91Ó°ÊÓ

When the dipole is in a homogeneous electric field, the electric field tends to orientate the dipole so that the dipole moment and electric field are parallel. To orientate dipole, the torque must act on it. The torque can be defined as:

$\begin{array}{r}\mathbf{T}\mathbf{=}\stackrel{\mathbf{r}}{\mathbf{p}}\mathbf{×}\stackrel{\mathbf{→}}{\mathbf{E}}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\\ \mathbf{T}\mathbf{=}\mathbf{\pm è·¡²õ¾\pm ²Ôθ}\mathbf{}\mathbf{}\mathbf{}\end{array}$

where $\mathrm{θ}$represents the angle between the dipole moment p and the electric field E.

Torque of the dipole will be zero when$\stackrel{→}{P}$ is in the same direction of$\stackrel{→}{E}$ or in opposite direction of$\stackrel{→}{E}$that is role="math" localid="1668231614082" $\mathrm{θ}=0^o{180}^o$.

If we take the dipole out of the equilibrium position, a torque will act to orient it in the direction of the external electric field. Since for $\mathrm{θ}$= 0 the dipole moment is already oriented in the direction of the electric field when we take the dipole out of this position, the electric field will tend to return it to the previous state which makes it a stable state.

A non-stable state is a state at which the dipole is oriented opposite to the direction of the electric field. So when the dipole is out of this state, the electric field will tend to orientate it to be in parallel. This state is when $\mathrm{θ}$=π.

A stable state is a state at which the dipole is oriented parallel to the direction of the electric field.

For stable orientation, the dipole moment is in parallel with the external electric field. Also, it is known that the dipole moment is a vector oriented from the negative toward the positive charge which means that the electric field of the dipole is oriented oppositely from the external electric field.

We can conclude that the electric field of the dipole opposes the external electric field.