02conductors, insulators, and electron flow basic concepts of electricity.mht

老外的硬件基础教材

easier=20
passage to electrons than the long one. This is analogous to water flow =
in a=20
pipe: a fat pipe offers easier passage than a skinny pipe, and a short =
pipe is=20
easier for water to move through than a long pipe, all other dimensions =
being=20
equal. </P><A name=3DSuperconductivity></A>
<P>It should also be understood that some materials experience changes =
in their=20
electrical properties under different conditions. Glass, for instance, =
is a very=20
good insulator at room temperature, but becomes a conductor when heated =
to a=20
very high temperature. Gases such as air, normally insulating materials, =
also=20
become conductive if heated to very high temperatures. Most metals =
become poorer=20
conductors when heated, and better conductors when cooled. Many =
conductive=20
materials become perfectly conductive (this is called =
<I>superconductivity</I>)=20
at extremely low temperatures. </P><A name=3D"Static electricity"></A><A =

name=3D"Dynamic electricity"></A><A name=3D"Electricity, static vs. "=20
dynamic??></A><A name=3DCurrent></A><A name=3D"Electric current"></A>
<P>While the normal motion of "free" electrons in a conductor is random, =
with no=20
particular direction or speed, electrons can be influenced to move in a=20
coordinated fashion through a conductive material. This uniform motion =
of=20
electrons is what we call <I>electricity</I>, or <I>electric =
current</I>. To be=20
more precise, it could be called <I>dynamic</I> electricity in contrast =
to=20
<I>static</I> electricity, which is an unmoving accumulation of electric =
charge.=20
Just like water flowing through the emptiness of a pipe, electrons are =
able to=20
move within the empty space within and between the atoms of a conductor. =
The=20
conductor may appear to be solid to our eyes, but any material composed =
of atoms=20
is mostly empty space! The liquid-flow analogy is so fitting that the =
motion of=20
electrons through a conductor is often referred to as a "flow." </P>
<P>A noteworthy observation may be made here. As each electron moves =
uniformly=20
through a conductor, it pushes on the one ahead of it, such that all the =

electrons move together as a group. The starting and stopping of =
electron flow=20
through the length of a conductive path is virtually instantaneous from =
one end=20
of a conductor to the other, even though the motion of each electron may =
be very=20
slow. An approximate analogy is that of a tube filled end-to-end with =
marbles:=20
</P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00007.png"> </P>
<P>The tube is full of marbles, just as a conductor is full of free =
electrons=20
ready to be moved by an outside influence. If a single marble is =
suddenly=20
inserted into this full tube on the left-hand side, another marble will=20
immediately try to exit the tube on the right. Even though each marble =
only=20
traveled a short distance, the transfer of motion through the tube is =
virtually=20
instantaneous from the left end to the right end, no matter how long the =
tube=20
is. With electricity, the overall effect from one end of a conductor to =
the=20
other happens at the speed of light: a swift 186,000 miles per second!!! =
Each=20
individual electron, though, travels through the conductor at a =
<I>much</I>=20
slower pace. </P><A name=3DWire></A>
<P>If we want electrons to flow in a certain direction to a certain =
place, we=20
must provide the proper path for them to move, just as a plumber must =
install=20
piping to get water to flow where he or she wants it to flow. To =
facilitate=20
this, <I>wires</I> are made of highly conductive metals such as copper =
or=20
aluminum in a wide variety of sizes. </P>
<P>Remember that electrons can flow only when they have the opportunity =
to move=20
in the space between the atoms of a material. This means that there can =
be=20
electric current <I>only</I> where there exists a continuous path of =
conductive=20
material providing a conduit for electrons to travel through. In the =
marble=20
analogy, marbles can flow into the left-hand side of the tube (and,=20
consequently, through the tube) if and only if the tube is open on the=20
right-hand side for marbles to flow out. If the tube is blocked on the=20
right-hand side, the marbles will just "pile up" inside the tube, and =
marble=20
"flow" will not occur. The same holds true for electric current: the =
continuous=20
flow of electrons requires there be an unbroken path to permit that =
flow. Let's=20
look at a diagram to illustrate how this works: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00008.png"> </P>
<P>A thin, solid line (as shown above) is the conventional symbol for a=20
continuous piece of wire. Since the wire is made of a conductive =
material, such=20
as copper, its constituent atoms have many free electrons which can =
easily move=20
through the wire. However, there will never be a continuous or uniform =
flow of=20
electrons within this wire unless they have a place to come from and a =
place to=20
go. Let's add an hypothetical electron "Source" and "Destination:" </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00009.png"> </P>
<P>Now, with the Electron Source pushing new electrons into the wire on =
the=20
left-hand side, electron flow through the wire can occur (as indicated =
by the=20
arrows pointing from left to right). However, the flow will be =
interrupted if=20
the conductive path formed by the wire is broken: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00010.png"> </P><A =

name=3DContinuity></A>
<P>Since air is an insulating material, and an air gap separates the two =
pieces=20
of wire, the once-continuous path has now been broken, and electrons =
cannot flow=20
from Source to Destination. This is like cutting a water pipe in two and =
capping=20
off the broken ends of the pipe: water can't flow if there's no exit out =
of the=20
pipe. In electrical terms, we had a condition of electrical =
<I>continuity</I>=20
when the wire was in one piece, and now that continuity is broken with =
the wire=20
cut and separated. </P>
<P>If we were to take another piece of wire leading to the Destination =
and=20
simply make physical contact with the wire leading to the Source, we =
would once=20
again have a continuous path for electrons to flow. The two dots in the =
diagram=20
indicate physical (metal-to-metal) contact between the wire pieces: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00011.png"> </P>
<P>Now, we have continuity from the Source, to the newly-made =
connection, down,=20
to the right, and up to the Destination. This is analogous to putting a =
"tee"=20
fitting in one of the capped-off pipes and directing water through a new =
segment=20
of pipe to its destination. Please take note that the broken segment of =
wire on=20
the right hand side has no electrons flowing through it, because it is =
no longer=20
part of a complete path from Source to Destination. </P>
<P>It is interesting to note that no "wear" occurs within wires due to =
this=20
electric current, unlike water-carrying pipes which are eventually =
corroded and=20
worn by prolonged flows. Electrons do encounter some degree of friction =
as they=20
move, however, and this friction can generate heat in a conductor. This =
is a=20
topic we'll explore in much greater detail later. </P>
<UL>
  <LI><B>REVIEW:</B>=20
  <LI>In <I>conductive</I> materials, the outer electrons in each atom =
can=20
  easily come or go, and are called <I>free electrons</I>.=20
  <LI>In <I>insulating</I> materials, the outer electrons are not so =
free to=20
  move.=20
  <LI>All metals are electrically conductive.=20
  <LI><I>Dynamic electricity</I>, or <I>electric current</I>, is the =
uniform=20
  motion of electrons through a conductor.=20
  <LI><I>Static electricity</I> is an unmoving (if on an insulator), =
accumulated=20
  charge formed by either an excess or deficiency of electrons in an =
object. It=20
  is typically formed by charge separation by contact and separation of=20
  dissimilar materials.=20
  <LI>For electrons to flow continuously (indefinitely) through a =
conductor,=20
  there must be a complete, unbroken path for them to move both into and =
out of=20
  that conductor. </LI></UL>
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