Finding the Net Electric Field of Two Point Charges

[Pruddy]

Homework Statement

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)".

Homework Equations

E1 = E2

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!

 

[Doc Al]

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?)

 

[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.

 

[Doc Al]

Ok the point where the electric field should be zero should be close to q2.

Yes, and in the region below q2.

Please check my diagram in the attachments and my solutions. I came up 1.49 m.

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

 

[Pruddy]

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

 

[Doc Al]

So will the coordinates then be (00.0 m, 1.49 m).

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.

 

[Pruddy]

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

 

[Doc Al]

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

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

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

 

[Pruddy]

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

 

[Pruddy]

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

 

[Doc Al]

coordinates q1 = (-12.0 cm, +15.6 cm)

OK

q2 and q1 have are in the same y - axis

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

 

[Doc Al]

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

No, but how did you arrive at this result?

 

[Pruddy]

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

 

[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

 

[Doc Al]

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

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.

 

[Pruddy]

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

 

[Doc Al]

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

Which is 1.49 m below Q2.

 

[Pruddy]

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

 

[Doc Al]

Still waiting for you to give the coordinates of Q2.

 

[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

 

[Doc Al]

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

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.

 

[Pruddy]

please can you give me the right answer and tell me how you got it. I still can't figure it out

 

[Pruddy]

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

 

[Pruddy]

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

 

[Doc Al]

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

Yes.

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

Yes again.

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

 

[Pruddy]

Thanks so much... I appreciate your assistance...

 

[Pruddy]

You are very gifted in the field of physics, please do you have any advice on how I can get a better understanding of the material?

 

[Doc Al]

One thing that helped me when I was first learning this stuff was to solve as many problems as possible. That's the only way you can be sure you really understand the material. Good luck in your studies!