How Is the Formula PEcos(θ) Derived for Dipoles in a Constant Electric Field?

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SUMMARY

The formula for the potential energy of a dipole in a constant electric field is derived as PE = -\vec{P} \cdot \vec{E}, where \vec{P} is the dipole moment and \vec{E} is the electric field. This derivation involves considering two charges of equal magnitude Q and opposite sign, positioned at the origin and at \vec{r}_0. As \vec{r}_0 approaches zero, the dipole moment is defined as \vec{P} = Q \vec{r}_0, leading to the established relationship for potential energy in a homogeneous electric field.

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electrohau5
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I know that the potential energy of a dipole in a constant electric field is

P(dot)E=PEcos(θ), but I can't seem to find how they got here; its not in my textbook.

If anyone knows why please tell me.
 
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A straight-forward way is to just use two charges of the same magnitude Q and opposite sign located at the origin and at \vec{r}_0 (negative charge in the origin). The total potential energy of this charge distribution in the homogeneous electric field is
V=-Q \vec{r}_0 \cdot \vec{E}.
Now the dipole moment is given for the limit \vec{r}_0 \rightarrow 0 such that Q \vec{r}_0=\vec{P}=\text{const}. This leads to
V=-\vec{P} \cdot \vec{E}.
QED.
 
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