section
item  ELECTRON-SPEED
The effective speed at which electrons travel through a conductor, is much smaller
than often assumed. It is in the order of a millimeter per second. An electric current of
ten ampere for instance, represents an electron displacement of about 6 x 10^19
electrons per second at which the symbol ^ stands for "to the power of". 10^19 is a
short notation for a one with nineteen zeros or 10 billion billion. This seems to be a lot
of electrons but a copper wire with a cross-cut of one square millimeter and a length
of one meter contains about 10^23 copper atoms, as much as the number of tennis
balls that would fit into the moon. When we assume that one atom of copper
participating in the conduction, provides no more than one free electron, the effective
drift velocity of the electric charge through the wire would be about (6 x 10^19)/(
10^23) meter/second or 0,6 mm/second. What ìs fast however, is the propagation of
a current change. For copper this propagation speed is about 250.000 km/sec so a
change in current travels around the globe in about 160 milliseconds. This explains
why we can make a fixed wire phone call towards the other side of the earth with only
a small notable delay while it would take an individual electron about 1000 years to
reach the other side of the globe.   building
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