# How far apart are two point charges...

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1. Jul 2, 2015

### snowjoe

1. How far apart must two point charges of 75.0 nC be to have a force of 1.00 N between them?

2. Relevant equations F = k Q1Q2/r2

3. 1N = 9e10 * 75e-10 squared/r squared
r2= 9e10 * 75e-10 squared/1N
r2= 9*75*75e-10
r2= 5.0625e-7
r= square root of 5.0625 * square root of e-7

if this is right so far then i just have an
algebra issue, what is the square root of e-7? can it be e-3.5?, if so, what is the value of e-3.5?

2. Jul 2, 2015

### Thyphon

e-7 means 1x10^(-7) so you are right sqrt(e-7)= e-3,5

3. Jul 2, 2015

### SammyS

Staff Emeritus
You also need to include the units for r .

4. Jul 3, 2015

### rude man

Better check your value of k. And Q.
nano = 10-9, not 10-10.

5. Jul 4, 2015

### snowjoe

rude man, isn't an nC a C/e9? which gives nC as 1.6 * e (19-9) 10? value of k i am using 9 as approximation, good enough, k= 9.0 e9

6. Jul 4, 2015

### SammyS

Staff Emeritus
Where does 1.6 come from?

7. Jul 4, 2015

### snowjoe

a Coulomb i take is 1.6 X e19 units of charge

8. Jul 4, 2015

### snowjoe

i grow confuseder, this radius is in meters, no? yet i'mgetting values that are out of the realm of possibility. square the point charges, which are nC, which is on the order of e10, so squared, e20. multiply this by k, which is on the order of e9, getting e29, so the square root of this is the r? That still gives e14.5, in meters, pretty sure no force exists between these charges at several trillion meters. obviously i've gone awry, where?

9. Jul 4, 2015

### ehild

1 nC = 10-9 C. C (coulomb) is the unit of charge, you mix it with the elementary charge, which is 1.6 `10-19 C.

10. Jul 4, 2015

### SammyS

Staff Emeritus
A single electron has a charge of -1.6×10-19 Coulombs .

A single proton has a charge of 1.6×10-19 Coulombs .

These values have nothing to do with your stated problem.

11. Jul 5, 2015

### snowjoe

then my problem is the relationship between electrons/protons and Coulombs. How would one calculate the number of e/p in a fraction of a Coulomb, such as one billionth, a nC?

12. Jul 5, 2015

### SammyS

Staff Emeritus
N protons have a total charge of N×(1.6×10-19)C .

So solve
N×(1.6×10-19)C = one billionth, a nC​

13. Jul 5, 2015

### snowjoe

thank you.

14. Jul 5, 2015

### snowjoe

so, a billionth of a Coulomb is 1.6e-10, and a billionth of the particles in a Coulomb is 6.25e-9?

15. Jul 5, 2015

### snowjoe

N(1.6e-19)=e-9
N=e-9C/1.6e-19C = 6.25e9 = nC ??

then what is the flaw in calculating a nC by dividing C by n, as 1.6e19/e9 = 1.6e10

16. Jul 5, 2015

### SammyS

Staff Emeritus
The question asks force between two point charges.

There is NO mention of how many electrons or how many protons are involved.

Use Coulomb's Law.

17. Jul 6, 2015

### snowjoe

Thanks, SS, But i'm asking about the math, where did i go wrong in calculating a nC by dividing 1.6e19 by e9?

18. Jul 6, 2015

### SammyS

Staff Emeritus
Forget the 1.6×10-19, 1.6×1019, etc. The charge of an electron has nothing to do with this.

nano is a prefix for metric units. nano means 10-9.

1 Coulomb is the basic unit of charge in the SI system of units. It is NOT the charge of an electron or proton.

1 nano-Coulomb is 1×109 Coulombs.

19. Jul 6, 2015

### snowjoe

is it not true that in a Coulomb there are 1.6 e19 units of charge, this unit of charge being the charge carried by one electron or proton?

20. Jul 6, 2015

### SammyS

Staff Emeritus
Yes that's true, but that fact is not needed anywhere in using Coulomb's Law.