الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

7.5 Evaluation of the resistance of a conductor
Lectures in General Physics
Consider a cylindrical conductor as shown in figure 7.2, of cross-sectional
area A and length l , carrying a current I. If a potential difference V is
connected to the ends the conductor, the electric field and the current
density will have the values
E
I
A
Figure 7.2
V
E = (7.10)
l
and
I
J =
A
The resistivity ρ is
E V I
ρ = =
(7.11)
J I A
But the V/I is the resistance R this leads to,
l
R = ρ
(7.12)
A
Therefore, the resistance R is proportional to the length l of the conductor
and inversely proportional the cross-sectional area A of it.
Notice that the resistance of a conductor depends on the geometry of the
conductor, and the resistivity of the conductor depends only on the
electronic structure of the material.
Dr. Hazem Falah Sakeek