# Electric Fields involving Disks

1. Sep 11, 2008

### demonhunter19

1. The problem statement, all variables and given/known data
A thin circular sheet of glass of diameter 6 meters is rubbed with a cloth on one surface, and becomes charged uniformly. A chloride ion (a chlorine atom which has gained one extra electron) passes near the glass sheet. When the chloride ion is near the center of the sheet, at a location 0.5 mm from the sheet, it experiences an electric force of 8e-15 N, toward the glass sheet.
It will be useful to you to draw a diagram on paper, showing field vectors, force vectors, and charges, before answering the following questions about this situation.

• The charged disk is the source of the electric field that causes the force on the chloride ion.

The force on the chloride ion is equal to the electric field of the glass sheet.

The electric field that acts on the chloride ion is due to the charge on the glass sheet and to the charge on the chloride ion.

The electric field of the glass sheet is equal to the electric field of the chloride ion.
The net electric field at the location of the chloride ion is zero.

2. Relevant equations
Electric Field of a Disk:
Electric FORCE due to some charge: F=qE

3. The attempt at a solution
I'm not sure which statement is correct, but it seems like the first three (The charged disk is the source of the electric field that causes the force on the chloride ion. The force on the chloride ion is equal to the electric field of the glass sheet. The electric field that acts on the chloride ion is due to the charge on the glass sheet and to the charge on the chloride ion) are correct.

I know the electric force exerted on the Chlorine atom, which is 8e-15N, and I set it equal to (-1.6e-19)(Q/(6^2*pi))/2*ε), which is really the relative equation between the electric force and the electric field, F=qE, q being the charge.

The answer I got for Q (the charge of the disk, which is what the 2nd part of the question is asking) is -.000025C. I'm not sure if it's right, but given the relative size of the disk, it makes sense.

And I'm still not certain about which statements above are correct (1st part of question)

Last edited: Sep 11, 2008
2. Sep 11, 2008

### Hootenanny

Staff Emeritus
Welcome to PF demonhunter19,

Let's take a look at the first three statements then:
I agree
What are the units of force? What are the units of the electric field?
Suppose we have a completely empty region of space, devoid of any matter, energy or fields. Suppose then we place a single point charge in this region. Would this create an electric field in the region?

3. Sep 11, 2008

### demonhunter19

Right, the 2nd statement cannot be true, since Force is in Newtons, whilst the electric field is in Newtons/Coulombs. As for the 3rd statement, I'm not entirely sure, but I would imagine that at any time or space, there will always be an electric field created by a charge.

4. Sep 11, 2008

### Hootenanny

Staff Emeritus
Correct
Again correct, does this electric field act on the point charge itself?

5. Sep 11, 2008

### demonhunter19

I don't think so. It would also act on other objects (assuming that there are other objects within that electric field).

6. Sep 11, 2008

### Hootenanny

Staff Emeritus
Correct . So how can the field that acts on the chloride ion be due to the chloride ion?

7. Sep 11, 2008

### demonhunter19

Ah, now I see the mistake! Thanks. So the only statement that is correct out of the 5 is the first one.

8. Sep 11, 2008

### Hootenanny

Staff Emeritus
No problem, sounds about right to me.

9. Sep 11, 2008

### demonhunter19

Now I'm just stuck on finding the charge of the glass disk.

We are given the amount of electric force acting on the chlorine ion, which has a charge of -1.6e-19.

And with the equation that relates electric force and electric field, F=qE, I can just set the equations up like this:

8e-15=(-1.6e-19)(Q/(3^2*pi))/(2*8.85e-12) and solve for Q, right? Then again, that's an approximation, and I'm not entirely sure if what I did right there is correct (since webassign is telling me that that is the wrong answer [-.000025]). Could it be the sign?

edit: Wait, it might be the sign, I just realized the chlorine ion has a negative charge, and the electric force is TOWARDS the glass disk. Which would also mean that the disk is positive? Need some help for this ;[

Last edited: Sep 11, 2008
10. Sep 11, 2008

### Hootenanny

Staff Emeritus
Your solution looks fine to me, except for the sign. As you correctly point out in your edit, the disk is positively charged. You also might want to check if there is a required number of significant figures, WebAssign is notoriously fickle about significant figures.

11. Sep 11, 2008

### demonhunter19

Yep, thank you for your help, Hootenanny!