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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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