The.
proportionality constant R is called the "resistance-, measured in Ohms (?). R is measured by applying a.
known voltage across the resistor and measuring the current through it. The reciprocal of resistance (R-1) is.
called conductance. Ohm's law is an empirical law, related to irreversible thermodynamics (Ilya Prigogine,.
Nobel Prize in Chemistry 1977), the flow I as a result of a gradient in potential leads to energy being.
dissipated (RI2 Joule s-1).
Not all materials obey Ohm's law. Gas discharges, vacuum tubes, semiconductors and what are termed onedimensional.
conductors (e.g.a linear polyene chain) generally all deviate from Ohm's law.
In Ohmic material the resistance is proportional to the length l of the sample and inversely proportional to the.
sample cross-section A:.
R =.
l A (2).
where is the resistivity measured in cm (in SI units m). Its inverse =-1 is the conductivity. The unit.
of conductance is the Siemens (S = -1). The unit of conductivity is S m-1.
FIGURE 1 Conductivity of conductive polymers compared to those of other materials, from quartz.
(insulator) to copper (conductor). Polymers may also have conductivities corresponding to those of.
semiconductors.
Conductivity depends on the number density of charge carriers (number of electrons n) and how fast they can.
move in the material (mobility µ):.
? = n µ e (3).
where -e is the electron charge. In semiconductors and electrolyte solutions, one must also add in Eq (3) an.
extra term due to positive charge carriers (holes or cations). Conductivity depends on temperature: it.
3.
generally increases with decreasing temperature for "metallic- materials (some of which become.
superconductive below a certain critical temperature Tc), while it generally decreases with lowered.
temperature for semiconductors and insulators.
FIGURE 2.
The conductivity of conductive polymers decreases with falling temperature in contrast to the conductivities.
of typical metals, e.g. silver, which increase with falling temperature.
