Kirchhoff’s law of heat from "summary" of Understanding Thermodynamics by H.C. Van Ness

Kirchhoff's Law of Heat is a fundamental law that states the total entropy of any isolated system remains constant, no matter what the changes in its state might be.

Kirchhoff’s Law of Heat holds true for systems consisting of both reversible and irreversible processes; it specifies an equilibrium where all heated areas are at the same temperature while losses due to friction between the parts of the system are also taken into account.

Essentially, this law states that the energy within an isolated system must remain constant and flow from one area to another creating an equal balance of heat energy throughout the entire system.

The main principle underlying Kirchhoff’s law is conservation of energy, which states that the total amount of energy in any closed system remains constant regardless of other transformations taking place inside it.

This law was formulated by German physicist Gustav Robert Kirchhoff in 1859 and it has since been recognized as one of the most important and fundamental laws in thermodynamics.

In physical terms, it can be defined mathematically as the ratio of the rate of change of the entropy over the rate of change of heat added per unit mass. This relation is often used to analyze heat transfer circuits and identify equilibrium or steady state conditions.

This law is useful when designing or analyzing devices such as engines, refrigerators or air conditioners - it can greatly affect internal efficiency as well as external output by allowing engineers to take into account the tradeoffs between energy inputs and outputs.

By understanding this law, we gain insights about how energy flows through a system and how its various components interact. It helps us understand how heat is conducted through materials, and why objects of different temperatures reach "thermal equilibrium" when brought together.