Current
Electricity
Prepared in Dec 1998
Second editing in march 2000
Learning objectives
At the end of this unit you should be able to :
1. use quantitatively the proportionality between
resistance and the length and the cross-sectional
area of a wire.
Factors
Factors
affecting
affecting Resistance
Resistance
• Temperature:
In general, for metallic wire, the higher the
temperature, the larger its resistance.
• Length :
Longer the length, larger its resistance.
• Cross-sectional area of wire:
Small the area (or diameter), larger its
resistance.
Importance
Importance Formula
Formula
• If the surrounding temperature is assumed
constant.
• we have,
R α L and R α 1/A
where R = resistance of the wire
L = length of the wire
A = cross-sectional are of the wire
imply that
R α L/A (continue on next slide)
Importance
Importance Formula
Formula
ρ
where ρ is known as
the resistivity of the material
unit of ρ is Ωm
Example
• The diameter of the copper wire used in a circuit is
2.0 mm. If the resistively for copper is 1.7 x 10-8
Ωm, what is the resistance for 50 cm of the wire ?
L = 50 cm = 0.5m
diameter = 2.0 mm = 0.002 m
A = π (d/2)2 = π (0.002/2)2 = π(0.001)2
R = (1.7 x 10-8)(0.5) / π(0.001)2
= 0.0027 Ω