Gustav Kirchhoff

Physicist — Gustav Robert Kirchhoff (Königsberg 1824 – Berlin 1887) was a German physicist who made fundamental contributions to electrical theory, spectroscopy and the theory of radiation. Still a student, he formulated the two circuit laws that bear his name, today basic tools for analysing any electrical network. Together with the chemist Robert Bunsen he developed spectral analysis, showing that every element emits and absorbs light at characteristic wavelengths: with this method the pair discovered two new elements, caesium and rubidium, and opened the way to the chemical analysis of stars. He also stated the law on the emission and absorption of bodies, introducing the concept of the “black body”, a problem whose solution would later lead Planck to quantum physics. He was also the author of important works on mechanics and thermochemistry (Kirchhoff’s law on the heat of reaction).

Trivia

When a banker remarked that the gold detected in the solar spectrum was useless because unreachable, Kirchhoff, having later received a gold medal as an award, replied: “Here is some gold I have taken from the Sun.”

Theories and contributions

• Junction rule — The algebraic sum of the currents at a node is zero: $\sum_k I_k = 0$.
• Loop rule — The algebraic sum of the voltages around a closed loop is zero: $\sum_k V_k = 0$.
• Law of thermal radiation — For any body in thermal equilibrium, the ratio between emissive power and absorptive power depends only on temperature: $\dfrac{e_\lambda}{a_\lambda}=f(\lambda,T)$.
• Spectral analysis — Every chemical element can be identified by its characteristic spectral lines; the basis of astronomical spectroscopy.

Present in chapters

15 18 Z

Linked atoms

12 linked atoms.

15 Circuiti elettrici

• Kirchhoff’s junction rule and loop rule
• Systematic procedure
• Law of conservation of energy
• The circuit as a model

18 Induzione elettromagnetica

• The fictitious generator
• Outline for solving an induction problem
• Why the current i is the same throughout the circuit
• The LC circuit and the oscillation equation
• RLC circuit: damped oscillations
• Differential equation of the forced RLC circuit
• Energy balance of the RLC circuit

Z Appendice — Maturità

• Ministerial syllabus and book map