# Homework Help: Direction of electric force

1. Feb 11, 2012

### nopescope

Hi all. I don't know if I'm completely over thinking this question or just not getting it. Please tell me if I'm correct.

A -8.0μC charge is located 0.30m to the left of a +6.0μC charge. What is the magnitude and direction of the electrostatic force on the positive charge?

F=KQaQb/r^2

F=(9x10^9)(-8.0x10^-6)(+6.0x10^-6)/(0.30m)^2 = -4.8N...so answer is 4.8, to the right.

So, I think that because the F is -4.8 means that the direction is going to the right because a negative means that it is an attractive force and the +6 charge will be attracted to the -8 charge. Am I correct?

2. Feb 11, 2012

### Staff: Mentor

Ask yourself: Do the charges attract or repel? Given that answer, in what direction must the force act?

3. Feb 11, 2012

### nopescope

The -8 and +6 will attract because they are opposite charges. So, the positive charge will go to the right.

4. Feb 11, 2012

### Staff: Mentor

Right, they will attract.
Why? If they attract, the force on the positive charge must be towards the negative charge.

5. Feb 11, 2012

### nopescope

Oh, I see. I think drawing a picture of the charges would have helped. So, because the -8 is to the left of the +6, they are attracted to each other the +6 will move to the left towards the -8.

I was originally just looking at the "left" in the question and automatically assuming, "ok, then it has to go right."

Thanks Doc

6. Feb 11, 2012

### nopescope

Also, does the amount of charge have an affect on the direction of the force? For example...say there was a -4 charge located to the left of a +6 charge...would that still be moving to the left?

7. Feb 11, 2012

### Staff: Mentor

No, just the signs.
Sure. The charges have opposite signs, so they attract.

(Of course, if there are more than 2 charges involved, you'll need to figure out the net force on any particular charge.)

8. Feb 11, 2012

### nopescope

Thank you so much! I actually get it now. My teacher was confusing me.