1. Jul 18, 2010

### Ush

1. The problem statement, all variables and given/known data

The question, along with my attempt is in the image attached. P

2. Relevant equations

E = KQ/r2
E = electric field
Q = source charge
r = distance

3. The attempt at a solution

see document attached

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thank you!

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• ###### attempt.jpg
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2. Jul 19, 2010

### Onamor

Well I haven't run through the numbers, but theoretically it seems possible.
The electric field at whatever point is what would act on an imagined point positive charge, so with that in mind:

You have q1 pulling the charge left, and q3 pushing it also left. q2 has no horizontal affect.

Then you've done the nice Pythagoras stuff and found that q1 pulls the charge down, but q2 and q3 push it upwards. Since q2 + q3 have a greater magnitude than q1, they will win.

Pythagoras-ing horizontal and vertical components gives your perfectly depicted arrow, and a magnitude that agrees in sign with this hand waving argument.
Was there something you didn't understand?

Last edited: Jul 19, 2010
3. Jul 19, 2010

### Ush

to me, it makes sense. But to the computer marking my answer, it does not =[

4. Jul 19, 2010

### Onamor

Hmm, do you know what the correct answer is?

I got a different answer to yours with just the superposition equation:

It may just be a computational mistake - when you are calculating the components, you have them on the order nC (x10^-9 in SI units). But the Coulomb constant is

So they should not be of this order.

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• ###### cimg336.gif
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5. Jul 19, 2010

### Ush

I checked over my calculations but i get the same number, I don't know what the correct answer is

I'm not sure how you did your superposition,
but that just looks like the equation for E, - the sum of the electric fields. Electric fields are vectors, you can't add them without breaking them into components- at least I don't know how to yet-

6. Jul 19, 2010

### Ush

7. Jul 19, 2010

### Staff: Mentor

8. Jul 19, 2010

### Ush

the computer keeps marking it wrong though =/ this seems like a simple question..

9. Jul 19, 2010

### Staff: Mentor

Often the online systems are fussy about significant figures--some of them insist on 3 sig figs no matter what.

10. Jul 19, 2010

### Ush

gosh.. thanks o_o it worked!