Erg (unit of energy)

Definition and conversion

In the CGS framework the basic force unit is the dyne, and the erg is the energy produced when a force of one dyne moves its point of application through one centimetre. Conversions commonly used are:

• 1 erg = 1 dyne·cm = 10⁻⁷ joules.
• 1 joule = 10⁷ erg.

Historical background

The erg arose with the development of the CGS system in the 19th century as scientists sought a coherent set of base units for mechanics and electromagnetism. The CGS system and its units such as the erg were widely used in theoretical and experimental physics. Over the 20th century the International System of Units (SI) gained broad international adoption, and the joule became the preferred standard for energy. For background on the CGS system see CGS system.

Practical scale and examples

An erg is extremely small by everyday standards. To give a sense of scale, it is often compared to tiny biological or microscopic actions: a common housefly performing a brief leg-bending motion that brings its mouth to the surface and back has been used as an illustrative example of the order of magnitude of an erg—an imprecise but helpful mental picture. For another popular reference see fly energy example and the described "press-up" motion at press-up description.

Uses and contemporary relevance

Although the erg is now largely supplanted by the joule in most contexts, it remains encountered in older literature and in some subfields such as astrophysics and plasma physics, where energies and luminosities were traditionally expressed in erg or erg·s⁻¹. Modern practice typically converts historical values to SI units for standardization, but awareness of the erg is useful when consulting classical papers and legacy datasets.

Notable distinctions

Unlike derived SI units, the erg is tied to the CGS base units (gram and centimetre). Its continued presence in certain scientific communities is mainly historical and practical: it can be convenient when working within a CGS-based formula set. For most modern measurement, communication, and engineering work the joule provides a unified SI alternative that avoids unit-system mixing.