Simple Coulomb's Law Question

[p.ella]

Simple Coulomb's Law Question Please Help!

Homework Statement

Two identical charged objects have charges of 6μC and -2μC. When placed a d distance apart, their forces of attraction is 2N. If the objects are touched together, then moved to a distance of separation of 2d, what will be the new force between them?

Homework Equations

F= k((q1)(q2))/r^2

The Attempt at a Solution

The answer is 0.17 (repulsive) in the back of the book. I think touching the objects together is supposed to have an affect, but not sure what it is.

Any help is much appreciated folks! :)

 

[Dick]

Homework Statement

Two identical charged objects have charges of 6μC and -2μC. When placed a d distance apart, their forces of attraction is 2N. If the objects are touched together, then moved to a distance of separation of 2d, what will be the new force between them?

Homework Equations

F= k((q1)(q2))/r^2

The Attempt at a Solution

The answer is 0.17 (repulsive) in the back of the book. I think touching the objects together is supposed to have an affect, but not sure what it is.

Any help is much appreciated folks! :)

Touching the objects together means allowing charge to flow between them. So after touching you will wind up with two objects of the same charge. What will be that final charge?

 

[p.ella]

Touching the objects together means allowing charge to flow between them. So after touching you will wind up with two objects of the same charge. What will be that final charge?

Would the final charge of each be 2C ?

 

[Dick]

Would the final charge of each be 2C ?

Yes. Well, 2μC anyway. But I know what you mean. The rest of the problem is just ratio and proportion, right?

 

[p.ella]

Yes. Well, 2μC anyway. But I know what you mean. The rest of the problem is just ratio and proportion, right?

Pretty much yea. Thank you very much! :)

 

[p.ella]

Yes. Well, 2μC anyway. But I know what you mean. The rest of the problem is just ratio and proportion, right?

hey sorry, but I'm still not getting the right answer for some reason, here's my solution (see picture)

 

[p.ella]

nevermind, I realized my method was wrong and got the answer. Thanks again!

 

[Dick]

aorry I just realized I never canceled out the r^2...is my method correct though?

It's simpler if you just pay attention to what changes. That's the ratio and proportion trick. k and r^2 and μC and all the other units don't change. Divide the two forces by each other and they all cancel. All that changes is (-6)(2) becomes (2)(2) and you get a 1/4 factor from the change in r. What's the ratio of the two forces?