الفيزياء لمدارس اللغات والتجريبي الباب الأول درس Physics| Parameters affecting The electric resistance

Physics| Parameters affecting The electric resistance

 

Parameters affecting The electric resistance:
The resistance of a conductor of a uniform cross sectional area at constant temperature depends upon the following parameters:-



The length of conductor (L):
The resistance of a resistor is inversely proportional to its crosssectional area.
R α L ,

The cross sectional area of the conductor (A):
The resistance of a resistor is inversely proportional to its cross sectional area.
R α 1/A



The kind of material of the conductor: The resistance of a resistor differs from one material to another.From the previous discussion we find that:-
R α L (1)
R α 1/A (2)
From (1) and (2) we find that: R α L/( A)



Where (ρe) is the proportionality constant (depends on the material ofthe resistor) called the resistivity of the material (its unit is Ohm. meter).



The resistivity of a material (ρe):
ρe = It is numerically equal to the resistance of a conductor of 1 m long and 1m2in cross sectional area.Thus, ρe = R, whenA = 1m2 , L = 1 m
N.B. The resistivity of material is a physical property of this material.



Question:
What is meant by the resistivity of copper = 1.7 X 10-8Ω.m?
Answer: This means that 1.7 X10-8 Ω is the resistance of a conductor of copper of 1m long and cross sectional area 1m2.



The electrical conductivity (σ ):
It is reciprocal of the resistivity of a material



∵ρ_e=RA/L ∴ σ=L/RA
The unit of elec. Conductivity: Ohm-1 m-1or Simon. The electrical conductivity of a material is a physical property of such material.



Question:
What is meant by the electrical conductivity of copper = 5.6X 107 Ω-1.m-1?



Answer:
This means that ( 1/(5.6X 〖10〗^7 )Ω) or (0.1786×10-7Ω) is the electrical resistance of a conductor of copper of 1m long and cross sect. area 1m2.
OR, it means that 5.6×〖10〗^7 Ω-1 is the reciprocal of the resistance of a conductor of copper of length 1m and cross sectional area 1m2.



Materials are divided into :-
Good conductors:Their electrical conductivity is relatively high, such as metals.

Physics| Parameters affecting The electric resistance

Semiconductors:Their electrical conductivity is relatively small (lies between conductors and insulators ), such as silicon , or germanium.
Bad conductors: Their electrical conductivity is very small such as Cadmium Sulfide and Arsenic Sulfide.