# Homework Help: Electromagnatics Disperate Help

1. Jan 23, 2005

### Berg

Hi i am in ap physics and we just finished the electric charges and electric fields chapter but i am completely clueless. I havent been keeping up with the problems and havent been to alot of classes and now it has come back to haunt me. Any good sites out there or can anyone answer some of my questions about these topics? I dont really get these chapters at all.

For example: 2 indentical conducting spheres, fixed in place, attract each other with an electrostatic force of .108 N when seperated 50cm. The spheres are then connected by a thin conducting wire. When the wire is removed, the spheres repel each other with an electorstatic force of .0360 N. What were the initial charges on the spheres? Answer is -1 and 3 mirco columbs but i dont know how to get it.

This is the easiest question out of all of my set and i hope if i can understand this the rest will click 2. Also, any good resources on Electirc field due to point, line, disk, charges? thx in advance

2. Jan 23, 2005

### Gokul43201

Staff Emeritus
Assume the charges are Q1 and Q2. Write the equation for the force between two charges (Coulomb's Law).

The effect of connecting the spheres by a wire is that this allows the charges to uniformly ditribute themselves, so that the charges on both spheres will now be equal. Since the total charge (Q1 + Q2) is conserved, each sphere must get half this total.

Now again write the force equation with the new charges.

You have 2 equations in 2 unknowns. Solve to find Q1 and Q2.

As far as study resources, your textbook is often better than most online resources. Nevertheless, I believe there's a thread at the top of this forum, listing useful online resources.

3. Jan 23, 2005

### futb0l

Try googling HyperPhysics, that site should help.

4. Jan 24, 2005

### Berg

Thx for the help guys, I understand charges and Columbs law but i still dont see how it applys to electric fields For example:

Two point charges q1= -5q and q2= +2q are seperated by distance d. Locate the point (or points) at which the electirc field due to the charges is zeo. Sketch the electric field lines.

I do 2 Field due to point Charges eqautions and set them equal with each other because they both equal 0 and then can equal each other but the 1/4pieo and d2 cancel out and i just end up with -5q=+2q... ?? hmm i dont get it.

Another one thats been really bothering me is....
In Millikan's experiment, an oil drop of radius 1.64 micro meters and density 0.851g/cm^3 is suspended in chamber C when a downward pointing electric field of 1.92 x10^5 N/C is applied. Find the charge on the drop in terms of e

Errr would this be a point charge?

Last edited: Jan 24, 2005
5. Jan 24, 2005

### Berg

well i figured out the milken experient but for that other charge one all my charges end up canceling out still i dont quite get it why cant i set them equal to each other?