# Homework Help: Electric Field

1. Apr 3, 2013

### Pruddy

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

Two point charges Q1 = -58.6 mC and Q2 = +35.4 mC are located so that Q1 is directly above Q2. The coordinates of Q1 is (-12.0 cm, +15.6 cm). The two charges are 42.8 cm apart. Determine the coordinates of the location where the net electric field due to Q1 and Q2 is zero. Give your answer in the form "(+/-ab.c m, +/-d.ef m)".

2. Relevant equations

E1 = E2

3. The attempt at a solution

58.6 x 10^(-3)/y^2 = +354 x 10^(-3)/(0.428 - y)^2

0.428 - y = +/- 0.77723y

0.428 = +/- 0.77723y + y

0.428 = 1.77723y

0.428/1.77723 = y

0.24 m = y

My final answer (-12.0 m, 0.24m) But its still wrong

I will be very gratefull if anyone helps me!!!

2. Apr 3, 2013

### Staff: Mentor

The first thing you need to ask yourself is in what region of the y-axis will E = 0. Look at each region--above Q1, between Q1 and Q2, below Q2--and figure out which way the field from each charge points.

(Your solution assumes the answer is between the charges. Does that make sense considering the direction of the fields?)

3. Apr 3, 2013

### Pruddy

Ok the point where the electric field should be zero should be close to q2. Please check my diagram in the attachments and my solutions. I came up 1.49 m. Please check the attachments to see my work.
Thanks a lot.

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4. Apr 4, 2013

### Staff: Mentor

Yes, and in the region below q2.

Good. What you solved for is the distance below q2. Now you must translate that into the x, y coordinates that they ask for.

5. Apr 4, 2013

### Pruddy

Thanks a lot Doc Al.
So will the coordinates then be (00.0 m, 1.49 m).

6. Apr 4, 2013

### Staff: Mentor

No.

You are given the coordinates of Q1. Figure out the coordinates of Q2 and then the coordinates of the E = 0 point that you found.

7. Apr 4, 2013

### Pruddy

I don't know how to do it. Please can you give me a hint.

8. Apr 4, 2013

### Staff: Mentor

You are given the coordinates of Q1. What are they?

Where is Q2 in relation to Q1? What must be the coordinates of Q2?

9. Apr 4, 2013

### Pruddy

coordinates q1 = (-12.0 cm, +15.6 cm)
q2 and q1 have are in the same y - axis

10. Apr 4, 2013

### Pruddy

so will the coordinates of q2 be (-12.0 cm, 1.49 m)

11. Apr 4, 2013

### Staff: Mentor

OK

They have the same x-coordinates. So what are the coordinates of q2?

12. Apr 4, 2013

### Staff: Mentor

No, but how did you arrive at this result?

13. Apr 4, 2013

### Pruddy

I guessed the answer, but I know its wrong because y coordinates of p cannot be thesame as the y coordinates of q2.

14. Apr 4, 2013

### Pruddy

But since we know the coordinates of y coordinates of Q1 and P. Will it be legit to subtract their distances to get Q2

15. Apr 4, 2013

### Staff: Mentor

I don't know what you mean by P.

You know where Q1 is. And you know that Q1 is directly above Q2 and you know their distance apart. That should tell you the coordinates of Q2.

16. Apr 4, 2013

### Pruddy

P is the point where the electric field is = 0. which is 1.49 m.

17. Apr 4, 2013

### Staff: Mentor

Which is 1.49 m below Q2.

18. Apr 4, 2013

### Pruddy

The question says "Determine the coordinates of the location where the net electric field due to Q1 and Q2 is zero"

19. Apr 4, 2013

### Staff: Mentor

Still waiting for you to give the coordinates of Q2.

20. Apr 4, 2013

### Pruddy

the coordinates of q2 will be (-12cm 1.92m)
I got 1.92 by taking the sum of 0.428m (which is the distance between q1 qnd q2) and 1.49m

21. Apr 4, 2013

### Staff: Mentor

No. The location of Q2 is (indirectly) given in the problem statement. (You don't need to know P to get the coordinates of Q2.) Draw yourself a diagram and figure out the coordinates of Q2.

22. Apr 4, 2013

### Pruddy

please can you give me the right answer and tell me how you got it. I still cant figure it out

23. Apr 4, 2013

### Pruddy

is Q2= (-12cm, -27.2cm)

24. Apr 4, 2013

### Pruddy

and P= (-12cm, -176.2cm)

25. Apr 4, 2013

### Staff: Mentor

Yes.

Yes again.

(Although they might want the answer in m, not cm.)