# 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.