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<html> <head> <meta content="en-us" http-equiv="Content-Language"> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../../css/ccna.css"> <script src="../../../../../js/framesetHelper.js" language="JavaScript"></script><script language="JavaScript"> window.scrollTo(0,0); function newPage(pageID) { alert("COMMING SOON!!\nThis feature is currently in development."); } function launchMoreInfo(oid) { dash = oid.indexOf('-'); sub1 = oid.substring(0,dash); sub2 = oid.substring(dash+1); // this is an URL base on the // system that generated the static // download of the curriculum. httpref = "http://nomar.sdf.cisco.com/servlet/org.cli.delivery.rendering.servlet.MIServlet/Static=true,LMSID=DTUI,Engine=dynamic,Theme=cnamstheme,Style=ccna,Language=en,Version=2,RootID=knet-v214aCV47051/CHAPID=null/RLOID=null/RIOID=null/"; newref = httpref + sub1 +"/"+ sub2 +"/moreinfoframeset.html"; //alert("new newref :\n" + newref); window.open(newref,'MoreInfo','toolbar=no,status=yes,menubar=no,location=no,scrollbars=yes,height=442,width=756,left=20,top=20,resizable=yes') } function launchContentResource(contentID) { top.currentSupport = 0; 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eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_crf.html"'); chapter = "knet-v214aCH47504"; ch_dash = chapter.indexOf('-'); ch_sub1 = chapter.substring(0,ch_dash); ch_sub2 = chapter.substring(ch_dash+1); params = "CHAPID=" + chapter + "/RLOID=" + "knet-v214aRLO47542" + "/RIOID=" + "knet-v214aRIO121150"; returnRef = "../../../../../"+ params + "/"+ ch_sub1 +"/"+ ch_sub2 +"/pageframeset.html"; top.prevPage = returnRef; } function launchFile(filepath) { newWindow = window.open(filepath,"","top=70,left=120,width="+(screen.width*.50)+",height="+(screen.height*.50)+",resizable=yes,scrollbars=yes,toolbar=yes,menubar=yes,status=yes"); /* if (is.ie && filepath.lastIndexOf(".pdf") != -1) { pageTimer = setInterval("winLoadCheck()", 1000); } */ } function winLoadCheck() { if (newWindow.document.readyState == 'complete') { clearInterval(pageTimer); newWindow.location.reload(); } } // BrowserCheck Object // provides most commonly needed browser checking variables // 19990326 // Copyright (C) 1999 Dan Steinman // Distributed under the terms of the GNU Library General Public License // Available at http://www.dansteinman.com/dynapi/ function BrowserCheck() { var b = navigator.appName if (b=="Netscape") this.b = "ns" else if (b=="Microsoft Internet Explorer") this.b = "ie" else this.b = b this.v = parseInt(navigator.appVersion) this.ns = (this.b=="ns" && this.v>=4) this.ns4 = (this.b=="ns" && this.v==4) this.ns5 = (this.b=="ns" && this.v==5) this.ie = (this.b=="ie" && this.v>=4) this.ie4 = (navigator.userAgent.indexOf('MSIE 4')>0) this.ie5 = (navigator.appVersion.indexOf('MSIE 5.0')>0) this.ie55 = (navigator.appVersion.indexOf('MSIE 5.5')>0) if (this.ie5) this.v = 5 this.min = (this.ns||this.ie) } // automatically create the "is" object is = new BrowserCheck()</script> </head> <body onLoad="window.focus();" alink="#000000" vlink="#000000" link="#000000" bgcolor="#E7E7E7" background="../../../../../images/ccna/common/bg.gif"> <table width="100%" cellspacing="0" cellpadding="2" border="0"><tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="rlohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="rlohdr">4.1</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Basics of Electricity</td> </tr> </table> </td> </tr> <tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="riohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="riohdr">4.1.4</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Electrical current including insulators, conductors, and semiconductors</td> </tr> </table> </td> </tr> <tr> <td colspan="4"> <table width="90%" cellspacing="0" cellpadding="0" border="0"> <tr> <td class="smtext"> <p> </p><table cellpadding="0" cellspacing="0" width="100%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td width="100%"><img height="10" width="1" border="0" src="../../../../../images/ccna/common/transdot.gif"></td> </tr> <tr> <td width="100%"> <table bgcolor="#000000" bordercolor="#000000" cellspacing="0" bordercolorlight="#000000" width="100%" border="0"> <tr> <td width="100%"> <table bgcolor="#FFFFFF" cellspacing="0" cellpadding="2" width="100%" border="0"> <tr> <td valign="middle"><img height="22" width="22" border="0" src="../../../../../images/ccna/common/inotes.gif"></td><td valign="middle"><span class="cstitle">Instructor Note</span></td> </tr> <tr> <td valign="top"> </td><td valign="top"><span class="cstext"> <p>The purpose of this target indicator is that students realize that the networking devices and components with which they will be working are based on very precise control of electron flow. Network devices use a combination of conductors (usually copper conducting paths), semiconductors (usually Silicon-based Integrated Circuits), and insulators (usually plastic or rubber to form plugs, connectors, cable jackets).</p> <p>There are many demonstrations possible with a digital multimeter. You can show that the electrical resistance of objects gives us a measure of whether a material is a conductor (low resistance), semiconductor (moderate resistance), or insulator (high resistance). You could demonstrate a low voltage series circuit with a 6 V lantern battery, a low voltage light bulb, a pencil, some plastic, some alligator clip leads, and a piece of copper. Make a complete circuit with the copper piece and the bulb burns brightly. Make a complete circuit with a short piece of pencil graphite and the bulb burns dimly. Make a complete circuit with a piece of plastic and the bulb does not glow.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>All materials can be classified as belonging to one of three groups. They are classified as insulators, conductors, or semiconductors, depending on how easily electricity flows through them.</p><p> <b>Electrical Insulators</b><img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47542/RIOID=knet-v214aRIO121150/knet/v214adataimage1/1.gif" width="12" height="12"><br><i>Electrical insulators</i>, or <i>insulators</i>, are materials that allow electricity to flow through them with great difficulty, or not at all. Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber, and helium gas.</p><p> <b>Electrical Conductors</b> <br><i>Electrical conductors</i>, or <i>conductors</i>, are materials that allow electrons to flow through them with great ease.</p><p>The best conductors are metals such as copper, silver, and gold.</p><p>Other conductors include solder, a mixture of lead (Pb) and tin (Sn), and water with <i>ions</i>. An ion is an atom that has more electrons, or fewer electrons, than a neutral atom. The human body is made of approximately 70% water with ions, which means that it, too, is a conductor. </p><p> <b>Electrical Semiconductors</b> <br><i>Semiconductors</i> are materials where the amount of electricity they conduct can be precisely controlled. Examples include carbon, germanium, and the alloy, gallium arsenide. The most important semiconductor, the one that makes the best microscopic-sized electronic circuits, is silicon.<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47542/RIOID=knet-v214aRIO121150/knet/v214adataimage2/2.gif" width="12" height="12"></p><p>Silicon is very common and can be found in sand, glass, and many types of rocks.</p><p>Whether materials are classified as insulators, conductors, or semiconductors, it is the way they work together in various combinations that is the basis for all electronic devices.</p><table cellpadding="0" cellspacing="4" width="63%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td valign="middle" width="8%"><img height="23" width="23" src="../../../../../images/ccna/common/icon2.gif"></td><td valign="middle" width="92%"><span class="cstitle">Web Links</span></td> </tr> <tr> <td valign="middle" width="8%"> </td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://easyweb.easynet.co.uk/~jesus.heals/java/basic/basic.htm">Forms of Energy</a></span></td> </tr> </table></td> </tr> </table> </td> </tr></table> <p> </p> <table width="100%" cellspacing="0" cellpadding="2" border="0" valign="bottom"> <tr> <td></td> </tr> </table> </body></html>⌨️ 快捷键说明
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