# Charged objects/potential

1. Jul 3, 2006

A few questions for my homework that I'm having trouble with. I'd like to really grasp these concepts, so any help is appreciated(especially good explainations).

1. When a previously uncharged object is charged to 1.602 microcoulombs....

A. It has an excess of electrons
B. one million electrons have been removed from it
C. 10^13 electrons have been added to it
D. None

---I'm thinking D right now.....as I can't find a way to make the others make sense. I know a microcoulomb is 10^-6 C. The sign isn't negative either....which means charged by protons??

2. If the potential is given by V = xy - 3z^-2, then the electric field has a y-component....

A. x
B. y
C. x + y
D. x + y - 6z^-3

I have no clue on this one.....need some guidance in getting started. I guess I don't understand how to come up with the individual components, using that equation.

Thank you for the help!!

2. Jul 3, 2006

### Staff: Mentor

Realize that for most things the net charge is changed by adding or subtracting electrons, not protons. (The protons are usually fixed in a lattice, while some of the electrons can be easily removed.) In any case, since the net charge is positive, electrons must have been removed. How many?

What's the relationship between electric field and potential? Hint: There's a derivative involved. (Look it up!)

3. Jul 3, 2006

Ok, so I've found this much anyways....not sure if it is helpful to me or not. But I think V(potential) = Intergal(Electric Field). Is this helpful to me at all......if so, I don't know how to do that intergual to find the y-component.

4. Jul 3, 2006

Doc, thanks for the good explaination on the electrons. So basically, they are easier to remove....and my 1.602 is positive, then the answer has to be B or D. I don't understand how it can be B....but assuming it is....how do I come up with 1 million?

I'm still not finding that equation(derivitive)....I've even looked on the net for it.....any help?

5. Jul 3, 2006

### Astronuc

Staff Emeritus
The charge on one electron is -1.602 x 10-19 C (coulombs). By convention the charge is negative, and the charge on the proton is + while being of the same magnitude.

Figure out how many electrons make 1.602 microcoulombs, and compare that to 1 million.

6. Jul 3, 2006

yeah ok, that is what I did beforehand. I took the 1.602 x 10^-19C/1.602 x 10^-6 C = 1 x 10 ^-13. This is not even close to a million, but now makes me wonder about answer C. I don't think it is answer C, due to the electrons need to be removed. So, I'm going to say it is answer D.

Thanks Astronuc!! Any help with that equation I need for the above question?(that is, if I have the correct answer for this question first).

7. Jul 3, 2006

### neutrino

Remember you're trying to figure out the number of electrons that make up 1.602 micro C. Therefore it is Total charge/charge on one electron = 10^13 electrons.

8. Jul 3, 2006

### Staff: Mentor

9. Jul 3, 2006

Ok, well my math is a little rusty on this one(actually, not sure I've been fully taught this stuff)......but from researching it a little more, I found that I just need to kinda break it up into coordinates(and take their derivative). So with that, do I just differentiate xy with respect to y?? If so, I'm not real sure how to do that, as I can't come up with the answer(but if that is what I do, I'll work on it for a bit longer).

10. Jul 3, 2006

### Staff: Mentor

Yes, you'll need to take the (negative of the) partial derivative of V with respect to y to find the y-component of the field.

11. Jul 3, 2006

So the partial derivative of V, with respect to y. That eliminates the z portion of the equation(I think). So I'm left with dy/dx=xy.

I'm going to say the answer is B, or Y.

12. Jul 3, 2006

### Staff: Mentor

Careful. It's true that (partial) dV/dy = x, but Ey = - dV/dy.

13. Jul 3, 2006

So....does that make it a -x(which I doubt). My math on these is terrible. If you don't want to explain it any more, that is fine Doc....I'm not really holding up my end of the deal real well....as my math seems to be failing me.

14. Jul 3, 2006

### Staff: Mentor

That's it.

Your math is OK. What I don't understand is why your homework includes stuff that you haven't covered. (For example, it seems like you haven't covered the mathematical relationship between field and potential.)

Another thing that bugs me: The correct answer for question 2 is not given as a choice. (And for question 1, why didn't they give the actual answer as a choice instead of "NONE"?)

15. Jul 3, 2006