# Joule effect

(Redirected from Joule's law)

Joule effect and Joule's law are any of several different physical effects discovered or characterized by English physicist James Prescott Joule. These physical effects are not the same, but all are frequently or occasionally referred to in literature as the "Joule effect" or "Joule law" These physical effects include:

• "Joule's first law" (Joule heating), a physical law expressing the relationship between the heat generated and current flowing through a conductor.
• Joule's second law states that the internal energy of an ideal gas is independent of its volume and pressure, depending only on its temperature.
• Magnetostriction, a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field.
• The Joule–Thomson effect (during Joule expansion), the temperature change of a gas (usually cooling) when it is allowed to expand freely.
• The Gough–Joule effect or the Gow–Joule effect, which is the tendency of elastomers to contract if heated while they are under tension.

## Joule's first law

Between the years 1840 and 1843 Joule made a careful study of the heat produced by an electric current. From this study, he developed Joule's laws of heating, the first of which is commonly referred to as the Joule effect. Joule's first law expresses the relationship between heat generated in a conductor and current flow, resistance, and time.[1]

## Magnetostriction

The magnetostriction effect describes a property of ferromagnetic materials which causes them to change their shape when subjected to a magnetic field. Joule first reported observing change in the length of ferromagnetic rods in 1842.[2]

## Joule expansion

In 1845, Joule studied the free expansion of a gas into a larger volume. This became known as Joule expansion.[3] The cooling of a gas by allowing it to expand freely is occasionally referred to as the Joule effect.[4]

## Gough–Joule effect

If an elastic band is first stretched and then subjected to heating, it will shrink rather than expand. This effect was first observed by John Gough in 1802, and was investigated further by Joule in the 1850s, when it then became known as the Gough–Joule effect.[5][6]
Examples in Literature:

• Popular Science magazine, January 1972: "A stretched piece of rubber contracts when heated. In doing so, it exerts a measurable increase in its pull. This surprising property of rubber was first observed by James Prescott Joule about a hundred years ago and is known as the Joule effect."[7]
• Rubber as an Engineering Material (book), by Khairi Nagdi: "The Joule effect is a phenomenon of practical importance that must be considered by machine designers. The simplest way of demonstrating this effect is to suspend a weight on a rubber band sufficient to elongate it at least 50%. When the stretched rubber band is warmed up by an infrared lamp, it does not elongate because of thermal expansion, as may be expected, but it retracts and lifts the weight."[8]

## References

1. ^ Crew, Henry (1910). General physics: an elementary text-book FOR colleges, 2nd Edition. The University of Michigan: The Macmillan Company. pp. 402–404.
2. ^ Joule, J.P. (1847). "On the Effects of Magnetism upon the Dimensions of Iron and Steel Bars". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. Taylor & Francis. 30, Third Series: 76–87, 225–241. Retrieved 2009-07-19. Joule observed in this paper that he first reported the measurements in a "Conversazione" in Manchester, England, in (Sturgeon's) Annals of Electricity, Magnetism, and Chemistry 8, 219-224 (1842)
3. ^ Longair, M. S. (2003). Theoretical concepts in physics: an alternative view of theoretical reasoning in physics, 2nd Ed. Cambridge University Press. p. 217. ISBN 978-0-521-52878-8.
4. ^ "Thermodynamics of cooling" (PDF). The University of Sydney. 2005. Archived from the original (PDF) on 2011-06-11. Retrieved 2009-07-22.
5. ^ "John Gough and his Observation of Rubber Thermodynamics". Yale University. 1998-10-06. Archived from the original on 2011-06-07. Retrieved 2009-07-19.
6. ^ Loadman, John (2005). Tears of the Tree: The Story of Rubber -- A Modern Marvel. Oxford University Press. p. 165. ISBN 978-0-19-856840-7.
7. ^ Kouhoupt, Rudy (January 1972). Heat Runs. Popular Science. Retrieved 2009-07-20.
8. ^ Nagdi, Khairi (1992). Rubber as an Engineering Material. Hanser Verlag. pp. 33–34. ISBN 978-3-446-16282-2.