# Debye

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

## Overview

The debye (symbol: D) is a non-SI, CGS unit of electrical dipole moment. It is defined as 1Template:E statcoulomb centimeter (or 1Template:E esu m, or 1Template:E Fr cm). In SI units, 1 D equals approximately 3.33564Template:E coulomb-meter (exactly 1Template:E C m2/s divided by c, the speed of light in vacuum). Conversely 1 C m = 2.9979Template:E D. It is named after the physicist Peter J. W. Debye.

Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb (generally called esu in older literature), which were separated by 1 angstrom (10-8 cm or 10-10m).

Dipole moment is defined:
$ \mathbf{p} = q \, \mathbf{r} $

1 debye = (10-10)(10-8) statcoulomb centimeter

Note that 10-10 statcoulomb is 0.48 units of elementary charge.

This gave a convenient unit for molecular dipole moments. Typical dipole moments for simple diatomic molecules are in the range of 0 - 11D, where symmetric homoatomic species, e.g. chlorine, Cl2, have a dipole moment of 0D and highly ionic molecular species such as gas phase potassium bromide, KBr have a dipole moment of 10.5D.[1]
The debye is still used in atomic physics and chemistry because SI units are inconveniently large, particularly since the smallest prefix is Template:E (e.g., 2.54 D = 8.47Template:E yCm). Note that SI disallows the application of prefixes to both members of a compound unit (e.g., 2.54 D = 8.47 fC·fm) or the compounding of prefixes (e.g., 2.54 D = 8.47 µyCm), so there is currently no satisfactory solution to this problem of notation.

## References

1. Physical chemistry 2d Edition (1966) G.M. Barrow McGraw Hill