In H.C Vermas concepts of physics vol-2 in the page.no.106 in Example 29.1, what is the rule behind taking E=Eacos@ + Ebcos@ ????Why cant sin@ be taken instead of cos@ ???Pls...help Pls i will approve the ans anyway help me aieee approaching n givin me the spine chills !!!Vikas waiting 4 ur reply !!! i know u knw this come on help me pls.....

electric field is a vector quantity so it can be broken into components ,

E_a is electric field by charge which is placed at vertex A

E_b is electric field by charge which is placed at vertex B

now , Ea & Eb both can be divided into two components ,

for Ea two components are Eacos@ & Easin@

for Eb two components are Ebcos@ & Ebsin@

now , it is clear that Eb sin@ is in opposite direction to Easin@ so they will cancel the effect of each other but

on other side Eacos@ & Ebcos@ are acting olong same direction so they will be add up to give total

electric field E = Eacos@ + Ebcos@

instead of all these one other method can be used for this particular problem , see both charges are of same magnitude

but opposite sign so this system will behave as a dipole & for this we can use direct formula

E(equitorial) = kp/(a²+x²)^3/2

here p(dipole moment) = q(2a)

     x = perpendicular distance

     2a is distance bw charges

drop a pependicular from PEa to PE now in triangle PEEa

there is no rule behind its just d simple trigonometry

tejaswi