What is the attractive electric force between the ions?

[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?

 

[Kurdt]

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

 

[predentalgirl1]

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

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?

 

[Kurdt]

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

 

[predentalgirl1]

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

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

 

[Kurdt]

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

 

[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?

 

[predentalgirl1]

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

Why do you have that as a negative?

 

[Kurdt]

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

 

[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

 

[Kurdt]

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.