# Homework Help: Calculating the electric field, multiple point charges

1. Oct 7, 2007

### aliaze1

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

What are the strength and direction of the electric field at the position indicated by the dot?

Part A:
Give your answer in terms of the horizontal and vertical components, separated by commas. Take the positive directions to be up and to the right.

Part B:
Specify the strength of the electric field.

2. Relevant equations

[1/(4πε0)]*(|q|/r2)

3. The attempt at a solution

[1/(4πε0)]=9*109

q1=-5*10-9
q2=10*10-9
q3=10*10-9

r1=0.02m
r2=0.04m
r3=√(0.002)=(0.002)0.5=~0.0447m

E1={[9*109]*[5*10-9]}/(0.02m)2
E2={[9*109]*[10*10-9]}/(0.04m)2
E3={[9*109]*[10*10-9]}/(0.002m)

E1=112500 j
E2=56250 i
E3=45000

E3_i=45000*(0.04/0.0447)=40249 i
E3_j=45000*(0.02/0.0447)=20134 j

Right is positive, therefore:
E2= -56250 i
E3_i= -40249 i

ETotal_i= -56250 + -40249 = -96499 i
ETotal_j= 20134 + 112500 = 133534 j

This is coming out as 'incorrect', and therefore I cannot proceed to part 2

Where am I going wrong?

Thanks!

Last edited: Oct 7, 2007
2. Oct 7, 2007

### neutrino

You need to redo E3.

3. Oct 7, 2007

### aliaze1

typo, sorry, E3 is actually 45000, unless you mean that that is also incorrect

Last edited: Oct 7, 2007
4. Oct 7, 2007

### neutrino

Oh, okay. 45000 is correct.

I just calculated the x-comp. of E3 and it comes out to 40,268.45638, while your answer is off by almost 20 N/C. I think it's a calculation error somewhere.

5. Oct 7, 2007

### aliaze1

yea idk for some reason my calculator is giving me the 40249.22359..., anyways, is my y-component correct? i have very few attempts left

thanks

6. Oct 7, 2007

7. Oct 7, 2007

8. Oct 7, 2007

### aliaze1

....still isnt working :(

9. Dec 13, 2007

### Tools_ink2

The answer is E(0,0.2) = (-964.15,-922.9) where E(0,0.2) = 1334.7

Last edited: Dec 13, 2007
10. Dec 13, 2007

### Tools_ink2

please correct the value of r3, then everything should fall into place.

11. Jan 25, 2008

### Nekura

I got (-96392,-92272)

your J value is incorrect. your e1 is incorrect. using the correct sign convention, since e1 is a negative charge, it pulls the dot towards it. not away, therefore e1 should be negative since up and right is postive