Ohm's Law | Resistance | Resistivity | Power Loss in Resistor

Electrical resistance is the property of a material by virtue of which it opposes the flow of electrons (or charge) through the material . It is represented by R. 

Resistance is so because when electrons flow through the material they collide with conductor atoms and other electrons thus losing all their kinetic energy in the form of heat.So, this shows resistance is the property responsible for heating of conductors and is the operating principle of equipment like room heater, immerson rod ,iron etc.

Resistance formula is   R= (ρ×L)/A

 Where, ρ=specific resistance/resistivity of the material  (Ω-m)

              L=length of the conductor (m)

              A=area of cross-section of the conductor     (m²)  

Ohm (Ω) is the unit of resistance. The Ohm unit was named after George Simon Ohm.

Specific resistance /Resistivity of a material :- 

 ρ = (R×A)/L     

                         The resistivity of a material is defined as the resistance between any two faces of a unit cube of that material. It depends only on temperature.

                                   Its unit is ohm metre (Ω-m)

Ohm’s Law :- Ohm’s law states that at a given temperature , the current passing through a conductor  will be directly proportional to the voltage  applied across it.

                              So, I ∝ V

                             Or   V= IR

Where,
R= proportionality constant called resistance between the points of                        application of voltage(V).

 I = current      

Ohm’s law is not followed by non-metallic materials like semiconductors.  

The voltage-current graph is a straight line  as  shown. 

Also, the slope of the graph gives the value of resistance i.e R = ΔV / ΔR  

Power Loss in a Resistor:-   As  explained above heat is produced in a resistor and therefore there will be power loss. The formula/expression for power loss is given by

                                                          P = V× I    

                                                             = (R× I) × I

                                                             = I² × R = V² / R

The unit is watt (W) 

Area under the V-I curve gives power loss in the resistor.  P = V× I