# Electrostatic force-Coulombs law

1. Jul 1, 2010

### Thrax

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

Hi, Im working on the following question.
A 64 µC charge is located 30cm to the left of a 16 µC charge. What is the resultant force on a -12 µC charge positioned exactly 50mm below the 16 µC charge?

2. Relevant equations
Coulombs law

3. The attempt at a solution
I see a right triangle So I tried to solve using the vector components as follows
Fac=9.0e9(65x10e-6 * 12x10e-6)/(.3^2+.05^2) = 74.72C
Fbc = 9.0e9(16x10e-6 * 12x10e-6)/(.05^2) = 691.2C

For the rest, I'm not sure if I'm attacking this thing properly to solve for the vectors? I've tried at least 6 times... but no luck.

I could use a little direction...I attached a picture of the problem. My be helpful if it show's up.

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2. Jul 2, 2010

### kuruman

You have calculated the magnitudes of the vectors. Now you need to add them as vectors to find the resultant force. If you tried this without success, you need to show what you did so we can point out where you went wrong. By the way, force is measured in Newtons, abbreviation N, not C.

3. Jul 2, 2010

### Thrax

OK...heres what I've done. Flipped the triangle a little setting C and B on the x axis. I'm new to all this stuff so please forgive my ignorance.
Fbc = 691.2
Fac = 74.72

Fbcx = 691.2
Fbcy = 0

Facx = 74.72N cos(80) = 12.28
Facy = 74.72 sin(80) = 73.58

Fbcx + Fbcy
691.2 + 12.28 = 703.48
Fbcy + Facy
0 + 12.28 = 73.58
Finally
Fc = Sqrt(703.48 ^2 + 73.58^2) = 707.32N
Not sure where I’m going wrong…but I know I’m incorrect.

4. Jul 2, 2010

### Staff: Mentor

I didn't check your arithmetic, but your basic method looks OK. How do you know it's incorrect?

5. Jul 2, 2010

### Thrax

The text book I have gave an answer. According to them the correct answer is 2650N, 113.3deg.

6. Jul 2, 2010

### kuruman

I get 707.3 for the resultant. Looks like you are OK here.

7. Jul 2, 2010

### Staff: Mentor

The textbook answer is way off. (What textbook?)

8. Jul 2, 2010

### kuruman

Unless both you and I made the same mistake exactly, the problem as stated has the answer that you gave. Are you sure you posted the problem as it appears in the book? If yes, then you need to consider that sometimes the back of the book has the wrong answers. In physics if you're right, you're right.

9. Jul 2, 2010

### Thrax

The text book is called. "Physics" 7th edition by Paul E. Tippens. Mc Graw Hill

10. Jul 2, 2010

### Thrax

Yes, I typed it exactly as it appears.

11. Jul 2, 2010

### Thrax

You spend so much on text books...You'd think the least they could do is edit!

12. Jul 2, 2010

### Thrax

If you guys think it’s OK...than its OK with me! Thanks for taking a look!