# Revision of 9 Basic Electrical Engineering Laws and Theorems

Are you done with your Basic Electrical Engineering Course at college?

Are you in 2nd, 3rd or 4th year of your electrical degree?

Are you a professional Engineer or an Electrical Technician.

If any of your answer to the above question is in ‘YES’, this post belongs you. This article presents basic definitions which help you strengthen your basic concepts. The revision is equally important if you appearing in competitive exams, job tests, and interviews. You’ll revise:

1. Ohm’s law
2. Kirchhoff’s current law
3. Kirchhoff’s voltage law
4. Superposition principle
5. Thevenin theorem
6. Norton theorem
7. Maximum power transfer theorem
8. Substitution theorem
9. Millman theorem

### Ohm’s Law

The simplest and basic law is obviously the Ohm’s law which relates current and voltage flowing through a resistor.

Statement: The current flowing through any resistor is directly proportional to the voltage applied to it.

Mathematically, V = IR

### Kirchhoff’s Current law

We all know that current is a basic feature of circuits which exists due to flow of charges. Kirchhoff’s current law explains the behavior of current at any junction.

Statement: The current flowing towards any junction (node) is equal to the current flowing away from that node.

Mathematically, ∑ Current In = ∑ Current out

### Kirchhoff’s Voltage law

Like Kirchhoff’s first law is focused on current at any junction, the voltage law explains the behavior of voltage around a loop.

Statement: The sum of voltage rise along a closed loop is equal to the sum of voltage drops around that loop.

Mathematically, ∑ Voltage rise = ∑ Voltage drop

### Superposition principle

Many electrical circuits contain a single source powering different resistors. Sometimes a circuit contain multiple current and voltage sources. A superposition principle is applied to all circuits having multiple sources.

Statement: The voltage or current appearing across any component is equal to the sum of individual voltage or current of all independent sources.

### Thevenin Theorem

The statement of Thevenin Theorem is self-explanatory and it requires no explanation:

Statement: Any complex electrical circuit can be reduced to a single voltage source having a single series resistor.

Practically, the Thevenin and Norton (which you’ll be reading below) theorems are used in analyzing the properties of electrical and electronic systems. They are employed in modeling transmission lines and large complex systems.

### Norton theorem

Statement: Any complex electrical circuit can be reduced to a single current source having a single parallel resistor.

### Maximum Power Transfer Theorem

Using Thevenin and Norton’s theorem we can reduce a complex electrical circuit to a single source and a single resistor. The Maximum power transfer theorem utilizes the Norton and Thevenin’s principle. Let’s revise it,

Statement: if the value of load resistor is equal to the single resistor ( as calculated from Norton or Thevenin theorem) the load resistor will receive the maximum power.

### Substitution Principle

Statement: Any electrical branch can be substituted with an equivalent electrical branch provided that current and voltage of both branches are same.

### Millman’s Theorem

The Millman’s theorem is applied to the circuits which have several voltage sources in the parallel configuration. According to this theorem, the voltages source in parallel branches can be replaced by equivalent current sources and parallel resistors which can then be reduced to a single voltage source and a series resistors.