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Stefan, Josef

law bodies physics vienna

[ shtef an] (1835–93) Austrian physicist: discovered the Stefan–Boltzmann black body radiation law.

After 4 years at the University of Vienna, Stefan became a school-teacher for 7 years, researching in physics in his spare time, but in 1863 he secured the professorship of physics at Vienna and remained there throughout his life.

A skilful experimentalist, Stefan measured the thermal conductivity of gases accurately and thereby gave early confirmation of kinetic theory. In 1879 he considered the heat losses of very hot bodies, which were reputed to cool faster than law of cooling predicted. Using results obtained with a platinum wire made incandescent by passing a current, Stefan showed that the rate of heat loss per unit area is E = s T 4 , a relation known as Stefan’s Law. Here s is now known as Stefan’s constant and T is the absolute temperature. In 1884 his ex-student used the kinetic theory and thermodynamics to derive this law, and showed that it only held for bodies radiating perfectly at all wavelengths, called black bodies. It became known as the Stefan–Boltzmann Law. Stefan used the law to make the first satisfactory estimate of the temperature of the Sun’s surface (the photosphere), arriving at 6000°C for this.

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