# Coulomb's law electric field charge on y axis

1. Feb 1, 2012

### kirby2

Is this question worded poorly or is it just me?

Three charges are on the y-axis. Charges -q are at y = ±d and charge + 2q is at y = 0.
(a) Determine the electric ﬁeld E along the x-axis.
(b) Verify that your answers to part a has the expected behavior as x
becomes very small and very large.

what i don't get is that the charge on y=0 can be anywhere on the x-axis so i don't know how to get the field on the x-axis when i can place that charge anywhere. can someone verify if this question is legit?

2. Feb 1, 2012

### SammyS

Staff Emeritus
All three charges are along the y-axis.

The +2q charge also lies on the x-axis.

That places it at a very specific location.

3. Feb 1, 2012

### kirby2

you said "All three charges are along the y-axis.

The +2q charge also lies on the x-axis." why did you say "also"?

4. Feb 1, 2012

### SammyS

Staff Emeritus
Because you said in your original post:
"what i don't get is that the charge on y=0 can be anywhere on the x-axis ..."​
This indicated to me that you must have thought that the +2q charge, the on at y = 0, could be anywhere on the x-axis.

5. Feb 1, 2012

### kirby2

(3,0) and (0,0) are both at y=0 yet they are in two different places.

6. Feb 1, 2012

### kirby2

just kidding you are right. i didnt read the first part that the charges are "on the y axis"

7. Feb 2, 2012

### kirby2

ok, i don't know how to start. my initial thought is that E on the x-axis is zero. the three charges pull in the y direction. i can't see anything that would pull/push in the x-direction. is this right?

8. Feb 2, 2012

### SammyS

Staff Emeritus
At any location on the x-axis, let's say at x = a, the +2q charge is closer to the point x = a, than the two -q charges are. This means that at that location, the field due to the two negative charges is less than the field due tho the positive charges. Also, the electric field due to each of the two negative is not entirely in the x-direction. These two facts taken together tell us that the E field from the negative charges does not entirely cancel the E field from the positive charge.