Nace CP 1, Course Manual

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Basic Electricity 1:3
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Resistance and Resistivity
Resistance
is the opposition that charges encounter when moving
through a material.
The ohm is the common unit of resistance measurement. It is the
resistance of a conductor when a voltage of one volt produces a
current flow of one ampere along the conductor. Resistance may also
be measured in milliohms (0.001 Ohm) or in megohms (1,000,000
Ohms).
Common symbols for resistance are:
•
R, r
• Ω (
Greek letter
omega
)
Resistance is important in such matters as cathodic protection
groundbeds, resistance of a structure to the electrolyte, and the linear
resistance of a long structure such as a pipe or cable.
Resistivity
is the resistance of a conductor of unit length and unit
cross-sectional area.
The symbol used for resistivity is
ρ
(Greek letter rho).
Resistivity is constant for a given material and is computed by the
formula:
L
A R
×
=
ρ
where
ρ
= Resistivity in ohm-cm
R = Resistance in Ohms
A = Cross-sectional area in cm
2
L = Length in cm
If the resistivity of a material is known (see Table 1.1), the resistance
of a conductor such as a cable or pipeline of known length and cross-
sectional area can be calculated from:
A
L
R
×
=
ρ