# Homework Help: Very simple charge question, please review

1. Jul 14, 2010

### Ush

1. The problem statement, all variables and given/known data
A -14.1 nC charge is located at position (x,y) = (2.48 cm, 1.24 cm). At what (x,y) position is the electric field -262600.0i? (Enter your x position first followed by the y position.)

2. Relevant equations
E = kQ/r2
E = electric field strength
k = 9 x 109
r = distance to the point from the charge

3. The attempt at a solution

E = kQ/r2
∴ r = √(kQ/E)
r = 0.02198

original position: (0.0248m, 0.0124m)
electric field: -262600.0i,
unit vector 'i' indicates only x component, no y component:
therefore y component is the same (electric field vectors go straight in toward -'ve charge)

therefore x-component is..
0.0248 + r
=0.0248 + 0.02198
= 0.04678m

(0.04678m, 0.0124m)

Originally what I did was
0.0248 - r
=0.0248 - 0.02198
= 2.82E-3

(2.82E-3m, 0.02198m)

the computer rejected this.

The computer only accepted (0.04678m, 0.0124m) as the correct answer

I was wondering... is the other answer equally as valid or am I missing something?

2. Jul 15, 2010

### graphene

For the other answer, E would be +262600.0i and not -262600.0i.

You could draw the field lines around the charge and see for yourself.

3. Jul 15, 2010

thanks!