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Based on the Ohm's law, the ohm is defined as follows: "the electric resistance, where one volt electric voltage produces one ampere electric current, is one ohm." Ohm's Law may be simply defined as: in three categories of electric quantities (electric voltage, electric current, and electric resistor), quantity is in direct proportion to total voltage, and in reverse proportion to total electric resistance. This is the most important principle in electronics. Its application is very comprehensive. This principle is very simple and easy to understand. At the same time, it is also the basis for the analysis of the highly complicated theories of electric circuit and system analysis.

Figure 1 Conversion table between Ohm's law and Joule Law.

In materials, the flow of electric charges may encounter the same types of resistance as mechanical frictions. This resistance is caused by the collision between electronic and lattice atoms or impurity atoms and leads to the conversion of electric energy into heat energy. Any electric circuits or devices will loss power due to heating. The heating by electric resistance and consume power may not be a bad thing, depending if it is useful. Some electric circuits use the properties of electric resistance to work. For example, we use electric heater to obtain heat energy. However, we do not want heating of transistors. It wastes energy. Electric resistance may be used to restrict electric current. It may also be used to adjust electric voltage. And, there are other functions. The devices that are used specifically to perform these functions are called electric resistance devices (resistor). When an electric circuit desires to prevent electric current from passing through, at the same time it converts electric energy into heat energy, it is called the electric resistor. The unit for electric resistance is ohm, or in short ohm. And Greek word Ω (omega) is used. Within a conductor, there are large amount of free electrons, when electric voltage is applied to the two ends of the conductor, it will generate electric current. However, the electric current cannot be increased unlimitedly. The reason is that, when electric charges passing through certain materials, they have to sustain the material's electric resistance and convert into heat energy. For a fixed electric current, the higher the electric resistance, the higher the requirement for electric voltage. Similarly, for a fixed electric voltage, the higher the electric resistance, the lower the electric current. Ohm's Law concerns three types of electric quantities

(1) Electric voltage, represents by E, unit is VOLTS.
(2) Electric current, represents by I, unit is AMPERES.
(3) Electric resistance, represents by R, unit is OHMS.
The law states that electric voltage equals to electric current multiply by electric resistor (E=ＩＲ)