The Poynting effect may refer to two unrelated physical phenomena. Neither should be confused with the

Poynting–Robertson effect The Poynting–Robertson effect, also known as Poynting–Robertson drag, named after John Henry Poynting and Howard P. Robertson, is a process by which solar radiation causes a dust grain orbiting a star to lose angular momentum relative to its or ...

. All of these effects are named after

John Henry Poynting John Henry Poynting Fellow of the Royal Society, FRS (9 September 185230 March 1914) was an English physicist. He was the first professor of physics at Mason Science College from 1880 to 1900, and then the successor institution, the University ...

, an English physicist.

Solid mechanics

In solid mechanics, the Poynting effect is a

finite strain theory In continuum mechanics, the finite strain theory—also called large strain theory, or large deformation theory—deals with deformations in which strains and/or rotations are large enough to invalidate assumptions inherent in infinitesimal str ...

effect observed when an elastic cube is sheared between two plates and stress is developed in the direction normal to the sheared faces, or when a cylinder is subjected to torsion and the axial length changes. The Poynting phenomenon in torsion was noticed experimentally by J. H. Poynting.

Chemistry and thermodynamics

In thermodynamics, the Poynting effect generally refers to the change in the

vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...

of a liquid substance when the total pressure of the liquid is varied. In particular this occurs when the vessel containing the vapor and liquid is pressurized by a non-condensable and non-soluble gas. In 1881Poynting, J. H., Change of State: Solid-Liquid, Phil. Mag., 12, 32-48, 232, 1881
/ref> Poynting generalized the

Kelvin equation The kelvin (symbol: K) is the SI base unit, base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute scale, absolute temperature scale that starts at the lowest possible temperature (absolute zero), ...

pointing out that vapor pressure was not only modified by

Laplace pressure The Laplace pressure is the pressure difference between the inside and the outside of a curved surface that forms the boundary between two fluid regions. The pressure difference is caused by the surface tension of the interface between liquid and ...

of curved surfaces, but in fact changes the same way due to any pressure source. In modern thermodynamics, this is understood as coming from the

Maxwell relation file:Thermodynamic map.svg, 400px, Flow chart showing the paths between the Maxwell relations. P is pressure, T temperature, V volume, S entropy, \alpha coefficient of thermal expansion, \kappa compressibility, C_V heat capacity at constant vo ...

for the

chemical potential In thermodynamics, the chemical potential of a Chemical specie, species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition. The chemical potent ...

shift due to pressure:

(\partial \mu/\partial P)_ = (\partial V/\partial N)_

, i.e.

\mathrm d\mu = v_\text \mathrm dP_\text

at constant temperature.Devoe "THERMODYNAMICS AND CHEMISTRY" - https://www2.chem.umd.edu/thermobook/ section 12.8 The

fugacity In thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of chemical equilibrium. It is equal to the pressure of an ideal gas which has the same tempe ...

shift from pressure is thus: :

\frac  = \exp\left(\frac \int_^ v_\text \, dp \right)

where the exponential on the right is known as the Poynting factor. If one assumes that the vapor is an ideal gas (fugacity = vapor pressure), and that the liquid is incompressible (

v_\text

constant), then: :

\frac  = \exp\left(\frac ( P - p_ ) \right)

where :

p

is the modified vapor pressure :

p_

is the vapor pressure as it would be with no other pressure sources applied :

v_

is the liquid molar volume :

R

is the molar gas constant :

T

is the temperature :

P

is the total pressure, i.e. the pressure of the liquid For a 1 atmosphere pressure, room temperature, and typical liquid densities, the vapor pressure change from Poynting effect is less than 1%. A common example is the production of the medicine

Entonox Nitrous oxide, as medical gas supply, is an inhaled gas used as pain medication, and is typically administered with 50% oxygen mix. It is often used together with other medications for anesthesia. Common uses include during childbirth, followin ...

, a high-pressure mixture of

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...

and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

. The ability to combine and at high

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

while remaining in the gaseous form is due to the oxygen exerting a Poynting effect on the nitrous oxide.

References

Elasticity (physics) Rubber properties Gases {{physics-stub