Poynting effect

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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.

Solid mechanics[edit]

In solid mechanics, the Poynting effect is a large strain 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.[1][2][3][4] The Poynting phenomenon in torsion was noticed experimentally by J. H. Poynting.[5][6][7]

Chemistry and thermodynamics[edit]

In 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:[8]


pv is the modified vapor pressure
pv,o is the unmodified vapor pressure
vliq is the liquid molar 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.


  1. ^ C. A. Truesdell, A programme of physical research in classical mechanics, Zeitschrift f¨ur Angewandte Mathematik und Physik 3 (1952) 79-95.
  2. ^ P. A. Janmey, M. E. McCormick, S. Rammensee, J. L. Leight, P. C. Georges, and F. C. MacKintosh, Negative normal stress in semiflexible biopolymer gels, Nature Materials 6 (2006) 48-51.
  3. ^ L. A. Mihai and A. Goriely, Positive or negative Poynting effect? The role of adscititious inequalities in hyperelastic materials, Proceedings of the Royal Society A 467 (2011) 3633-3646.
  4. ^ L. A. Mihai and A. Goriely, Numerical simulation of shear and the Poynting effects by the finite element method: An application of the generalised empirical inequalities in nonlinear elasticity, International Journal of Non-Linear Mechanics 49 (2013) 1-14.
  5. ^ J. H. Poynting, Radiation-pressure, Philosophical Magazine 9 (1905) 393-406.
  6. ^ J. H. Poynting, On pressure perpendicular to the shear-planes in finite pure shears, and on the lengthening of loaded wires when twisted, Proceedings of the Royal Society A 82 (1909) 546-559.
  7. ^ J. H. Poynting, The changes in length and volume of an Indian-rubber cord when twisted, India-Rubber Journal, October 4 (1913) p. 6.
  8. ^ Wark, Kenneth Advanced Thermodynamics for Engineers. New York: McGraw-Hill, 1995