05resistance basic concepts of electricity.mht

老外的硬件基础教材

inside of a clear glass bulb, which glows white-hot ("incandesces") with =
heat=20
energy when sufficient electric current passes through it. Like the =
battery, it=20
has two conductive connection points, one for electrons to enter and the =
other=20
for electrons to exit. </P>
<P>Connected to a source of voltage, an electric lamp circuit looks =
something=20
like this: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00027.png"> </P><A =

name=3DResistance></A>
<P>As the electrons work their way through the thin metal filament of =
the lamp,=20
they encounter more opposition to motion than they typically would in a =
thick=20
piece of wire. This opposition to electric current depends on the type =
of=20
material, its cross-sectional area, and its temperature. It is =
technically known=20
as <I>resistance</I>. (It can be said that conductors have low =
resistance and=20
insulators have very high resistance.) This resistance serves to limit =
the=20
amount of current through the circuit with a given amount of voltage =
supplied by=20
the battery, as compared with the "short circuit" where we had nothing =
but a=20
wire joining one end of the voltage source (battery) to the other. </P>
<P>When electrons move against the opposition of resistance, "friction" =
is=20
generated. Just like mechanical friction, the friction produced by =
electrons=20
flowing against a resistance manifests itself in the form of heat. The=20
concentrated resistance of a lamp's filament results in a relatively =
large=20
amount of heat energy dissipated at that filament. This heat energy is =
enough to=20
cause the filament to glow white-hot, producing light, whereas the wires =

connecting the lamp to the battery (which have much lower resistance) =
hardly=20
even get warm while conducting the same amount of current. </P>
<P>As in the case of the short circuit, if the continuity of the circuit =
is=20
broken at any point, electron flow stops throughout the entire circuit. =
With a=20
lamp in place, this means that it will stop glowing: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00028.png"> </P><A =

name=3D"Open circuit"></A><A name=3D"Closed circuit"></A><A=20
name=3D"Circuit, open"></A><A name=3D"Circuit, closed"></A>
<P>As before, with no flow of electrons, the entire potential (voltage) =
of the=20
battery is available across the break, waiting for the opportunity of a=20
connection to bridge across that break and permit electron flow again. =
This=20
condition is known as an <I>open circuit</I>, where a break in the =
continuity of=20
the circuit prevents current throughout. All it takes is a single break =
in=20
continuity to "open" a circuit. Once any breaks have been connected once =
again=20
and the continuity of the circuit re-established, it is known as a =
<I>closed=20
circuit</I>. </P><A name=3DSwitch></A>
<P>What we see here is the basis for switching lamps on and off by =
remote=20
switches. Because any break in a circuit's continuity results in current =

stopping throughout the entire circuit, we can use a device designed to=20
intentionally break that continuity (called a <I>switch</I>), mounted at =
any=20
convenient location that we can run wires to, to control the flow of =
electrons=20
in the circuit: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/00029.png"> </P>
<P>This is how a switch mounted on the wall of a house can control a =
lamp that=20
is mounted down a long hallway, or even in another room, far away from =
the=20
switch. The switch itself is constructed of a pair of conductive =
contacts=20
(usually made of some kind of metal) forced together by a mechanical =
lever=20
actuator or pushbutton. When the contacts touch each other, electrons =
are able=20
to flow from one to the other and the circuit's continuity is =
established; when=20
the contacts are separated, electron flow from one to the other is =
prevented by=20
the insulation of the air between, and the circuit's continuity is =
broken. </P>
<P>Perhaps the best kind of switch to show for illustration of the basic =

principle is the "knife" switch: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/50010.jpg"> </P>
<P>A knife switch is nothing more than a conductive lever, free to pivot =
on a=20
hinge, coming into physical contact with one or more stationary contact =
points=20
which are also conductive. The switch shown in the above illustration is =

constructed on a porcelain base (an excellent insulating material), =
using copper=20
(an excellent conductor) for the "blade" and contact points. The handle =
is=20
plastic to insulate the operator's hand from the conductive blade of the =
switch=20
when opening or closing it. </P>
<P>Here is another type of knife switch, with two stationary contacts =
instead of=20
one: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/50006.jpg"> </P>
<P>The particular knife switch shown here has one "blade" but two =
stationary=20
contacts, meaning that it can make or break more than one circuit. For =
now this=20
is not terribly important to be aware of, just the basic concept of what =
a=20
switch is and how it works. </P>
<P>Knife switches are great for illustrating the basic principle of how =
a switch=20
works, but they present distinct safety problems when used in high-power =

electric circuits. The exposed conductors in a knife switch make =
accidental=20
contact with the circuit a distinct possibility, and any sparking that =
may occur=20
between the moving blade and the stationary contact is free to ignite =
any nearby=20
flammable materials. Most modern switch designs have their moving =
conductors and=20
contact points sealed inside an insulating case in order to mitigate =
these=20
hazards. A photograph of a few modern switch types show how the =
switching=20
mechanisms are much more concealed than with the knife design: </P>
<P><IMG src=3D"http://sub.allaboutcircuits.com/images/50019.jpg"> </P><A =

name=3D"Switch, open"></A><A name=3D"Switch, closed"></A>
<P>In keeping with the "open" and "closed" terminology of circuits, a =
switch=20
that is making contact from one connection terminal to the other =
(example: a=20
knife switch with the blade fully touching the stationary contact point) =

provides continuity for electrons to flow through, and is called a =
<I>closed</I>=20
switch. Conversely, a switch that is breaking continuity (example: a =
knife=20
switch with the blade <I>not</I> touching the stationary contact point) =
won't=20
allow electrons to pass through and is called an <I>open</I> switch. =
This=20
terminology is often confusing to the new student of electronics, =
because the=20
words "open" and "closed" are commonly understood in the context of a =
door,=20
where "open" is equated with free passage and "closed" with blockage. =
With=20
electrical switches, these terms have opposite meaning: "open" means no =
flow=20
while "closed" means free passage of electrons. </P>
<UL>
  <LI><B>REVIEW:</B>=20
  <LI><I>Resistance</I> is the measure of opposition to electric =
current.=20
  <LI>A <I>short circuit</I> is an electric circuit offering little or =
no=20
  resistance to the flow of electrons. Short circuits are dangerous with =
high=20
  voltage power sources because the high currents encountered can cause =
large=20
  amounts of heat energy to be released.=20
  <LI>An <I>open circuit</I> is one where the continuity has been broken =
by an=20
  interruption in the path for electrons to flow.=20
  <LI>A <I>closed circuit</I> is one that is complete, with good =
continuity=20
  throughout.=20
  <LI>A device designed to open or close a circuit under controlled =
conditions=20
  is called a <I>switch</I>.=20
  <LI>The terms <I>"open"</I> and <I>"closed"</I> refer to switches as =
well as=20
  entire circuits. An open switch is one without continuity: electrons =
cannot=20
  flow through it. A closed switch is one that provides a direct (low=20
  resistance) path for electrons to flow through. </LI></UL>
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