the equipotential surfaces.
Electronic charge is the charge of one electron. Electric charge is any charge.