|  | 4.1 | Basics of Electricity
| 4.1.5 | Electrical measurement terms
These are the terms that describe networking media. Voltage Voltage, sometimes referred to as electromotive force (EMF), is an electrical force, or pressure, that occurs when electrons and protons are separated. The force that is created pushes toward the opposite charge and away from the like charge. This process occurs in a battery, where chemical action causes electrons to be freed from the battery's negative terminal, and to travel to the opposite, or positive, terminal. The separation of charges results in voltage. Voltage can also be created by friction (static electricity), by magnetism (electric generator), or by light (solar cell). Voltage is represented by the letter "V", and sometimes by the letter "E", for electromotive force. The unit of measurement for voltage is volt (v), and is defined as the amount of work, per unit charge, needed to separate the charges. Current Electrical current, or current, is the flow of charges that is created when electrons move. In electrical circuits, current is caused by a flow of free electrons. When voltage (electrical pressure) is applied, and there is a path for the current, electrons move from the negative terminal (which repels them), along the path, to the positive terminal (which attracts them). Current is represented by the letter "I". The unit of measurement for current is ampere (amp), and is defined as the number of charges per second that pass by a point along a path. Resistance Materials through which current flows, offer varying amounts of opposition, or resistance, to the movement of the electrons. Materials that offer very little, or no, resistance, are called conductors. Those that do not allow the current to flow, or severely restrict its flow, are called insulators. The amount of resistance depends on the chemical composition of the materials. Resistance is represented by the letter "R". The unit of measurement for resistance is the ohm (W ). The symbol comes from the Greek capital letter "W" - omega. Alternating Current (AC) This is one of the two ways in which current flows. Alternating current (AC) and voltages vary with time, by changing their polarity, or direction. AC flows in one direction, then reverses its direction, and repeats the process. AC voltage is positive at one terminal, and negative at the other, then it reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive. This process repeats itself continuously. Direct Current (DC) This is the other one, of the two ways, in which current flows. Direct current (DC) always flow 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. Impedance Impedance is the total opposition to current flow (due to AC and DC voltages). The term resistance is generally used when referring to DC voltages. Impedance is the general term, and is the measure of how the flow of electrons is resisted, or impeded. Impedance is represented by the letter "Z". Its unit of measurement, like that for resistance, is the ohm (W ). Voltage, Current, Resistance Relationship Currents only flow in closed loops called circuits. These circuits must be composed of conducting materials, and must have sources of voltage. Voltage causes current to flow, while resistance and impedance oppose it. Knowing these facts allows people to control a flow of current. Ground The term ground can be difficult concept to understand, completely, because people use the term for many different purposes. Ground can refer to the place on the earth that touches your house (probably via the buried water pipes), eventually making an indirect connection to your electric outlets. When you use an electric appliance that has a plug with three prongs, the third prong is the ground. It gives the electrons an extra conducting path to flow to the earth, rather than through your body. Ground can also mean the reference point, or the 0 volts level, when making electrical measurements. Voltage is created by the separation of charges, which means that voltage measurements must be made between two points. A multimeter (which measures voltage, current, and resistance) has two wires for that reason. The black wire is referred to as the ground, or reference ground. A negative terminal on a battery is also referred to as 0 volts, or reference ground. Note: A multimeter is test equipment used for measuring voltage, current, resistance, and possibly other electrical quantities and displaying the value in numeric form.
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