Two conducting spheres with initial charges attracted then repelled

[Berg]

Hi i am in ap physics and we just finished the electric charges and electric fields chapter but i am completely clueless. I haven't been keeping up with the problems and haven't been to a lot 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 don't 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 separated 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 don't 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? thanks in advance

 

[Gokul43201]

For example: 2 indentical conducting spheres, fixed in place, attract each other with an electrostatic force of .108 N when separated 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 don't know how to get it.

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.

 

[futb0l]

Try googling HyperPhysics, that site should help.

 

[Berg]

Thx for the help guys, I understand charges and Columbs law but i still don't see how it applys to electric fields For example:

Two point charges q1= -5q and q2= +2q are separated 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 equations 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 don't get it.

Another one that's 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?

 

[Berg]

well i figured out the milken experient but for that other charge one all my charges end up canceling out still i don't quite get it why can't i set them equal to each other?