Dipole Definition in Chemistry and Physics

Close up of a direction finder antenna
This direction finder antenna is made of a 16 dipole element array.

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A dipole is a separation of opposite electrical charges. A dipole is quantified by its dipole moment (μ).

A dipole moment is the distance between charges multiplied by the charge. The unit of the dipole moment is the debye, where 1 debye is 3.34×10−30 C ·m. The dipole moment is a vector quantity that has both magnitude and direction.

The direction of an electric dipole moment points from the negative charge toward the positive charge. The larger the difference in electronegativity, the greater the dipole moment. The distance separating opposite electrical charges also affects the magnitude of the dipole moment.

Types of Dipoles

There are two types of dipoles:

  • Electric dipoles
  • Magnetic dipoles

An electric dipole occurs when positive and negative charges (like a proton and an electron or a cation and an anion) are separate from each other. Usually, the charges are separated by a small distance. Electric dipoles may be temporary or permanent. A permanent electric dipole is called an electret.

A magnetic dipole occurs when there is a closed loop of electric current, such as a loop of wire with electricity running through it. Any moving electric charge also has an associated magnetic field. In the current loop, the direction of the magnetic dipole moment points through the loop using the right-hand grip rule. The magnitude of the magnetic dipole moment is the current of the loop multiplied by the area of the loop.

Examples of Dipoles

In chemistry, a dipole usually refers to the separation of charges within a molecule between two covalently bonded atoms or atoms that share an ionic bond. For example, a water molecule (H2O) is a dipole.

The oxygen side of the molecule carries a net negative charge, while the side with the two hydrogen atoms has a net positive electrical charge. The charges of a molecule, like water, are partial charges, meaning they don't add up to the "1" for a proton or electron. All polar molecules are dipoles.

Even a linear nonpolar molecule like carbon dioxide (CO2) contains dipoles. There is a charge distribution across the molecule in which charge is separated between the oxygen and carbon atoms.

Even a single electron has a magnetic dipole moment. An electron is a moving electrical charge, so it has a small current loop and generates a magnetic field. Although it may seem counter-intuitive, some scientists believe a single electron may also possess an electric dipole moment.

A permanent magnet is magnetic because of the magnetic dipole moment of the electron. The dipole of a bar magnet points from its magnetic south to its magnetic north.

The only known way to make magnetic dipoles is by forming current loops or via quantum mechanics spin.

The Dipole Limit

A dipole moment is defined by its dipole limit. Essentially, this means the distance between charges converges to 0 while the strength of the charges diverges to infinity. The product of the charge strength and separating distance is a constant positive value.

Dipole as an Antenna

In physics, another definition of a dipole is an antenna that is a horizontal metal rod with a wire connected to its center.

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Helmenstine, Anne Marie, Ph.D. "Dipole Definition in Chemistry and Physics." ThoughtCo, Aug. 29, 2020, thoughtco.com/definition-of-dipole-605031. Helmenstine, Anne Marie, Ph.D. (2020, August 29). Dipole Definition in Chemistry and Physics. Retrieved from https://www.thoughtco.com/definition-of-dipole-605031 Helmenstine, Anne Marie, Ph.D. "Dipole Definition in Chemistry and Physics." ThoughtCo. https://www.thoughtco.com/definition-of-dipole-605031 (accessed June 9, 2023).