Gustav Robert Kirchhoff A Brief Digest of His

Gustav Robert Kirchhoff A Brief Digest of His Life and Work by Mike Rasmussen

Vital Statistics l l l l Born: 12 March 1824, Königsberg, Prussia Christened: 11 April 1824 Father: Friedrich Kirchhoff (lawyer) Mother: Johanna Henriette Wittke Married: 1847, to Clara Richelot (his math professor’s daughter – good reason to study math!) Died: 17 October 1887, Berlin Temple work: Done several times, 1929 -2001. (Often just sealed to Mr. and Mrs. Kirchhoff – good reason to study physics!)

Scientific contributions in brief l l Kirchhoff’s Law (Thermodynamics) Kirchhoff’s Circuit Laws Spectroscopy (with Bunsen) Fresnel-Kirchhoff diffraction formula.

Kirchhoff’s Law (Thermodynamics) l l At thermal equilibrium, light emitted equals light absorbed. (Otherwise it wouldn’t be at equilibrium. ) So, black bodies are great emitters. And “Space Blankets” are good at not emitting.

Kirchhoff’s Circuit Laws l l l The Sum of voltage rises and drops around a loop equals zero. (Energy is conserved. ) Currents entering a junction are equal to currents leaving a junction. (Charge is conserved, similar to thermodynamic law. Remember, a lot of people thought charge was like heat, and might accumulate at a junction. ) So, we can do circuit analysis. Obvious, but useful – maybe we can all get laws named after us someday!

Kirchhoff and Bunsen

Spectroscopy “Two old telescopes, a prism, and a cigar box”

A Great Spectroscopy Experiment l l l Burned highly purified sodium, observed two strong yellow lines Lit a Drummond light, which is a stick of lime heated by a flame to white hot – it radiates nearly as a black body, with no dark lines Lit a sodium flame in between the “limelight” and the spectroscope, and observed two dark lines – the sodium absorbed more energy than it emitted

Spectroscopy results l l Identified Fraunhofer’s yellow lines in several lamp spectra as sodium emission Identified Fraunhofer’s dark lines in the solar spectrum as absorption Identified the chemical content of the sun Identified Cesium and Rubidium

Fresnel-Kirchhoff diffraction formula. l l Fresnel derived a formula for diffraction by applying Newton’s calculus to Huygen’s wavelets. In 1887, Kirchhoff used Maxwell’s Equations to justify Fresnel’s formula. Hence, it was named after both Fresnel and Kirchhoff. Apparently, it killed him, as he died that year.

References l l l http: //en. wikipedia. org/wiki/Gustav_Kirchhoff www. familysearch. org http: //dbhs. wvusd. k 12. ca. us/webdocs/Chem. History/Kirchhoff-Bunsen-1860. html http: //www. neatherd. org/astronomy/Fingerpri nts%20 of%20 light. htm http: //optics. byu. edu/Textbook. htm