Poynting effect

The Poynting effect may refer to two unrelated physical phenomena. Neither should be confused with the Poynting–Robertson effect. All of these effects are named after John Henry Poynting, an English physicist.

Chemistry / Thermodynamics

The Poynting effect generally refers to the change in the vapor pressure of a liquid when a non-condensable gas is mixed with the vapor at saturated conditions. If one assumes that the vapor and the non-condensable gas behave as ideal gases and an ideal mixture, it can be shown that:

$\ln {\frac {p_{v}}{p_{v,o}}}={\frac {v_{liq}}{RT}}(P-p_{v,o})\!$

where

p_v is the modified vapor pressure

p_v,o is the unmodified vapor pressure

v_liq is the liquid specific volume

R is the liquid/vapor's gas constant

T is the temperature

P is the total pressure (vapor pressure + non-condensable gas)

As a common example, the ability to combine nitrous oxide and oxygen at high pressure while remaining in the gaseous form is due to the Poynting effect.

Entonox is a 50:50 combination of the anesthetic gas nitrous oxide and oxygen. This combination is useful because it can provide a sufficient concentration of nitrous oxide to provide analgesia (pain relief) in sufficient oxygen so that the risk of hypoxemia is eliminated. This makes it safe to use by para-medical staff such as ambulance officers. However the ability to combine these two gases at the temperature and pressure in the cylinder while remaining in the gaseous form is unexpected based on the known properties of the two gases.

The Poynting effect involves the dissolution of gaseous O₂ when bubbled through liquid N₂O, with vaporisation of the liquid to form a gaseous O₂/N₂O mixture.

Material science

In material science, the Poynting effect is a non-linear elastic 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 ;