# 4A: Existing Laws

### 3.   Existing Laws

The list of some of the well known laws of conservation is (a) Law of conservation of mass, (b) Law of conservation of energy, (c) Law of conservation of linear momentum (d) Kirchhoff’s current law (e) Kirchhoff’s voltage law etc. We briefly describe them in the next section with the goal of highlighting an unifying concept behind them.

#### Conservation of mass law

This is one of the most fundamental laws of nature. It says [Beiser, p.104] that the total mass in a closed system remains constant. It can only be transformed from one form to another. This is widely used in chemistry in the analysis of chemical reactions.

The closed system is an assumption. It is not necessary. The mass will never be destroyed, and therefore the law will remain valid even in open system. For the purpose of analysis and verification a closed system will be necessary.

#### Conservation of energy law

This is another version of the sigma law. It says that we can neither create nor destroy energy [Serway, p.199], energy is always conserved. The total energy remains constant in a closed system. This is also known as the first law of thermodynamics. The book [Crowell, p.146] says – “The law of conservation of energy is also known as the first law of thermodynamics”. Usually the laws of thermodynamics are stated in a different way. You may have to interpret it carefully to see that it is indeed the sigma law.

#### Kirchhoff’s Current law

This law says that the sum of the currents entering any junction in an electric circuit must equal the sum of the currents leaving that junction [Serway, p.869]. This is same as the sigma of all currents at any junction is zero. This law is valid for all kinds of through or flow variables for all types of networks.