# Electric field on a point charge

1. Jan 31, 2005

### Electro

Hello guys,
I have a question which I think I solved it right but I don't understand why the final answer is wrong.
Here's the problem: Consider three charges as shown. k is 8.98755 * 10^9 N m^2/c^2. What is the electric field at a point 1.01 cm to the left of the middle charge? Ans. in N/C.
For the figure: O---------O-------O; seems like this. The charges are: (beginning from the left) 7.35, 4.32, -3.42 microCulombs. Distances The left to Middle is 4.92 cm and Middle to Right is 4 cm.
So, the most left is positive, the middle is positive and the one in the right is negative. The point P is 1.01 cm to the left of the middle particle.

What I did:

E1 = [k*(7.35*10^-6)]/(0.0492-0.0101)^2
E2 = the same thing, with its parameters (but this is toward -x-axis)
E3= the same, toward + x-axis

Then I add this up and I get approx. -3.2517*10^8 N/C
Actually this is not the answer (at least the System says so)
Please take a look at this thing, maybe there's sth tiny I don't get.
Thanks

2. Jan 31, 2005

### vincentchan

what is the distance you use for E2 and E3?

3. Jan 31, 2005

### Electro

For E2 i.e the middle one, I used just 1.01 cm (actually 0.0101 m) and for E3 4cm + 1.01 cm, which is 0.0501 m.