Electric Forces Homework: Calculate Two Point Charges Using Coulomb's Law

[dwn]

Homework Statement

Two point charges are 3 m apart and their combined charge is 20µC. If they
repel each other with a fore of 0.075 N, what are the two charges? Ans: 15 μC and 5μC


Homework Equations

F=k(q1*q2)/r^2

(could someone mention in their response how we use the math text in our posts, so that I can make it more convenient for the reader)

The Attempt at a Solution

I'm really not so sure where to begin here, but I made a few attempts by manipulating both q₂ and q₂. I will post one of them below:

9*10⁹ (q₁+q₂)/3²=0.075N

 

[Doc Al]

F=k(q1*q2)/r^2

Good.

(could someone mention in their response how we use the math text in our posts, so that I can make it more convenient for the reader)

Check out the FAQ, under "SITE INFO/Frequently Asked Questions" at the top of every page.

The Attempt at a Solution

I'm really not so sure where to begin here, but I made a few attempts by manipulating both q₂ and q₂. I will post one of them below:

9*10⁹ (q₁+q₂)/3²=0.075N

The one mistake you made was to use q₁+q₂ instead of q₁*q₂.

The second equation you'll need is q₁+q₂ = ??

 

[dwn]

I tried that in one of my attempts, but I don't see the "bridge" between the two equations.

If I solve Coulombs equation for q1*q2, I don't see where I'm supposed to plug in q1+q2 = 20 into the other equation...?

 

[Doc Al]

I tried that in one of my attempts, but I don't see the "bridge" between the two equations.

If I solve Coulombs equation for q1*q2, I don't see where I'm supposed to plug in q1+q2 = 20 into the other equation...?

You'll have two equations and two unknowns. Solve them simultaneously, using any method you like. (Such as substitution.)