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Electron Flow and Temperature
Name: Jean K.
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002
Question:
I am an education student studying to be a elementary
education teacher. I want to know where I can find information on the
following for lessons I am developing.
Do electrons move faster in cold or warm power lines? I have read
conflicting statements on this topic.
Replies:
Hi, Jean !!!
As you know, the electrical resistance of a wire
is proportional to the temperature. In other words,
the more heated, the greater will be the resistance.
In power lines, the electron flow occurs somehow
concentrated at the external surface of the wires.
Nevertheless, this does not invalidate this argument.
So that the conclusion should be that warm power
lines make electrons move slower than under the
same conditions in cold lines.
Regards
Alcir Grohmann
Dear Jean-
Remember Ohm's Law: V=IR. This can be re-arranged to I=V/R. This states
that
the current (I) through a circuit is equal to the voltage across the
circuit (V)
divided by the resistance in the circuit (R). Since I is inversely
proportional
to R, then for a fixed V, the current will decrease as the resistance
increases.
One of the things which increases electrical resistance is the increased
motion
of the molecules and free electrons of a conductor caused by
heating. This is
due to their random collisions with the electrons making up the current which
tends to interfere with their forward progress. So, it is true that
heating of
the power line causes the free electrons to move faster, but in random
directions. As a result, the net electron current will be diminished due
to the
increased frequency of random collisions.
Do not confuse this everyday state of affairs with what happens in a
superconductor. When certain metals are cooled to extremely low
temperatures,
they exhibit virtually no resistance to electrical current. This is due to
quantum mechanical coupling of pairs of electrons in a fashion that causes
them
not to interact with the atomic electrons in the conductor. For further
information on electrical current at ambient temperatures, you might want to
consult any encyclopaedia with entries for 'conduction' and
'superconductivity',
or the Electricity section of any introductory physics text in your school
library.
Best wishes,
JGW
Electrons move faster in a warm wire, but they do not travel as far
along the wire as they would if it were cold.
The motion of an electron can be separated into two components:
1) rapid motion driven by temperature. This motion produces no net
transfer of charge along the wire.
2) slow drift driven by the electric field in the wire
Because the resistance of a wire increases with temperature, less
current flows in a warm wire than would flow in a cold wire with the
same applied voltage. The drift speed of electrons in the wire is
proportional to the current.
Tim Mooney
Jean,
I do not know a web site that addresses this particular question, but I
believe I may be able to explain the conflict. You must more clearly state
your question. Are you asking whether individual electrons jiggle faster or
whether the overall current move faster?
At higher temperatures, individual electrons jiggle faster, but in a less
organized fashion. The current speed is effectively the average motion of
the electrons. In a wire with zero current, the electrons move very fast;
however, there is no preferred direction. In a wire with current, there is
a slight preference for an electron to move toward the higher voltage
(actually opposite the current, since electrons have negative charge). In a
standard current, electrons actually move trough about 4 meters of wire per
second. Individual electrons move at speeds approaching that of light.
At high temperatures, the true speed of an electron is higher because the
electrons have more energy from the heat. At high temperatures, current
tends to be less because the electron motion is less organized, more random.
For a more day-to-day example, it is easier to direct the motion of a calm
cow than a raging bull. Still, the raging bull moves much faster.
Dr. Ken Mellendorf
Illinois Central College
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