# Homework Help: Two point charges

1. Jan 26, 2013

### phunphysics2

I was wondering if someone could help me with my posted problem. All the necessary details from the template are included in the screenshot on the right.

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• ###### phunphysics.png
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2. Jan 26, 2013

### Staff: Mentor

From the second attachment (the one on the right) it looks as though you need only cross multiply and solve for x to finish.

3. Jan 26, 2013

### phunphysics2

I don't know how to do so mathematically...

4. Jan 26, 2013

### rude man

Draw a picture of the two charges along a line, then equate Coulomb attraction of a unit (positive) test charge to qB to repulsion of that test charge from qA.

Hint: the zero-field point might be between qA and qB, or it might not ...

5. Jan 26, 2013

### phunphysics2

Coulomb's law is not used. My professor said to only use the E=k[q]/rsquared formula...

6. Jan 26, 2013

### phunphysics2

I know that the order of the point charges goes as followed

qb -------------------qa--------------------P

7. Jan 26, 2013

### rude man

Never mind, you called x the distance from p to qB and d the distance between qA and qB, so what you wrote is fine.

8. Jan 26, 2013

### Staff: Mentor

Really?

If $\frac{a}{b} = \frac{c}{d}$ then $a \cdot d = b \cdot c$

You've never seen this?