Page 6 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

FUNDAMENTALS OF ELECTRIC SYSTEMS

FUNDAMENTALS OF ELECTRIC SYSTEMS 1.5

It represents the rate of variation of resistivity with temperature. Its units are 1/°C (or 1/°F).
Conductivity ( ), is used more commonly than resistivity. It is the inverse of conductivity,
given by
1

1
The units for conductivity are (
m) .
Across a resistor, the voltage and current have this relationship:
V iR

The power dissipated across the resistor (conversion of electric energy to heat) is given by
2
P i R
or
V 2
P
R
where P is in watts, i in amperes, V in volts, and R in ohms.

TABLE 1.2 Properties of Metals as
Conductors
Temperature
coefficient
Resistivity of resistivity
(at 20°C), , per C°
5 *
Metal 10 8
m ( 10 )
Silver 1.6 380
Copper 1.7 390
Aluminum 2.8 390
Tungsten 5.6 450
Nickel 6.8 600
Iron 10 500
Steel 18 300
Manganin 44 1.0
Carbon † 3500 50
* This quantity, defined from
1 d

dT
is the fractional change in resistivity d / per unit
change in temperature. It varies with temperature, the
values here referring to 20°C. For copper ( 3.9
10 3 /°C) the resistivity increases by 0.39 percent for
a temperature increase of 1°C near 20°C. Note that for
carbon is negative, which means that the resistivity
decreases with increasing temperature.
† Carbon, not strictly a metal, is included for com-
parison.

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