text: The ability of the director to align along an external field is caused by the electric nature of the molecules. Permanent electric dipoles result when one end of a molecule has a net positive charge while the other end has a net negative charge. When an external electric field is applied to the liquid crystal, the dipole molecules tend to orient themselves along the direction of the field.
  Even if a molecule does not form a permanent dipole, it can still be influenced by an electric field. In some cases, the field produces slight re-arrangement of electrons and protons in molecules such that an induced electric dipole results. While not as strong as permanent dipoles, orientation with the external field still occurs.
  The response of any system to an external electrical field is
  where ,  and  are the components of the electric field, electric displacement field and polarization density. The electric energy per volume stored in the system is
  (summation over the doubly appearing index ). In nematic liquid crystals, the polarization, and electric displacement both depend linearly on the direction of the electric field. The polarization should be even in the director since liquid crystals are invariants under reflexions of . The most general form to express  is
  (summation over the index ) with  and  the electric permittivity parallel and perpendicular to the director . Then density of energy is (ignoring the constant terms that do not contribute to the dynamics of the system)
  (summation over ). If  is positive, then the minimum of the energy is achieved when  and  are parallel. This means that the system will favor aligning the liquid crystal with the externally applied electric field. If  is negative, then the minimum of the energy is achieved when  and  are perpendicular (in nematics the perpendicular orientation is degenerated, making possible the emergence of vortices).
  The difference  is called dielectrical anisotropy and is an important parameter in liquid crystal applications. There are both  and  commercial liquid crystals. 5CB and E7 liquid crystal mixture are two  liquid crystals commonly used. MBBA is a common  liquid crystal.
  The effects of magnetic fields on liquid crystal molecules are analogous to electric fields. Because magnetic fields are generated by moving electric charges, permanent magnetic dipoles are produced by electrons moving about atoms. When a magnetic field is applied, the molecules will tend to align with or against the field. Electromagnetic radiation, e.g. UV-Visible light, can influence light-responsive liquid crystals which mainly carry at least a photo-switchable unit.