Kirchhoff gave two laws for current and voltage. First law is called KCL and second law is called KVL.

**Kirchhoff’s current law (KCL):** This is the first law of Kirchhoff’s about current. How current divide in a circuit this law explains.

The branch of physics where relation between heat and work done is discussed called thermodynamics. In thermodynamic process firstly we have to know about thermodynamic system. What is thermodynamic system? The system is a closed area where thermodynamic substance observed. Two types of system are i) close system and ii) open system.

**Basic gates **

There are three basic gates in digital electronics. These are NOT gate, AND gate & OR gate.

** NOT gate / Inverter:** NOT gate is called inverter also. Operation of NOT gate is if input is high output is low and if input is low then output is high. Another words output is always inverse or toggle of input. So it is called inverter. Circuit diagram of inverter is given below Read more on Logic gates…

American engineer Norton explained his theorem in 1926 after 43 years publishing Thevenin’s theorem. Norton’s theorem is similar to Thevenin’s theorem.

Nortons theorem states that **“a linear two terminal circuit can be replaced by an equivalent current source I _{N} in parallel with a resistance R_{N} where R_{N} is the input or equivalent resistance at the terminals and I_{N} is the short circuit current through the terminals when all the independent sources are turned off”.**

Practically always occurs that a particular element like load is always variable when the other elements are fixed in a circuit. If we consider our household as example a household outlet terminal may be connected to different appliances constituting a variable load. Every time the variable is changed, the complete circuit has to be analyzed again. This is harassment for electrical engineer. To avoid this problem we can use the method of Thevenin’s theorem. The fixed circuit is replaced by an equivalent circuit and according to this method.

The device which raises or lowering voltage is called transformer. Previously I have discussed about ideal transformer and practical transformer, working principal of transformer also. Transformer gives same output power which we give as input power but raise or down voltage corresponding current decrease or increase. Input and output frequency is same. We also know transformer is used only ac current.

## Practical transformer

Practical transformer is different from ideal transformer in many respects. Ideal transformer has no losses but practical transformer have

- Iron losses
- Magnetic leakage
- Winding resistances

**Ideal transformer on no load**

A transformer is ideal if it has

- No leakage flux
- No windings resistance
- No iron loss in core

Ideal transformer can not possible physically. But it gives powerful tool in the analysis of a practical transformer. Properties of ideal and practical transformer is very close each other.