Gustav Robert Kirchhoff (March 12, 1824–October 17, 1887) was a German physicist. He is best known for developing Kirchhoff’s laws, which quantify the current and voltage in electrical circuits. In addition to Kirchhoff’s laws, Kirchhoff made a number of other fundamental contributions to physics, including work on spectroscopy and blackbody radiation.

### Fast Facts: Gustav Kirchhoff

**Full Name:**Gustav Robert Kirchhoff**Occupation:**Physicist**Known For**: Developed Kirchhoff's laws for electrical circuits**Born:**March 12, 1824 in Königsberg, Prussia**Died:**October 17, 1887 in Berlin, Germany**Parents’ Names:**Carl Friedrich Kirchhoff, Juliane Johanna Henriette von Wittke**Spouses' Names:**Clara Richelot (m. 1834-1869), Benovefa Karolina Sopie Luise Brömmel (m. 1872)

## Early Years and Education

Born in Königsberg, Prussia (now Kaliningrad, Russia), Gustav Kirchhoff was the youngest of three sons. His parents were Carl Friedrich Kirchhoff, a law counselor devoted to the Prussian state, and Juliane Johanna Henriette von Wittke. Kirchhoff’s parents encouraged their children to serve the Prussian state as best as they were able. Kirchoff was an academically strong student, so he planned to become a university professor, which was considered a civil servant role in Prussia at that time. Kirchhoff attended Kneiphofische High School with his brothers and received his diploma in 1842.

After graduating high school, Kirchhoff began studying in the Mathematics-Physics department at the Albertus University of Königsberg. There, Kirchhoff attended a mathematics-physics seminar from 1843 to 1846 developed by the mathematicians Franz Neumann and Carl Jacobi.

Neumann in particular had a profound impact on Kirchhoff, and encouraged him to pursue mathematical physics – a field which focuses on developing mathematical methods for problems in physics. While studying with Neumann, Kirchhoff published his first paper in 1845 at age 21*.* This paper contained the two Kirchhoff’s laws, which allow for the calculation of the current and voltage in electrical circuits.

## Kirchhoff's Laws

Kirchhoff’s laws for current and voltage are at the foundation of analyzing electrical circuits, allowing for the quantification of current and voltage within the circuit. Kirchhoff derived these laws by generalizing the results of Ohm’s law, which states that the current between two points is directly proportional to the voltage between those points and inversely proportional to the resistance.

**Kirchhoff’s first law **says that at a given junction in a circuit, the current going into the junction must equal the sum of the currents leaving the junction. **Kirchhoff’s second law** says that if there is a closed loop in a circuit, the sum of the voltage differences within the loop equals zero.

Through his collaboration with Bunsen, Kirchhoff developed three Kirchhoff’s laws for spectroscopy:

- Incandescent
*continuous*spectrum of light: they emit light at all wavelengths. - A hot, low-density gas produces an
*emission-line*spectrum: the gas emits light at specific, discrete wavelengths, which can be seen as bright lines in an otherwise dark spectrum. - A continuous spectrum traversing through a cooler, low-density gas produces an
*absorption-line*spectrum: the gas*absorbs*light at specific, discrete wavelengths, which can be seen as dark lines in an otherwise continuous spectrum.

Because atoms and molecules produce their own unique spectra, these laws allow for the identification of atoms and molecules found in the object being studied.

Kirchhoff also performed important work in thermal radiation, and proposed Kirchhoff’s law of thermal radiation in 1859. This law states that the emissivity (ability to emit energy as radiation) and absorbance (ability to absorb radiation) of an object or surface are equal at any wavelength and temperature, if the object or surface is at static thermal equilibrium.

While studying thermal radiation, Kirchhoff also coined the term “black body” to describe a hypothetical object which absorbed all incoming light and thus emitted all of that light when it was maintained at a constant temperature to establish thermal equilibrium. In 1900, the physicist Max Planck would hypothesize that these black bodies absorbed and emitted energy in certain values called “quanta.” This discovery would serve as one of the key insights for quantum mechanics.

## Academic Career

In 1847, Kirchhoff graduated from Königsberg University, and became an unpaid lecturer at Berlin University in Germany in 1848. In 1850, he became an associate professor at Breslau University and in 1854 a professor of physics at Heidelberg University. At Breslau, Kirchhoff met the German chemist Robert Bunsen, after whom the Bunsen burner was named, and it was Bunsen who arranged for Kirchhoff to come to Heidelberg University.

In the 1860s, Kirchhoff and Bunsen showed that each element could be identified with a unique spectral pattern, establishing that spectroscopy could be used to experimentally analyze the elements. The pair would discover the elements cesium and rubidium while investigating the elements in the sun using spectroscopy.

In addition to his work in spectroscopy, Kirchhoff would also study blackbody radiation, coining the term in 1862. His work is considered fundamental to the development of quantum mechanics. In 1875, Kirchhoff became the chair of mathematical physics at Berlin. He later retired in 1886.

## Later Life and Legacy

Kirchhoff died on October 17, 1887 in Berlin, Germany at the age of 63. He is remembered for his contributions to the field of physics as well as his influential teaching career. His Kirchhoff's laws for electrical circuits are now taught as part of introductory physics courses on electromagnetism.

## Sources

- Hockey, Thomas A., editor.
*The Biographical Encyclopedia of Astronomers*. Springer, 2014. - Inan, Aziz S. “What did Gustav Robert Kirchhoff Stumble Upon 150 Years Ago?”
*Proceedings of 2010 IEEE International Symposium on Circuits and Systems*, pp. 73–76. - “Kirchhoff’s Laws.” Cornell University, http://astrosun2.astro.cornell.edu/academics/courses/astro201/kirchhoff.htm.
- Kurrer, Karl-Eugen.
*The History of the Theory of Structures: from Arch Analysis to Computational Mechanics*. Ernst & Sohn, 2008. - “Gustav Robert Kirchhoff.”
*Molecular Expressions: Science, Optics, and You*, 2015, https://micro.magnet.fsu.edu/optics/timeline/people/kirchhoff.html. - O’Connor, J. J., and Robertson, E. F. “Gustav Robert Kirchhoff.”
*University of St. Andrews, Scotland*, 2002. - Palma, Christopher. “Kirchoff’s Laws and Spectroscopy.”
*The Pennsylvania State University*, https://www.e-education.psu.edu/astro801/content/l3_p6.html.