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quietrain
#3
Aug22-11, 01:14 AM
P: 651
ii) the electric field at a point is the superposition of the point charges everywhere else

so question wants at origin.

at origin, the E-field from +Q1 is towards the -ve x direction.
at origin, the E-field from -Q2 is towards the +ve z direction.

so your resultant direction is not (6,0,4.5), i think your magnitude is wrong too

E = kQ1 r1 / r13 + -kQ2r2 /r23

so k = 9x109
Q1 = +2x10-6
Q2 = -2x10-6
r13 = 33=27
r1=(-3,0,0)
r23 = 43=64
r2=(0,0,4)

resultant direction = r1+r2=(-3,0,4)
magnitude = (666, 0 , 281)
E-field at origin = (-3*666, 0 , 4*281) N/C