James Prescott Joule

Physicist — James Prescott Joule (Salford, 1818 – Sale, 1889) was an English physicist, son of a wealthy brewer, who remained for most of his life an independent, self-taught researcher, a pupil for a time of John Dalton. His meticulous experiments in the 1840s showed that heat and mechanical work are two forms of the same quantity, energy, by measuring the mechanical equivalent of heat. This result was decisive for the formulation of the principle of conservation of energy and hence the first law of thermodynamics. He also studied the heating of current-carrying conductors (the Joule effect) and collaborated with William Thomson (Lord Kelvin) on the Joule–Thomson effect, underlying refrigeration and gas liquefaction. In his honour the SI unit of energy is named the joule.

Trivia

Joule was so obsessed with precise measurement that, according to a famous anecdote, during his honeymoon in the Alps he tried to measure the temperature difference between the top and bottom of a waterfall, to check the heating of the water caused by the fall.

Theories and contributions

• Mechanical equivalent of heat — Work and heat are forms of the same energy; the constant linking them is about $1\,\text{cal} \approx 4{.}186\,\text{J}$.
• Joule effect — A current-carrying conductor dissipates heat proportional to $Q = R I^2 t$.
• First law of thermodynamics — The change in internal energy is $\Delta U = Q - L$, with total energy conserved.

Featured in chapters

04 10 12 15 18 19 Z

Linked atoms

17 linked atoms.

04 Energia e lavoro

• Joule and the mechanical equivalent of heat
• Energy: a currency that is conserved

10 Trasformazioni irreversibili e bilancio energetico

• Joule’s frustration
• Heat: from invisible fluid to the ledger of energy accounts

12 Cicli e macchine termiche

• The named cycles

15 Circuiti elettrici

• Ohm’s first law
• The Joule effect
• Energy and power in circuits
• Power dissipated by resistors

18 Induzione elettromagnetica

• Lenz’s law as magnetic inertia
• Eddy currents
• Three industrial applications
• When eddy currents are the enemy
• Energy balance of the RLC circuit
• Lenz’s minus sign

19 Le equazioni di Maxwell e le onde elettromagnetiche

• The transformer

Z Appendice — Maturità

• Rod in a magnetic field: induction, dynamics and energy