Gustav Kirchhoff, physicist who investigated electrical circuits, plate theory, chemistry, optics, spectroscopy, and blackbody radiation emission

March 12, 1824, in Königsberg (now Kaliningrad) – October 17, 1887, in Berlin (Germany)

Gustav Robert Kirchhoff, from a very young age showed academic faculties which led him to enroll in the Albertus University (founded in 1544 by Albert, the first Duke of Prussia) where he attended a mathematical physics seminar for three years to introduce his students in research methods. It is during this time that he became interested in electrical induction and electrical currents.

He graduated in 1847 and moved to Berlin in a very tense situation, mainly due to the poor conditions of the German Confederation. There were riots and Louis Philippe I of France was dethroned in an uprising in 1848 that sparked revolutions in several German states and conflicts in Berlin.

He worked as a teacher in an unpaid position between 1848 and 1950 while rectifying what was believed until then regarding electrical and electrostatic currents.

He was appointed extraordinary professor at the University of Wroclaw (currently Wroclaw) and moved to another city. In that same year, he solves various problems concerning the deformation of elastic plates and meets the chemist Robert Bunsen, becoming friends.

In 1854, Bunsen works in Heidelberg and offers Kirchhoff to join him as a professor of physics. From that moment on, they collaborated very fruitfully.

In 1862, Kirchhoff proposed the name "black body radiation" and postulated two sets of fundamental laws, in classical electric circuit theory and in thermal emission. Although both are known as "Kirchhoff's Laws", this denomination is probably more common in the field of electrical engineering:

1- Kirchhoff's First Law or law of knots (or nodes): the sum of currents entering a node is equal to the sum of those leaving (All currents entering and leaving a node add up to 0). For a metal, in which the charge carriers are electrons, the above statement is equivalent to saying that the electrons entering a knot at a given instant are numerically equal to those leaving. The knots do not accumulate charge (electrons).
2- Kirchhoff's Second Law or mesh law: the sum of voltage drops in a section between two nodes is equal to the sum of voltage drops in any other section between said nodes.

His investigations into blackbody radiation were fundamental to the development of quantum theory. The astronomer and physicist Joseph von Fraunhofer had observed the bright lines in the spectrum produced by flames and noticed that they appeared at similar frequencies to the dark lines in the Sun's spectrum. To make further progress, the pure forms of these substances were required. , because by containing impurities, a confused image of the lines was produced. Kirchhoff was able to make this important breakthrough by producing the pure forms of the substances studied, and by 1859 he was able to realize that each element had unique features in the spectrum. He presented his law of radiation stating what he discovered, saying that for a given atom or molecule, the emission and absorption frequencies are the same.

He proposed the three empirical laws that describe the emission of light by incandescent objects:

  • A hot solid object produces light in a continuous spectrum.
  • A tenuous gas produces light with spectral lines at discrete wavelengths that depend on the chemical composition of the gas.
  • A solid object at high temperature surrounded by a tenuous gas at lower temperatures produces light in a continuous spectrum with gaps at discrete wavelengths whose positions depend on the chemical composition of the gas.

The justification for these laws was given later by the physicist Niels Bohr, decisively contributing to the birth of quantum mechanics.

In 1861, Kirchhoff and Bunsen studied the spectrum of the Sun, identifying the chemical elements in the solar atmosphere and discovering two new elements in the course of their investigations, cesium and rubidium.

Kirchhoff is known for being the first to explain the dark lines in the Sun's spectrum as a result of the absorption of particular wavelengths as light passes through gases in the Sun's atmosphere, thereby revolutionizing astronomy.

As his health deteriorated due to a disability that forced him to spend much of his life on crutches or in a wheelchair, it became more difficult for him to practice experimentation, so when in 1875 he was offered the chair of mathematical physics at Berlin, he accepted it so that he could continue to make contributions to teaching and conduct theoretical research.

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