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Electrical Conductivity and Temperature
Name: Mark
Status: student
Grade: other
Location: Outside U.S.
Country: USA
Date: Winter 2011-2012
Question:
Is it true that generally electrical conductivity of metals first increases with temperature, then decreases? (one of my teachers said that mobility of electrons increases rapidly and thus conductivity increases but with the increasing temperature, their movement is restricted by vibrating atoms, so the conductivity decreases.. )
Replies:
Mark
From the following Wikipedia article:
http://en.wikipedia.org/wiki/Electrical_resistance_and_
conductance#Temperature_dependence
"Near room temperature, the resistivity of metals typically increases as
temperature is increased, while the resistivity of semiconductors typically
decreases as temperature is increased."
What this means is that current is reduced as temperature and resistivity
increase.
Please note the word "typically." That means in most situations as there
are a few situations where this is not true as this article discusses.
Sincere regards,
Mike Stewart
Mark,
In general, this is not true for metals as can be seen in the
Handbook of Chemistry and Physics. This reference book gives
resistivity (1/conductivity) for a wide range of metals and over a
range from almost absolute zero to 900 Kelvin. In general,
conductivity decreases with increases in temperature from low
temperature to high. That said, we can explain this general trend by
noting that good conductors (metals) have many electrons that are
free to move in the material. These are called "conduction"
electrons and act much like they are a flowing like water in a pipe.
Occasionally, these electrons will collide with the fixed atoms of
the metals (nuclei) and be deflected, or "scatter." They are slowed
in this process and have to return to the overall flow of electrons
resulting in what we see as "resistance" to electron flow or a
decrease in conductance. Increasing temperature increases the
agitation of this flow, the electron collisions, and the scattering
amount resulting in a decrease in conductance over temperature.
What your teacher probably meant to say (or you were meant to hear)
is that the conductance of semiconductors will increase initially
with temperature then decrease. Electrons in a semiconductor are
more tightly bound to their respective atoms and are not free to
roam the material and participate in current flow. Agitation of the
atoms by increasing the temperature will cause electrons bound to
the atoms to escape, roam the material and participate in current
flow. Increased temperature increases the number of these available
electrons and causes conductivity to decrease. As temperature
increases, scattering from the vibration of the atoms will take
over, and the semiconductor will act much like a metal--the
conductivity will decrease over temperature.
Kyle J. Bunch
There may be a mix of misunderstanding and / or incomplete explanation. The
comparison you make describe the behavior of both metals and semiconductors.
The resistivity of a METAL increases with increasing temperature. In
contrast the resistivity of a SEMICONDUCTOR decreases with increasing
temperature. The "reason" for this is the different mechanisms for the
transport of electrons. But this is an apples and oranges comparison. I
suppose it is possible that in a temperature range a substance behaves like
a METAL and in another temperature range behaves like a semiconductor, but I
don't know of an example. In any case it would be misleading to describe
such a substance as both a metal and semiconductor. Physicists would
consider these as different PHASES of the substance. An analogy would be the
resistivity of a solid phase and liquid phase of a given substance. The
mechanism for electron transport in those two phases is different. You can
see details at the website:
http://en.wikipedia.org/wiki/Electrical_resistivity_and_
conductivity#Semiconductors
Vince Calder
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Update: June 2012
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