Qu'est-ce qu'un/une Dyne (dyn) ?
Formal Definition
The dyne (symbol: dyn) is the unit of force in the centimeter-gram-second (CGS) system of units. One dyne is defined as the force required to accelerate a mass of one gram at a rate of one centimeter per second squared: 1 dyn = 1 g·cm·s⁻². In SI units, one dyne equals exactly 10⁻⁵ newtons (0.00001 N), or equivalently 10 micronewtons.
The dyne is a very small unit of force. The gravitational force on a one-gram mass at Earth's surface is approximately 980.665 dynes (or approximately 1 gram-force). The dyne's small magnitude makes it well-suited for measuring surface tension, viscous forces, and other phenomena involving small forces, but impractical for engineering applications.
CGS System Context
The CGS system, which uses centimeters, grams, and seconds as base units, was the dominant system in physics from the mid-19th century until the adoption of SI in 1960. While SI has largely replaced CGS, several CGS units — including the dyne — survive in specialized scientific fields where they provide convenient magnitudes or where decades of published data use CGS units.
Etymology
Greek Origin
The word "dyne" derives from the Greek "δύναμις" (dynamis), meaning power, strength, or force. The same root gives us "dynamic," "dynamo," and "dynasty." The name was chosen to reflect the unit's role as the fundamental measure of force in the CGS system.
The dyne was introduced as part of the CGS system developed by the British Association for the Advancement of Science in the 1870s. The committee, which included Lord Kelvin and James Clerk Maxwell, sought to create a coherent system where mechanical, thermal, and electromagnetic quantities could be expressed in terms of centimeters, grams, and seconds.
Naming Convention
Unlike most modern metric units, the dyne is not named after a scientist. It follows the pattern of CGS units derived from Greek or Latin roots (erg for energy, from Greek "ergon" = work; poise for viscosity, named after Poiseuille). The dyne's simple, descriptive name has helped it persist in scientific usage even as SI units replaced most other CGS units.
Precise Definition
CGS Definition
The dyne is defined as: 1 dyn = 1 g·cm·s⁻². This is the force that accelerates a mass of one gram by one centimeter per second squared. The definition follows directly from Newton's second law applied in CGS units.
SI Equivalence
1 dyn = 10⁻⁵ N = 10 μN (micronewtons). The conversion follows from the CGS-to-SI unit relationships: 1 g = 10⁻³ kg and 1 cm = 10⁻² m, so 1 g·cm/s² = 10⁻³ kg × 10⁻² m/s² = 10⁻⁵ kg·m/s² = 10⁻⁵ N.
Key Conversions
1 dyn = 10⁻⁵ N = 10 μN = 0.0000022481 lbf = 0.0000010197 kgf = 1.0197 × 10⁻⁶ gf (gram-force) is incorrect; actually 1 dyn = 1.0197 × 10⁻³ gf. More usefully: 1 N = 100,000 dyn = 10⁵ dyn, and 1 kgf = 980,665 dyn.
Histoire
The CGS System
The centimeter-gram-second system was proposed by Carl Friedrich Gauss in 1832 and formally developed by the British Association for the Advancement of Science (BAAS) starting in 1874. The BAAS committee, which included William Thomson (Lord Kelvin), James Clerk Maxwell, and other leading physicists, established the CGS mechanical units: the dyne (force), the erg (energy), and the barye (pressure).
The CGS system became the dominant system in physics and chemistry for nearly a century. It was particularly favored because the electromagnetic equations took simpler forms in CGS than in the original MKS (meter-kilogram-second) system. Generations of physicists learned and published in CGS units, creating a vast body of literature denominated in dynes, ergs, and gauss.
Competition with MKS
The MKS (meter-kilogram-second) system, proposed by Giovanni Giorgi in 1901, competed with CGS throughout the early 20th century. The MKS system offered more practical unit sizes for engineering: the newton (= 10⁵ dyn) was better suited for everyday forces, and the joule (= 10⁷ erg) was more appropriate for practical energy measurements. The establishment of the SI in 1960 formalized the MKS approach, relegating CGS to legacy status.
Survival in Specific Fields
Despite SI's dominance, the dyne survives in several scientific contexts. Surface tension is frequently reported in dynes per centimeter (dyn/cm) in chemistry and materials science. Viscosity in the CGS unit poise (dyn·s/cm²) remains common. Astronomical literature, particularly older references, uses CGS units extensively. The dyne per square centimeter (barye) appears in some atmospheric science contexts.
Utilisation actuelle
Surface Tension Measurement
The dyne per centimeter (dyn/cm) is the most common unit for expressing surface tension in chemistry, materials science, and industry. Water at 20 °C has a surface tension of approximately 72.8 dyn/cm. Ethanol has about 22 dyn/cm. Mercury has about 487 dyn/cm. The SI equivalent unit is millinewtons per meter (mN/m), and the conversion is conveniently simple: 1 dyn/cm = 1 mN/m exactly.
Viscosity
The CGS unit of dynamic viscosity, the poise (P), is defined as 1 dyn·s/cm². The centipoise (cP) — one hundredth of a poise — is the most commonly used viscosity unit in industry. Water at 20 °C has a viscosity of approximately 1 cP. The SI equivalent, the millipascal-second (mPa·s), has the same numerical value: 1 cP = 1 mPa·s.
Astrophysics
Some astrophysical calculations continue to use CGS units, including the dyne. Radiation pressure, magnetic pressure, and gravitational stresses in stellar interiors may be expressed in dynes per square centimeter. While newer publications increasingly use SI, the legacy of CGS in astrophysics persists.
Polymer and Colloid Science
Polymer science and colloid chemistry frequently use dyn/cm for interfacial tension measurements. The critical surface tension of wetting for polymer surfaces — a key parameter in adhesion and coating technology — is traditionally reported in dyn/cm.
Everyday Use
An Extremely Small Force
The dyne is far too small for everyday force measurement. Holding this text in front of your face, the weight of a single eyelash is about 0.5–1 dyne (5–10 micronewtons). A mosquito landing on your arm exerts about 2–5 dynes. A grain of sand weighs about 25–50 dynes. These examples illustrate why the dyne never found practical everyday application.
Surface Tension in Daily Life
Although most people do not use the dyne directly, surface tension measured in dyn/cm governs many everyday phenomena. The fact that water has a surface tension of about 73 dyn/cm (much higher than most liquids) is why water forms droplets, why small insects can walk on water, and why a slightly overfilled glass holds water above its rim.
Soap and detergents work by reducing water's surface tension from about 73 dyn/cm to about 25–30 dyn/cm, allowing water to wet surfaces and penetrate fabrics more effectively. This is a direct application of the science measured in dynes per centimeter.
Printing and Coatings
The printing and coatings industry uses surface tension (in dyn/cm) to ensure proper ink adhesion. Plastic films must have a surface energy of at least 38–42 dyn/cm for ink to adhere properly. Corona treatment or flame treatment increases surface energy by oxidizing the polymer surface. Quality control checks surface energy using dyne test pens — markers calibrated to specific dyn/cm values.
In Science & Industry
Electromagnetic Theory (CGS)
In Gaussian CGS units (a variant of CGS used in electromagnetism), the dyne appears in Coulomb's law as: F = q₁q₂/r² (in statcoulombs and centimeters, giving force in dynes). This form is simpler than the SI equivalent, which requires the constant 1/(4πε₀). Many theoretical physics textbooks, particularly older ones, use this formulation.
Molecular Biology
Forces at the molecular level are often in the piconewton range (10⁻¹² N = 10⁻⁷ dyn). While piconewtons have largely replaced dynes in modern biophysics publications, the conversion is straightforward. The force to unzip DNA is about 10–15 pN (10⁻⁶ dyn). The force generated by a single kinesin motor protein stepping along a microtubule is about 6 pN.
Fluid Dynamics
The CGS system provides a particularly convenient framework for fluid dynamics at small scales. The Reynolds number — the key dimensionless parameter governing fluid flow behavior — takes simple forms in CGS. Viscous drag on small particles (Stokes' law) gives force directly in dynes when using CGS inputs: F = 6πηrv, where η is in poise, r in centimeters, and v in cm/s.
Surface Chemistry
Surface and interfacial tension measurements are central to surface chemistry. The Wilhelmy plate method, Du Noüy ring method, and pendant drop method all yield results in dyn/cm or mN/m. Contact angle measurements are interpreted using Young's equation, where surface energies are in dyn/cm (= erg/cm²).
Interesting Facts
The dyne is so small that Earth's gravitational force on a single grain of rice is about 25,000 dynes (0.25 N). A dyne is roughly the weight of 1/1000 of a grain of rice.
Water's surface tension (about 73 dyn/cm at 20 °C) is unusually high because of hydrogen bonding between water molecules. Only mercury (about 487 dyn/cm) has a substantially higher surface tension among common liquids.
Dyne test pens, used in the printing industry, contain liquids calibrated to specific surface tensions (typically 30–56 dyn/cm). When drawn across a plastic surface, the ink either beads up (surface energy too low) or spreads evenly (surface energy adequate for printing).
The CGS system in which the dyne lives was partly developed by James Clerk Maxwell — the same physicist who unified electricity, magnetism, and optics. Maxwell preferred CGS because it simplified electromagnetic equations.
In Gaussian CGS units, the speed of light appears explicitly in electromagnetic equations, making the connection between electricity and magnetism more transparent than in SI. This pedagogical advantage kept CGS alive in physics education for decades after SI adoption.
The erg (the CGS energy unit) equals 1 dyne-centimeter, just as the joule equals 1 newton-meter. One erg = 10⁻⁷ joules — another illustration of the CGS system's small-scale orientation.