flow. If the electric current is in an AC circuit, then the amount of current will depend on how much impedance is present. If the electric current is in a DC circuit, then the amount of current will depend on how much resistance is present. The pump is like a battery. It provides pressure to keep the flow moving. The relationship among voltage, resistance, and current is voltage (V) = current (I) multiplied by resistance (R). In other words, V=I*R. This is Ohm’s law, named after the scientist who explored these issues. Two ways in which current flows are Alternating Current (AC) and Direct Current (DC). Alternating current (AC) and voltages vary over time by changing their polarity, or direction. AC flows in one direction, then reverses its direction and flows in the other direction, and then repeats the process. AC voltage is positive at one terminal, and negative at the other. Then the AC voltage reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive. This process repeats itself continuously. DC always flows in the same direction, and DC voltages always have the same polarity. One terminal is always positive, and the other is always negative. They do not change or reverse. An oscilloscope is an electronic device used to measure electrical signals relative to time. An oscilloscope graphs the electrical waves, pulses, and patterns. An oscilloscope has an x-axis that represents time, and a y-axis that represents voltage. There are usually two y-axis voltage inputs so that two waves can be observed and measured at the same time. Power lines carry electricity in the form of AC because it can be delivered efficiently over large distances. DC can be found in flashlight batteries, car batteries, and as power for the microchips on the motherboard of a computer, where it only needs to go a short distance. Electrons flow in closed circuits, or complete loops. Figure shows a simple circuit. The chemical processes in the battery cause charges to build up. This provides a voltage, or electrical pressure, that enables electrons to flow through various devices. The lines represent a conductor, which is usually copper wire. Think of a switch as two ends of a single wire that can be opened or broken to prevent electrons from flowing. When the two ends are closed, fixed, or shorted, electrons are allowed to flow. Finally, a light bulb provides resistance to the flow of electrons, causing the electrons to release energy in the form of light. The circuits involved in networking use a much more complex version of this very simple circuit. For AC and DC electrical systems, the flow of electrons is always from a negatively charged source to a positively charged source. However, for the controlled flow of electrons to occur, a complete circuit is required. Remember, electrical current follows the path of least resistance. Figure shows part of the electrical circuit that brings power to a home or office. Lab Activity Lab Exercise: Series CircuitsIn this lab, the student will build and explore the basic properties of series circuits. Web Links Circuits http://www3.iptv.org/exploremore/energy/ Energy_In_Depth/sections/ circuits.htm
Content 3.1 Copper Media 3.1.6 Cable specifications Cables have different specifications and expectations pertaining to performance: Some examples of Ethernet specifications which relate to cable type include: 10BASE-T refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The T stands for twisted pair. 10BASE5 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 5 represents the capability of the cable to allow the signal to travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE5 is often referred to as Thicknet. Thicknet is actually a type of network, while 10BASE5 is the cabling used in that network. 10BASE2 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 2, in 10BASE2, represents the capability of the cable to allow the signal to travel for approximately 200 meters, before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE2 is often referred to as Thinnet. Thinnet is actually a type of network, while 10BASE2 is the cabling used in that network. Web Links Networking Guide: Making Your Own Cable http://www.tuplay.com/display. asp?i=41&p=1
Content 3.1 Copper Media 3.1.7 Coaxial cable Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements, located in the center of the cable, is a copper conductor. Surrounding the copper conductor is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield reduces the amount of outside electro-magnetic interference. Covering this shield is the cable jacket. For LANs, coaxial cable offers several advantages. It can be run longer distances than shielded twisted pair, STP, and unshielded twisted pair, UTP, cable without the need for repeaters. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known. It has been used for many years for many types of data communication, including cable television. When working with cable, it is important to consider its size. As the thickness of the cable increases, so does the difficulty in working with it. Remember that cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable, because it has a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, this type of cable can be too rigid to install easily in some situations. Generally, the more difficult the network media is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used anymore, except for special purpose installations. In the past, ‘thinnet’ coaxial cable with an outside diameter of only 0.35 cm was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since thinnet was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet. The outer copper or metallic braid in coaxial cable comprises half the electric circuit and special care must be taken to ensure a solid