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v t e

The electric field due to a point dipole (upper left), a physical dipole of electric charges (upper right), a thin polarized sheet (lower left) or a plate capacitor (lower right). All generate the same field profile when the arrangement is infinitesimally small.
Common symbols	p
SI unit	Coulomb-meter (C m)
In SI base units	m⋅s⋅A
Dimension	LTI

The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system: that is, a measure of the system's overall polarity. The SI unit for electric dipole moment is the coulomb-meter (C⋅m). The debye (D) is another unit of measurement used in atomic physics and chemistry.

Theoretically, an electric dipole is defined by the first-order term of the multipole expansion; it consists of two equal and opposite charges that are infinitesimally close together, although real dipoles have separated charge.^{[notes 1]}

Elementary definition

Animation showing the electric field of an electric dipole. The dipole consists of two point electric charges of opposite polarity located close together. A transformation from a point-shaped dipole to a finite-size electric dipole is shown.

A molecule of water is polar because of the unequal sharing of its electrons in a "bent" structure. A separation of charge is present with negative charge in the middle (red shade), and positive charge at the ends (blue shade).

Often in physics, the dimensions of an object can be ignored so it can be treated as a pointlike object, i.e. a point particle. Point particles with electric charge are referred to as point charges. Two point charges, one with charge +q and the other one with charge −q separated by a distance d, constitute an electric dipole (a simple case of an electric multipole). For this case, the electric dipole moment has a magnitude $${\displaystyle p=qd}$$ and is directed from the negative charge to the positive one. Some authors may split d in half and use s = d/2 since this quantity is the distance between either charge and the center of the dipole, leading to a factor of two in the definition.

A stronger mathematical definition is to use vector algebra, since a quantity with magnitude and direction, like the dipole moment of two point charges, can be expressed in vector form $${\displaystyle \mathbf {p} =q\mathbf {d} }$$ where d is the displacement vector pointing from the negative charge to the positive charge. The electric dipole moment vector p also points from the negative charge to the positive charge. With this definition the dipole direction tends to align itself with an external electric field (and note that the electric flux lines produced by the charges of the dipole itself, which point from positive charge to negative charge, then tend to oppose the flux lines of the external field). Note that this sign convention is used in physics, while the opposite sign convention for the dipole, from the positive charge to the negative charge, is used in chemistry.^[1]

An idealization of this two-charge system is the electrical point dipole consisting of two (infinite) charges only infinitesimally separated, but with a finite p. This quantity is used in the definition of polarization density.

Energy and torque

An object with an electric dipole moment p is subject to a torque τ when placed in an external electric field E. The torque tends to align the dipole with the field. A dipole aligned parallel to an electric field has lower potential energy than a dipole making some non-zero angle with it. For a spatially uniform electric field across the small region occupied by the dipole, the energy U and the torque ${\displaystyle {\boldsymbol {\tau }}}$ are given by^[2] $${\displaystyle U=-\mathbf {p} \cdot \mathbf {E} ,\qquad \ {\boldsymbol {\tau }}=\mathbf {p} \times \mathbf {E} .}$$

The scalar dot "⋅" product and the negative sign shows the potential energy minimises when the dipole is parallel with the field, maximises when it is antiparallel, and is zero when it is perpendicular. The symbol "×" refers to the vector cross product. The E-field vector and the dipole vector define a plane, and the torque is directed normal to that plane with the direction given by the right-hand rule. A dipole in such a uniform field may twist and oscillate, but receives no overall net force with no linear acceleration of the dipole. The dipole twists to align with the external field.

However, in a non-uniform electric field a dipole may indeed receive a net force since the force on one end of the dipole no longer balances that on the other end. It can be shown that this net force is generally parallel to the dipole moment.

Expression (general case)

More generally, for a continuous distribution of charge confined to a volume V, the corresponding expression for the dipole moment is: $${\displaystyle \mathbf {p} (\mathbf {r} )=\int _{V}\rho (\mathbf {r} ')\left(\mathbf {r} '-\mathbf {r} \right)d^{3}\mathbf {r} ',}$$

where r locates the point of observation and d³r′ denotes an elementary volume in V. For an array of point charges, the charge density becomes a sum of Dirac delta functions: $${\displaystyle \rho (\mathbf {r} )=\sum _{i=1}^{N}\,q_{i}\,\delta \left(\mathbf {r} -\mathbf {r} _{i}\right),}$$

where each r_i is a vector from some reference point to the charge q_i. Substitution into the above integration formula provides: $${\displaystyle \mathbf {p} (\mathbf {r} )=\sum _{i=1}^{N}\,q_{i}\int _{V}\delta \left(\mathbf {r} _{0}-\mathbf {r} _{i}\right)\,\left(\mathbf {r} _{0}-\mathbf {r} \right)\,d^{3}\mathbf {r} _{0}=\sum _{i=1}^{N}\,q_{i}\left(\mathbf {r} _{i}-\mathbf {r} \right).}$$

This expression is equivalent to the previous expression in the case of charge neutrality and N = 2. For two opposite charges, denoting the location of the positive charge of the pair as r₊ and the location of the negative charge as r₋: $${\displaystyle \mathbf {p} (\mathbf {r} )=q_{1}(\mathbf {r} _{1}-\mathbf {r} )+q_{2}(\mathbf {r} _{2}-\mathbf {r} )=q(\mathbf {r} _{+}-\mathbf {r} )-q(\mathbf {r} _{-}-\mathbf {r} )=q(\mathbf {r} _{+}-\mathbf {r} _{-})=q\mathbf {d} ,}$$

showing that the dipole moment vector is directed from the negative charge to the positive charge because the position vector of a point is directed outward from the origin to that point.

The dipole moment is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges or a neutral conductor in a uniform electric field. For such a system, visualized as an array of paired opposite charges, the relation for electric dipole moment is:

Zdroj:https://en.wikipedia.org?pojem=Electric_dipole
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