Calculate the component of the electric field

[nokia8650]

http://img367.imageshack.us/img367/5459/69671206ju9.th.jpg

See image above. Point charges A, of +2.0nC, and B, of –3.0 nC, are 200mm apart in a vacuum, as shown by the figure. The point P is 120mm from A and 160mm from B.

(i) Calculate the component of the electric field at P in the direction AP.
(ii) Calculate the component of the electric field at P in the direction PB.

The markscheme uses E= kQ/(r^2)

where k = 1/4*pi*epsilon

However, for part (i), uses only the 2nC charge, while uses only the -3nC charge for part ii - can someone please explain why this is the case.

Thanks

 

[Doc Al]

In what direction is the field at P due to A? Due to B?

 

[nokia8650]

ahh yes, i understand now, thanks