Finding the electric field at a point midway between two charges

[Bryster77]

Find the electric field at a point midway between the two charges of +30.0 x 10^-9 C and +60.0 x 10^-9 C separated by a distance of 30.0cm


My work:

E= Kq / r^2

E= (9 x 10^9 NxM^2/C^2)(30.0 x 10^-9 C)/ (0.30m)^2

i get 3000 N/C

E= (9x10^9 NxM^2/C^2)(60 x 10^-9 C)/(0.30m)^2

i get 6000 N/C

i added the two together and got 9000N/C, but its telling me the answer is "12.0 x 10^3N/C toward 30 nC"

i am confused on what i am doing wrong. please help

 

[hage567]

I believe your r value is wrong. The distance from a charge to the midpoint is NOT 0.30m. 0.30m is the distance from one charge to the other. Also, electric field is a vector quantity, so directions matter when you sum up contributions from multiple sources.

 

[Bryster77]

I completely understand it now, thank you. =D