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Resistance


Figure 1
    It may be defined as the property of a substance due to which it opposes the flow of electricity (Read: The Ide of Electric Potential) throug it. 
     Metal (as a class), acids and salt solutions are good conductors of electricity. amongst pure metals, silver, copper, and aluminium are vey good conductors in the given order. This, as discussed earlier (Read: Modern Electron Theory), is due to the presence of a large number of free or loosely-attached electrons in their atoms. These vagrant electrons assume a derected motion on the application of an electric potential difference. These electrons while flowing pass throug the molecules or the atoms of the conductor, collide with other atoms and electrons, thereby producing heat.
               Figure 2 metal ( https://www.imperiummultiniaga.com)

   Figure 3 copper (http://www.hoo-tronik.com)
    These substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.v.v (polyvinly chloride ) and dry wood etc. amongst good insulators can be included fibrous substance such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelin and asbestos and many other plastics besides p.v.c. 
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Modern Electron Theory

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Effect of Temperature on Resistance and Temperature-Coefficient of Resistance

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The Unit of Resistance

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