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W = pE [-cosθf -(-cosθi)] ....eq(1)

p = moment of the dipole

θi = initial position which is when the dipole is aligned with electric field i.e θi = 0

θf = final position of the dipole.

i.e eq(1) becomes,

W= pE(1-cosθf) ....eq(2)

But for convenience, θi is taken when the dipole is perpendicular to electric field, i.e θi= 90°

. This is where I didn't understand. In book, the explanation is just that , it is convenient to take θi=90°, as cos 90°=0, but why and how there is no explanation.

i.e eq(1) now becomes,

W = -pE cosθf ...eq(3)

eq(2) and eq(3) gives different values, so how could they both give the same potential energy of the dipole in an electric field.