# Homework Help: What is the attractive electric force between the ions?

1. Feb 5, 2008

### predentalgirl1

[SOLVED] Electric Force Question

The distance between neighboring singly charged sodium and chloride ions in crystals of table salt (NaCl) is 2.82 x 10^-10 m. What is the attractive electric force between the ions?

By coulomb’s law,
Electric force F = [1/4πεo] q1 q2/ r^2
= 9.0 x 10^9 (1.6 x 10^-19) / (2.82 x 10^-12)^2
= 2.9 x 10^9 N (force of attraction)

Correct????

2. Feb 5, 2008

### Kurdt

Staff Emeritus
One is positively charged and the other is negatively charged. Is the distance 2.82x10-10m or 2.82 x10-12m?

3. Feb 5, 2008

### predentalgirl1

The distance is supposed to be 2.82 x 10^-10. So........ the answer should be 1.26 x 10^10 N??? Did I work this question correctly or is it all wrong?

4. Feb 5, 2008

### Kurdt

Staff Emeritus
Your first answer was in the right area, but I got it to be -2.9x10-9N.

5. Feb 5, 2008

### predentalgirl1

Could you show me how you got that answer? Because I'm still coming up with the same answer.

6. Feb 5, 2008

### Kurdt

Staff Emeritus
I think you've just forgot to square the charge. Remember its $q_1q_2=(1.6x10^{-19})^2$.

7. Feb 5, 2008

### predentalgirl1

I'm just not getting this for some reason and it seems so simple...now i'm getting 2.89 x 10^-1. Where am I making a mistake? Could you please help me work through this problem?

8. Feb 5, 2008

### predentalgirl1

Why do you have that as a negative?

9. Feb 6, 2008

### Kurdt

Staff Emeritus
An attractive force is negative. One of the ions will be negatively charged and the other will be positively charged giving a negative answer.

(9x109*1.6x10-19*1.6x10-19)/(2.82x10-10)2

10. Feb 6, 2008

### predentalgirl1

Ok...I reworked it and got 2.9 x 10^-11 (so do I just add a negative since an attractive force is negative?) Is this right?

Edit: 2.9 x 10^-9...or -2.9 x 10^-9

Last edited: Feb 6, 2008
11. Feb 6, 2008

### Kurdt

Staff Emeritus
Yes. That would be correct. the negative comes from the fact that one of the charges has to be negative. Rather than include it in the calculations i just remember that the answer will be negative once I've plugged all the numbers in.