Dipôles électriques induits

When a permanent dipole is placed in an external electric field, the field exerts a torque that orients the dipole along the field direction. The potential energy of this orientation depends on the angle between the dipole moment and the field. At parallel alignment, potential energy is minimum and stable; at antiparallel alignment, it is maximum and unstable.

The potential energy of a dipole in an external field equals the negative dot product of its dipole moment and the field vector. By convention, potential energy is set to zero when the dipole is perpendicular to the field. This relationship allows visualization of how dipole orientation affects energy state.

Carbon dioxide has a symmetric molecular structure where the positive and negative charge centers coincide at the same location. Since there is no separation between charge centers, no net dipole moment exists. This makes CO2 a non-polar molecule in the absence of an external field.

In an external electric field, positive charges are repelled away from the field while negative charges are pulled toward it, creating charge separation. This induced charge separation generates a net dipole moment even in non-polar molecules like methane. The induced dipole moment arises from the relative displacement of charge centers.

An induced dipole creates its own electric field that opposes the external field in its vicinity, while reinforcing it at large distances. The net electric field is the vector sum of both fields. This opposition near the dipole reduces the electric field inside dielectric materials, a key mechanism in how dielectrics respond to external fields.

Polar molecules like water attract opposite charges within their structure. When dissolving ionic compounds such as sodium chloride, water's polar nature attracts positive and negative ions toward opposite charge centers, breaking apart the crystal lattice. Non-polar molecules lack this ability, making them poor solvents for ionic compounds.

Stable equilibrium occurs when a dipole aligns parallel to the external field, with minimum potential energy. Unstable equilibrium occurs at antiparallel alignment, with maximum potential energy. Small perturbations cause a dipole at unstable equilibrium to rotate away, while those at stable equilibrium return to alignment.