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Find electric field due to individual charges at the center and do their vector sum. That will give you the resultant field.
P.S. you mustn't make any mistake while determining the direction of electric field due to individual charges.|dw:1340434465357:dw|
uhm okay. i think thats it.
|dw:1340434775644:dw|The electric fields due to charges at B and D seems to cancel each other. So, you have got only A and C.
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calculate the fields due to individual charges and do the vector sum.
break the problem into parts:
force(here electric field) always acts through the line joining the charges so it is enough if u consider forces along AC and BD
imagine placing a positive test charge(with no electric field of its own at the centre.
along BD , since u have two equal charges attracting a test charge(+ve) equally the effect due to them is zero
same way think wat should happen along AC
then use formulae q/(4 pi epsilonnt r^@)
u can see that we are taking only charge q in the eqn as electric field is nothing but force/unit charge
first u have to draw a diagram like ujjwal for clarity