dipole

(noun)

A separation of positive and negative charges.

Related Terms

Examples of dipole in the following topics:

  • Dipole Moments

    • The electric dipole moment is a measure of polarity in a system.
    • There are many different types of dipole moments, including electric dipole moments, magnetic dipole moments, and topological dipole moments.
    • Among the subset of electric dipole moments are transition dipole moments, molecular dipole moments , bond dipole moments, and electron electric dipole moments.
    • This torque rotates the dipole to align it with the field.
    • Relate the electric dipole moment to the polarity in a system

  • A Physical Aside: Multipole Radiation

    • It is possible to calculate the radiation field to higher order in $L/(c\tau)$.This is necessary if the dipole moment vanishes, for example.
    • where $k\equiv\omega/c$$n=0$ gives the dipole radiation, $n=1$ gives the quadrupole radiation and so on.

  • Selection Rules

    • We can determine the selection rules for dipole emission by examining the definition of the dipole matrix element
    • First let's calculate the dipole matrix element after a parity transformation that takes ${\bf r} \rightarrow -{\bf r}$.
    • The first condition holds because the dipole operator does not couple to the spin of the electrons and the final condition exists because a photon carries away one unit of angular momentum.
    • On the other hand transitions that don't follow these rules can proceed through magnetic dipole or higher order multipole interactions.

  • Polarization

    • This separation creates a dipole moment, as shown in .
    • On the molecular level, polarization can occur with both dipoles and ions.
    • One example of a dipole molecule is water, (H2O), which has a bent shape (the H-O-H angle is 104.45°) and in which the oxygen pulls electron density away from the H atoms, leaving the H relatively positive and the O relatively negative, as shown in .
    • Water is an example of a dipole molecule, which has a bent shape (the H-O-H angle is 104.45°) and in which the oxygen pulls electron density away from the H atoms, leaving the H relatively positive and the O relatively negative.
    • The atom's dipole moment is represented by M.

  • Total Polarization

    • The physical mechanism for this can be qualitatively understood from the manner in which electric dipoles in the media respond to p-polarized light (whose electric field is polarized in the same plane as the incident ray and the surface normal).
    • One can imagine that light incident on the surface is absorbed, and then re-radiated by oscillating electric dipoles at the interface between the two media.
    • The refracted light is emitted perpendicular to the direction of the dipole moment; no energy can be radiated in the direction of the dipole moment.
    • Thus, if the angle of reflection θ1 (angle of reflection) is equal to the alignment of the dipoles (90 - θ2), where θ2 is angle of refraction, no light is reflected.

  • Ferromagnetism

    • Ferromagnetism arises from the fundamental property of an electron; it also carries charge to have a dipole moment.
    • This dipole moment comes from the more fundamental property of the electron—its quantum mechanical spin.
    • When these tiny magnetic dipoles are aligned in the same direction, their individual magnetic fields combine to create a measurable macroscopic field.
    • However, in materials with a filled electron shell, the total dipole moment of the electrons is zero, as the spins are in up/down pairs.
    • (According to Hund's rules, the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment. )

  • Dipole Approximation

  • Maxwell's Equations

    • Gauss's law for magnetism states that there are no "magnetic charges (or monopoles)" analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles.
    • Such dipoles can be represented as loops of current, but in many ways are similar in appearance to positive and negative "magnetic charges" that are inseparable and thus have no formal net "magnetic charge."
    • Thus, the total magnetic flux through a surface surrounding a magnetic dipole is always zero.
    • The field lines created by this magnetic dipole either form loops or extend infinitely.

  • Paramagnetism and Diamagnetism

    • Constituent atoms or molecules of paramagnetic materials have permanent magnetic moments (dipoles), even in the absence of an applied field.
    • In pure paramagnetism, the dipoles do not interact with each other and are randomly oriented in the absence of an external field due to thermal agitation; this results in a zero net magnetic moment.
    • When a magnetic field is applied, the dipoles will tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field.

  • Radiation from Systems of Particles

    • Let's examine the spectrum of dipole radiation.
    • To make things easier, let us assume that the dipole lies in a single direction and varies in magnitude (imagine a negative charge moving up and down a wire).