Resistor

        Resistor is a semiconductor device.when a current flows in a material, the free electrons move through the material and collide with other atoms.these collisions cause the electrons to lose some of their energy. this loss of energy per unit charge is the drop in potential across the material . the amount of energy lost by the electrons is related to the physical property of the material. These collisions restrict the movement of electrons. The property of a material to restrict the flow of electrons is called resistance, denoted by R. The symbol for the resistor is shown in fig.

The unit of resistance is ohm. ohm is defined as the resistance offered by the material when a current of one ampere flows between two terminals with one volt applied across it.

     According to ohm's law, the current is directly proportional to the total resistance of the circuit, i.e.

I=V/R

or i= V/R

that 

I= current

V= voltage 

R= resistance