r/AskScienceDiscussion • u/redditomguser • 7d ago

General Discussion Does conductivity and length have anything to do with each other?

im so confused rn, ive googled this and some say that based on ohms law, conductivity decrease as length and resistance increase, while others say that length and conductivity have nothing to do with each other, can someone please explain

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u/whateverusername 7d ago

Conductivity is a intrinsic property of the material, so it is constant. It is also inversely proportional to the resistivity (ρ).
Resistance (R) is proportional to resistivity and length, and inversely proportional to the cross-sectional area of the conductor, so the resistance increases with length and the conductance (not the conductivity) decreases with length.
In Ohm's law (I=V/R), the current (I) depends on the resistance, not the opposite.

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u/PeculiarAlize 1d ago

Generally, conductivity decreases, and resistance increases with length. However, the cross-section of the conductor also plays a role in determining resistance. Hence, a long fat wire and a short skinny wire may have the same resistance.

The simplest analogy is a straw. The longer your straw, the harder it is to suck up liquid. However, the diameter of the straw also plays a key role in the sucking resistance.

This analogy is not perfect because the cross-sectional area only plays a role in conductivity to a certain degree. This is because surface area/cross-section perimeter is actually more important in determining the resistance of a conductor because of the way electrons flow mostly along the surface. That's why sometimes high current high voltage conductors use a flat oval to save material.