# Electric Field Strength problem.

1. Dec 13, 2011

### Darth Frodo

1. The problem statement, all variables and given/known data
2 point charges are situated 45nm apart in a vacuum. Charge A is +8μC and charge B is -25μC. X is a point 8nm from A.

Calculate the electric field strength at point X only due to A (Ans is 1.2 x 10^9 N/C)
Calculate the overall electric field strength at point X due to A and B. (Ans is 1.32 x 10^9 N/C)

2. Relevant equations
E = $\frac{1}{4\pi\epsilon}$ X $\frac{Q}{d^{2}}$

3. The attempt at a solution
Electric field strength due to A.
$\frac{1}{4\pi(8.9X10^-12)}$ X $\frac{8X10^-6}{(8X10^-9)^{2}}$

= 1.117X10^21
= 1.12X10^21 N/C

Electric Field Strength due to B.
E = $\frac{1}{4\pi(8.9X10^-12)}$ X $\frac{25X10^-6}{(37X10^9)^{2}}$

= 1.632X10^20
= 1.63X10^20 N/C

1.12X10^21 + 1.63X0^20 = 1.283X10^21 N/C

Where am I going wrong?
I would appreciate any help.

2. Dec 13, 2011

### Staff: Mentor

It doesn't look like you've done anything wrong. The "answers" given seem to be well out of whack in terms of order of magnitude.

3. Dec 13, 2011

### netgypsy

I don't see anything wrong with a and b either but the combined field is a vector sum and one of the charges is negative and the other, positive.