The opposition offered by a substance to the flow of electric current through them is called resistance All materials oppose the flow of electric current through them i,e they have resistance Since the current is the flow of free electrons, resistance is the opposition offered by the substance to the flow of free electrons.this opposition occures because atoms and molecules of the substance obstruct the flow of these electrons .certain substances offer very little opposition to the flow of electric current are called conductors e,g copper ,aluminium ,silver etc On the otherhand substances which offers high opposition to the flow of electric current are called insulators e,g glass, rubver mica etc The resistance is the friction offered by the substance and causes production of heat with the flow of current.the moving electrons collide with the atoms or molecules of the substance, each collision resulting in the liberationof minute quantity of heat The unit of resistance...
About basic concepts of electronics circuits and electronic data sheets By; AAFAQ

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