Wed May 02, 2012 By: Manisha Mohanty

Shaleen sir,Why resistance is directly proportional to the length of the conductor and inversely proportional to the area of its cross-section?Please explain it to me.

Expert Reply
Thu May 03, 2012
Resistance is caused due to the collision of the moving free electrons in a conductor with the fixed positive ions in the metal when a potential difference is applied across the conductor. As the length increases, the number of collisions by the moving free electrons with the fixed positive ions increases as more number of fixed positive ions are present in an increased length of the conductor. As a result, resistance increases.

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The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance.
The answer is surface area. Current travels on the surface of a conductor.
Since surface area is where current travels, the greater the cross sectional area, the greater the surface area, the lower the resistance.
In fact, a copper pipe makes just as good a conductor as a copper bar.

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