Resistance is the property of a substance that offers opposition to the flow of electric current. It denotes by Uppercase letter R.

Explanation of Resistance

Metals like Gold, Silver, Copper, Aluminium etc. are good conductors of electricity. This is due to the presence of a large number of free electrons in their atoms. These electrons flow in a particular direction when a potential difference is applied. When current flows through the conductor, the electrons collide with the molecules or atoms of the conductor thereby producing heat. Heat depends on the presence of number molecules in the conductors, if a large number of molecules are present more heat is produced and vice versa. Gold, Silver, Copper etc. metals offer low restriction to the flow of electric current. So we can say that these materials having low resistance.

Carbon Resistance
Carbon Resistor

The materials (like bakelite, mica, glass, etc.) are poor conductors of electricity. This is due to loosely-attached or free electrons are not present in materials. Hence, no current flow even voltage is applied. These materials offer large resistance to the flow of electric current.

Unit of Resistance

The unit of resistance is ohm. A conductor said to have a resistance one ohm if it allows one ampere current flow to through it when one volt is applied across its terminal. 

The resistance of insulators is very high, in that cases much bigger unit is used, i.e. mega-ohm, kilo-ohm. In the case of very small resistances, smaller unit is used like milli-ohm, micro-ohm, etc.

Factor Affecting Resistance

The resistance of a conductor, depends on the following factors:

  • It is directly proportional to the length.
  • It is inversely proportion to the area of cross section.
  • It depends on the nature of a material.
  • It almost varies directly with the temperature.

Resistivity

Resistivity of material may be defined as, the resistance between the opposite faces of a metre  cube of that material. The unit of resistivity is ohm-metre.