Lecture Two
Resistance
The resistance of any material depends on
A property of Material called resistivity,
Length, l
Cross Sectional Area, A
Temperature, T
The unit of measurement is ohm for which the symbol is . At a fixed temperature of 20ºC, the
resistance is
l
R
A
The higher the resistivity, the more resistance
The longer the length, the more resistance
The smaller the area, the more resistance
The higher the temperature, the more resistance
Units:
Consider the figure 1 mil (abbreviation for milli inch) = 10-3 inch.
A wire of diameter one mil has an area of one circular mil (CM). Thus a wire with d= 20 mil has
an area of 400 CM. Area of circular cross section is defined as
2 2
4
A r d
If d= 1 mil then
2
(1mil) sq.mil 1 CM
4 4
A
The resistivity of every material is different. It is the resistance of a length of wire 1 feet long
having a cross sectional area 1 mil. Unit of
is
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