Tue May 01, 2012 By:

What would be the effect of resistance of a metal wire on increasing its length,diameter and temperature?

Expert Reply
Tue May 01, 2012
when you apply voltage across any piece of material, the electrons in it (off course they are there!) start moving in the direction opposite to magnetic field created because of the voltage applied. If there are enough number of electrons, its a metal else insulator. 
But when electrons move, they get scattered from the nuclei present in the material. This is what causes the material to show "resistance" towards the flow of the current.
so...based on the above information, one can easily understand the following.

1. The more is the length of the sample (wire, if you wish) there are more number of scatterers in the direction electron moves to, larger resistance. So 
"R is proportional to L"

2. If the cross section is large, a large number of electrons can move Parallel to cause current, and as current is total sum of charge flowing per second, it make easy to get large current for a given voltage, hence less resistance. So
"R is inversely proportional to A"

3. Lastly if the electron density is itself very high, the current will be overall large. So larger density of mobile electrons should decrease resistance.
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