Homework Help: Where would you place a proton to produce this field at the origin?

1. Aug 26, 2008

cowmoo32

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

You want to create an electric field = < 0, 5e4,0 > N/C at location < 0, 0, 0>.
Where would you place a proton to produce this field at the origin?

2. Relevant equations

$$\vec E = q\hat{r} / 4\pi\varepsilon r^2$$

3. The attempt at a solution

I'm not sure how to treat $$\hat{r}$$ in the equation. Obviously, I know it's equal to r/rmag, but rmag is what's throwing me.

2. Aug 26, 2008

LowlyPion

An electric field is a vector field. So where you place the proton will determine both the magnitude and the direction of the electric field.

3. Aug 26, 2008

Staff: Mentor

That's just a unit vector (magnitude = 1) giving the direction of the field. The field from a positive charge is radially outward.

4. Aug 26, 2008

cowmoo32

Ok, I thought that unit vector might be 1, so I solve for $$r^2$$, correct? I tried that and didn't get the right answer.

5. Aug 26, 2008

LowlyPion

What values did you use?

6. Aug 26, 2008

cowmoo32

5e4 = [(1.6e-19)(9e9)] / r^2

I got r = 1.69707e-7

7. Aug 26, 2008

LowlyPion

Why do you think this is incorrect?

8. Aug 26, 2008

cowmoo32

Because when I submit it online it tells me that's the wrong answer. I'm not sure what I'm doing wrong.

9. Aug 26, 2008

LowlyPion

Is it possible it's your coordinates for placing the proton? They are asking you where you would place the proton to affect this electric field vector at 0,0,0.