# Homework Help: Electrostatics homework help

1. Apr 18, 2012

### domyy

1. The problem statement, all variables and given/known data

two equal charges repel one another with a f = 4.0x 10^-4N when they are 10cm apart. If they are moved until the separation is 5.0 cm, the repulsive force is...

2. Relevant equations

f= kq1q2/r^2

3. The attempt at a solution

I already calculated and I just would like someone to tell me if 16x10^-4 seems correct.
I calculated q to then find force at 5.0cm. Is that right?

Last edited: Apr 18, 2012
2. Apr 18, 2012

### domyy

Re: electrostatics

Anyone?
I just would like to know if my answer is correct.

3. Apr 18, 2012

### domyy

Re: electrostatics

if it´s not correct just say no. that´s all I want. I want to know if my answer is correct. that´s all ) PLease?!

Last edited: Apr 18, 2012
4. Apr 18, 2012

### darkxponent

hello domyy!
This is not the way one asks help at physicsforums. We are not here to solve your problem and check whether the answer is correct or not. We can help through the problem. Answer would definitely come out correct if your approach is right. You show us your attempt then only we can help. We are not going to solve and check the answer.

5. Apr 18, 2012

### domyy

Re: electrostatics

I calculated for q using the force given when they were 10cm apart and got 44.4 x 10^-13. With the value, I used it to calculate the repulsive force when they were 5.0cm apart.

6. Apr 18, 2012

### domyy

Re: electrostatics

Would that be correct? would that be the correct approach?
q = fr^2/k = 4.0x10^-4 x 1.0 x 10^-2m/9x10^9Nm^2/c^2 = 0.44 x 10^-15.
f = (9x10^9)(0.44 x 10^-15)/0.0025m = 1584 x 10^-6

Ok, i just changed my answer significantly. i think i am supposed to use m instead of cm, so i just converted. =/
Now, that´s my final answer. What do you think?

Last edited: Apr 18, 2012
7. Apr 18, 2012

### SammyS

Staff Emeritus
Re: electrostatics

That's a lot more work than is necessary.

Simply use the fact that the electric force follows an inverse square law.

The force exerted upon one charge by another charge is inversely proportional to the square of the distance of separation.

8. Apr 18, 2012

### domyy

Re: electrostatics

ok, the square of the distance of separation = 0.0025m. Is that what you mean?
So my answer makes no sense?

9. Apr 18, 2012

### SammyS

Staff Emeritus
Re: electrostatics

My point is that the force becomes 4 times as great as it was initially, because the separation distance is halved.

10. Apr 19, 2012

### domyy

Re: electrostatics

Hello, thank you for the help.
Do I have to convert cm to m when calculating the distance or not?

11. Apr 19, 2012

### domyy

Re: electrostatics

If the force becomes 4 times a greater, that means 16x10^-4 would be correct.
You havent yet said whether my answer is right or wrong. That´s all asked. That´s all i needed to know.
For the answer above, i didnt convert cm to m for distance. Do i need to convert? because that would serve the answers for some other problems about the same topic.

12. Apr 19, 2012

### SammyS

Staff Emeritus
Re: electrostatics

Yes, of course ... unless you use a value for k that is in units of N∙cm2/C2 .
Your final answer is correct, but your intermediate values would not be correct, if you used centimeters rather than meters.

Furthermore, When you were calculating the charge, q, what you were finding was q2.

If this was work I was grading, I would deduct points for these incorrect steps.

13. Apr 20, 2012

### domyy

Re: electrostatics

To find q1, I would use the value of q2 if I tried to find all the values?

Also, I am confused because if my final answer is correct which would be 1584 x 10^-6, and I didn´t calculate q1, then, how would it be correct?

In case I don´t have both q´s, I thought I could have: q1q2 = q. Someone solved this problem by using this concept.

Is it correct?

Last edited: Apr 20, 2012
14. Apr 20, 2012

### domyy

Re: electrostatics

I am still confused about how I should leave my answer. This all I need to finish this chapter and start the homework from another. I still have a long way to go. Please, can I leave 1584 x 10^-6 as my answer?