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How does an electric dipole lose its energy while aligning with a uniform electric field--through heat or light?

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Electric dipole has different potential energy in different direction in electric field . It has lowest energy in direction of constant electric field because it is most stable there as net force is zero. So if electric dipole aligned in any direction ( having some energy) is aligned in direction of electric field its final potential energy decreases.

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  • $\begingroup$ yeah but since energy is conserved, what happens to the potential energy that is lost? (I think is what the OP is asking) $\endgroup$ – pentane Feb 11 '18 at 9:08
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The potential energy a molecular dipole loses when aligning to an electric field is either converted to rotational kinetic energy in the free dipole which eventuallt converts back to potential energy (oscillation) or it converts into heat when friction with other molecules occurs. The latter is responsible for the microwave heating of water. Some energy might also be converted to emitted EM radiation due to the rotational dipole oscillaton.

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In an electric field ($\vec{E}$), the electric dipole ($\vec{P}$) experience a torque approximated by $\vec{\tau}=\vec{P}\times \vec{E}$. Due to this torque, the dipole gains angular momentum, which is what the potential energy is converted to when the dipole aligns with the field.

When the dipole is finally aligned with the fields, instead of stopping it starts oscillating due to the angular velocity it has gained. But there is also a simultaneous loss of energy through radiation, which is because the charges are accelerating while rotating. Thus, the oscillation completely stops after a finite time, because all the energy has been lost through radiation.

To conclude, the energy is lost in 2 ways:

  1. Rotational Energy
  2. Radiation
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