OHM's LAW

In general, materials have a characteristics behavior of resisting the flow of electric charge. This physical property or ability to resist current is known as resistance and is represented by the symbol R. In other words, resistance is the capacity of materials to impede the flow of or, more specifically, the flow of electric charge. The circuit element used to model this behavior is the resistor.

Conceptually, we can understand the resistance of material, if we think about moving electrons that make up electric current interacting with and being resisted by the atomic structure of the material through which they are moving. Because of the course of these interactions, some amount of electric energy is converted to thermal energy and dissipated in the form of heat. This effect is not desirable but many useful electrical appliances take advantage of resistance heating, including space heaters, irons, stores and toasters.

Figure below shows a material with uniform cross-sectional area A, length l and resistivity ρ(ohm-meters).

We can represent resistance in mathematical form,

The amount of resistance depends on the material.

Good conductors such as copper and aluminium have low resistivity and hence have small value of resistance. They are good choices for wiring used to conduct electric current. In a circuit diagram, copper or aluminium wiring is not usually modeled as a resistor. The resistance of the wire is so small compared to the resistance of other elements in the circuit that we can neglect the wiring resistance to simplify the diagram. For the purpose of constructing circuits, resistors are usually made from metallic alloys and carbon compound.

The circuit symbol of resistor is shown in figure below:-

It is denoted by R. The resistance is the simplest passive element.

Resistivity of common material is show in table below:-

Material	Resistivity	Usage
Gold	2.45*10-8	Conductor
Silver	1.64*10-8	Conductor
Copper	1.72*10-8	Conductor
Aluminum	2.8*10-8	Conductor
Silicon	6.4*102	Semiconductor
Carbon	4*10-5	Semiconductor
Germanium	47*10-2	Semiconductor
Mica	5*1011	Insulator
Paper	1*1010	Insulator

 Georg Simon Ohm (1784-1854), a German physicist, who established the relationship between current and voltage for a resistor. This relationship is known as ohm's law. Ohm's law is the algebraic relationship between voltage and current for a resistor. Ohm's law states that the voltage "v" across the resistor is directly proportional to the current "i" flowing though the resistor.

 i.e. v ∝ i

or, v=i.R

Above  equation is the mathematical form of ohm's law.

Ohm defined the constant of proportionality to be the resistance R. The resistance can change if the external or internal conditions of the element are altered, for example, if there are changes in the temperature.

If current flows from a higher potential to a lower potential, v=i.R and if a current flows from a lower potential to a higher potential, v=-i.R. Since the value of R can vary from ZERO to INFINITY, we must consider the two extreme possible values of R.

A element with R=∞ is called an open circuit as shown in figure(a).