Jumat, 12 Juli 2013
Electricity is magical. Electricity is the flow of electrical power or charge. Electricity is produced by spinning copper between magnets, which produces electrons that move as current through metal (copper) conductors. Mechanical energy is converted into electrical energy with the physical force usually provided by using steam to spin the copper. This is done through the conversion of primary sources of energy like coal, natural gas, oil, and nuclear power.
Electricity is produced from the following sources by the following amounts in the United States: coal 50 percent, nuclear 20 percent, natural gas 20 percent, Hydropower 9 percent, with the rest coming from wind, geothermal, solar and garbage.
There is the misconception that electricity is invariable made only of negatively-charged particles called electrons. This error also leads people to wrongly imagine that electrical current animation is always a flow of negative particles. Actually, in some situations electric currents can really be a flow of positive particles. In other situations, the flows are negative particles. And sometimes, they're both positive and negative flowing at once, but in opposite directions. The true direction of the flowing particles depends on the type of conductor. Electrons and protons have a property called “charge” that is the same size, but opposite in polarity for the proton and electron. The proton has 1836 times the mass of the electron, but exactly the same size charge, only positive rather than negative. Even the terms “positive” and “negative” are arbitrary, but well-entrenched historical labels. The bottom line is that the proton and electron will strongly attract each other.
Electricity is not made only of electrons. Charge actually comes in two varieties: positive and negative particles. Most people think the electrons ARE the electricity, and they think that protons are not electrical. Some text and reference books even state this outright, saying that electricity is composed of electrons. Nope. In reality the electrons and protons carry electrical charges of equal strength. If electrons are “electricity,” then protons are electricity too. Positive charge comes from having more protons than electrons; negative charge comes from having more electrons than protons.
It is generally believed that the protons within wires cannot flow, while the electrons can. Yes, this is true, but only for metals, and only for metals that are not liquid. Metals are composed of positively charged atoms immersed in a sea of movable electrons. When an electric current is created within a solid copper wire, the “electron sea” moves forward, but the protons within the positive atoms of copper do not. However, solid metals are not the only conductors, and in many other substances the positive atoms “do” move, and they “do” participate in the electrical current animation. These various non-electron conductors are nothing exotic. They are all around us. Two-way positive/negative electric currents can exist in: batteries, human bodies, all living organisms, the ground, the ocean, the sky (ionosphere), mercury and solder, ion-based smoke detectors, air cleaners, and the vertical “sky current” in the atmosphere, among others.1
Most of the electricity in the United States is produced in steam turbines. A turbine converts the kinetic energy of a moving fluid (liquid or gas) to mechanical energy. Steam turbines have a series of blades mounted on a shaft against which steam is forced, thus rotating the shaft connected to the generator. In a fossil-fueled steam turbine, the fuel is burned in a furnace to heat water in a boiler to produce steam. In a nuclear steam turbine,the heat from the fission of uranium atoms is produced in a reactor vessel heat water to produce steam.
The electricity generation sequence involves taking charge from the Earth, doing work on it to give it energy (expressed in terms of voltage), transporting the energy via a distribution system, using the energy, and dumping the spent charge back to the Earth.2
The illustration above shows the sequence. Three-phase electric power is common method of electric power transmission. It is a type of polyphase system mainly used to power motors and many other devices. A three-phase system uses less conductor material to transmit electric power than equivalent single-phase, two-phase, or direct-current systems at the same voltage.
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current animation. This delay between “phases” has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.33
Why is alternating current (AC) used for power distribution over direct current (DC), and why is three-phase used rather than single phase?
AC generators cost less and can produce more power than an equivalent size DC generator. AC in general is easier to distribute because very efficient transformers can be used to step up and step down distribution voltage. The step up helps reduce distribution losses by substantially reducing the electrical current animation carried by the long distance lines. The voltage on high tension transmission lines may range anywhere between 230kV to as much as 500kV with currents up to 450 Amperes. Transmitting that amount of power at low voltage (240 volts) would mean extremely high currents resulting in unacceptably high ohmic losses. There are several reasons why electricity is distributed in three phase:
- Polyphase generators can generate more power and cost less to maintain than single phase generators
- 3-phase distribution is more efficient than single phase AC
- Many industries require 3-phase power
Individual homes are usually wired for single phase 120/240 in North America. Street level distribution is about 13.8 kilovolts three-phase at 200 amps or so. Industries are some of the largest consumers of electricity. The primary supplier of electricity in the Washington, DC metropolitan area is Potomac Electric Power Company (PEPCO). There is no large industry in the DC area, so it turns out that the largest commercial user of electricity is PEPCO itself. They have all kinds of machinery to operate: pumps, grinders, conveyers, etc. It takes electricity to make electricity. The Blue Plains Sewage Treatment Plant is a very large user too. Homes and businesses don't take a lot of power in the grand scheme of things. Industries often run large motors. Three-phase motors, like generators, are also cheaper and cost less to operate. 4
In three-phase electricity, all three wires carry the same electrical current animation. Secondly power transfer into a linear balanced load is constant. Most domestic loads are single phase electricity. Generally three-phase electricity either does not enter domestic houses at all, or where it does, it is split out at the main distribution board. One voltage cycle of a three-phase electrical system is labeled zero to 20 along the time axis in the image at right. The plotted line represents the variation of instantaneous voltage (or current) with respect to time. This cycle will repeat 60 times per second, depending on the power system frequency. The colors of the lines represent the American color code for three-phase. That is yellow (or black), red and blue. 5
Step Down Transformer on Pole
What is Lenz's law? This law states that the direction of an induced current is always such as to oppose, by its magnetism, the action that produced it.
What is meant by an ampere (amp)? Just as a current of water is measured in gallons per second, so we could measure an electric current in electrons per second. But the electron has such a tiny charge that enormous numbers of them are needed to make even a small current. For example, a pressure of one volt pumps 6.25 billion billion electrons per second through a resistance of 1ohm. A current of this size is known as an ampere. A current of a little less than 1 ampere flows through a common 100 watt light bulb. The total electric charge of 6.25 billion billion electrons is called onecoulomb (koo-lowm). An ampere is also defined as a current that carries a charge of one coulomb per second.
Ohm's Law. The unit of electrical resistance is the ohm. To give you an idea of what the ohm is, note that 1,000 feet of No. 10 copper wire (1/10th inch thick) has a resistance of 1 ohm. If it were twice as thick, its resistance would be only 1/4th ohm, and it could carry four times as much current under the same conditions.
If 1 volt pushes 1 amp through 1 ohm, then 5 volts would be needed for five times as much current, or 5 amps. If the resistance were now doubled to 2 ohms, twice as much pressure or voltage would be needed. That is, it would take a 10-volt pressure to push a 5 amp current through a 2 ohm resistance.
Ohm's Law: Volts = amperes x ohms
In the case above above, 5 amps x 2 ohms equals 10 volts.
SAMPLE PROBLEM
(a) How larege a current will 120 volts send through a resistance of 2 ohms?
(b) What is the resistance of an electric toaster if 120 volts sends a current of 5 amps through it?
SOLUTION:
(a) Step 1. We re-arrange the Ohm's Law formula to read: Amps = volts/ohms. Step 2. Amps = 120/2 = 60
(b) Step 1. Re-arranging the formula, we have: Ohms = volts/amps. Step 2. Ohms = 120/5 = 24
Electricity is measured in units of power called watts. It was named to honor James Watt, the inventor of the steam engine. One watt is a very small amount of power. It would require nearly 750 watts to equal one horsepower. A kilowatt represents 1,000 watts. A kilowatt hour (kWh) is equal to the energy of 1,000 watts working for one hour. The amount of electricity a power plant generates or a customer uses over a period of time is measured in kilowatt hours (kWh). Kilowatt hours are determined by multiplying the number of kW's required by the number of hours of use. For example, if you use a 40-watt light bulb five hours a day, you have used 200 watt hours, or 0.2 kilowatt hours, of electrical energy. See Energy Calculator for converting units.6
Substation
SAMPLE PROBLEM
Find the to total power of the following appliances: a toaster that draws a current of 5 amps on a 115-volt line; an electric range that uses a 5-amp current on a 230-volt line; and ten 100-watt light bulbs.
SOLUTION
Step 1. The toaster uses 5 amps at 115 volts. Power (watts) = volts x amps = 115 x 5 = 575 watts
Step 2. The range uses 5 amps at 230 volts. Power = volts x amps = 230 x 5 = 1150 watts
Step 3. Ten lamps at 100 watts each. Power = 10 x 100 = 1000 watts
Step 4. The total power is 575 + 1150 + 1000 = 2725 watts, or 2.725 kilowatts.
The electric meter records the number of kilowatt-hours used. This is multiplied by the rate per kWh. If the rate is five cents per kilowatt-hour and you use 100 kWh, your bill will be 500 cents, or $5.00.
Electricity Meter
Source: Modern Physical Science
Nikola Tesla, left, and Thomas Edison, right, revolutionized American society by developing the first electricity distribution systems. Edison developed and promoted direct current (DC) and Tesla developed and promoted alternating current (AC) and they became rivals in commercializing their respective electrical systems. Tesla won. Unlike DC, AC could be stepped up to very high voltages with transformers, sent over thinner and less expensive wires, and stepped down again at the destination for distribution to users. Thomas Edison failed because he could not practically and economically send DC very far. Nikola Tesla developed and patented much of the AC power generation and distribution technology used today.
Edison's company established the first investor-owned electric utility in 1882 and his generating station's electrical power distribution system provided 110 volts DC to 59 customers in lower Manhattan. George Westinghouse and Edison became adversaries because of Edison's promotion of direct current for electric power distribution instead of the more easily transmitted AC systems invented by Tesla and promoted by Westinghouse. George Westinghouse purchased Tesla's patents and profited from them. Even with these patents, the company Edison founded, General Electric, is many times the size of Westinghouse. Tesla fell into relative obscurity, he is rarely mentioned in the history books. Nikola Tesla does not get the kind of recognition he truly deserves, even though he is the creator of polyphase transformers and machinery. Nikola Tesla is the real reason why we use three-phase distribution.
Edison did not invent the first electric light bulb, but instead invented the first commercially practical incandescent light. Edison patented an electric distribution system in 1880, which was essential to capitalize on the invention of the electric lamp. Nearly all of Edison's patents were utility patents but the phonograph patent was unprecedented as the first device to record and reproduce sounds. The key to Edison's fortunes was the telegraph. This allowed him to make his early fortune with the stock ticker, the first electricity-based broadcast system. Edison was also granted a patent for the motion picture camera. (Wikipedia: Tesla, Edison) (Madhu Siddalingaiah)
Transformers
No machine can be 100 percent efficient so the output energy of a transformer must be less than the input energy. This means that if you doublethe voltage, you get less than half the current (power in watts = volts x amps).
A step-down transformer, right, has fewer turns on the secondary side, and thus reduces the voltage. If the secondary coil has 1/10th as many turns as the primary coil, the output voltage will be only 1/100th as much. However, the current becomes nearly 10 times as large.
How do we use transformers? Electric energy can be transported economically over transmission lines when voltage is high and the current is low. This is because transmission losses are proportional to the square of the current. Thus, high voltages permit the use of low amperages which, in turn, result in less heat loss in the transmission line. Hence, the electrical current animation from the generators is sent to step-up transformers, where the voltage may go to 300,000 or 400,000 volts. At the other end of the line, other transformers step the voltage down for distribution to the consumer.
Step-down transformers may be used whenever low voltages are needed. On the other hand, you also may use one whenever a large current is desired. For example, the large current needed for electric welding is obtained from step-down transformers.
Direct Current Electric Motor Animation
Source: Modern Physical Science
Direct current or DC electricity is the continuous movement of electrons from an area of negative (-) charges to an area of positive (+) charges through a conducting material such as a metal wire. Whereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, DC electricity is the continuous movement of the electrons through a wire. A DC circuit is necessary to allow the current or steam of electrons to flow. Such a circuit consists of a source of electrical energy (such as a battery) and a conducting wire running from the positive end of the source to the negative terminal. Electrical devices may be included in the circuit. DC electricity in a circuit consists of voltage, current and resistance. The flow of DC electricity is similar to the flow of water through a hose. 7
Direct and Alternating Current
There are two different ways that electricity is produced, and they are used in most cases for very different purposes. They can also be converted from one form to another.
The first and simpler type of electricity is called direct current, abbreviated “DC.” This is the type of electricity that is produced by batteries, static and lightning. A voltage is created, and possibly stored, until a circuit is completed. When it is, the current flows directly, in one direction. In the circuit, the current flows at a specific, constant voltage (this is oversimplified somewhat but goo enough for our needs). When you use a flashlight, pocket radio, portable CD player or virtually any othte type of portable or battery-powered device, you are using direct current. Most DC circuits are relatively low in voltage; for example, your car's battery is approximately 12 V, and that's about as high a DC voltage as most people ever use.
An idealized 12 V DC current. The voltage is considered positive because its potential is measured relative to ground or the zero-potential default state of the earth. (This diagram drawn to the same scale as the AC diagram below.)
The other type of electricity is called alternating current, or “AC”. This is the electricity that you get from your house's wall and that you use to power most of your electrical appliances. Alternating current is harder to explain than direct current. The electricity is not provided as a single, constant voltage, but rather as a sinusoidal (sine) wave that over time starts at zero, increases to a maximum value, then decreases to a minimum value, and repeats A representation of an alternating current's voltage over time is shown in the diagram below.
While simple direct current circuits are generally described only by their voltage, alternating current circuits require more detail. First of all, if the voltage goes from a positive value to a negative value and back again, what do we say is the voltage? Is it zero, because it averages out to zero? That would seem to imply that there is no energy there at all. But imagine, if you will, a wave of water flowing across the surface of the sea. The peaks and troughs of the wave seem to “cancel each other out,” but the wave clearly exists and has energy. The same is true of alternating current.
The way the science world measures the energy in an AC signal is to compute what is called the root mean square (RMS) average of the voltage. In simple terms, the RMS value of an electrical current is the number which represents the same energy that a DC current at that voltage would produce; it is in essence an average of the alternating current waveform. Whenever you see an AC voltage specification, they are giving you the RMS number unless they say otherwise specifically. So for example, in North America, most homes have 115 VAC electricity. This is AC electricity equivalent in energy to a 115 V CD circuit. (This is an approximate number, and standard household electricity in North America is also sometimes called 110VAC or 120VAC; it's the same thing). Other parts of the world use different voltages ranging from 100VAC to 240 VAC, and of course, heavy equipment anywhere can use much higher voltages.
The other key characteristic of AC is its frequency, measured in cycles per second (cps) or, more commonly, Hertz (Hz). This number describes how many times in a second the voltage alternates from positive to negative and back again, completing one cycle. In North America, the standard is 60Hz, meaning 60 cycles from positive to negative and back again in one second. In other parts of the world the standard is 50 Hz.
Three cycles of an idealized North American 115 VAC, 60 Hz alternating current signal (black curve). Note that each cycle represents 16.67 milliseconds of time, because that is 1/60th of a second. The curve actually goes from -170 V to +170 V in order to provide the average (RMS) value of 115 V. The RMS equivalent is shown as a green horizontal line. To demonstrate what RMS means, look at the blue shaded area, which shows the total energy in the signal for one cycle. The green shading is the area between the RMS line and the zero line for one cycle, and represents the energy in an equivalent 115 V DC signal. The definition of the RMS value is that which makes the green and blue areas equal. (This diagram drawn to the same scale as the DC diagram above.)
Why does standard electricity come only in the form of alternating current? There are a number of reasons, but one of the most important is that a characteristic of AC is that it is relatively easy to change voltages from one level to another using a transformer, while transformers do not work for DC. This capability allows the companies that generate and distribute electricity to do it in a more efficient manner, by transmitting it at high voltage for long lengths, which reduces energy loss due to the resistance in the transmission wires. Another reason is that it may be easier to mechanically generate alternating current electricity than direct current.
PCs use only direct current, which mans that the alternating current provided by your utility must be converted to direct current before use. This is the primary function of your power supply.
Source: The PC Guide
Magnetism
Magnetism is caused by the spinning and orbiting motion of electrons. A magnet has two kinds of poles, north and south. Like poles repel; unlike poles attract. The source of magnetism in any material is believe to come from spinning electrons within the atoms. Each electron generates a magnetic field as it spins on its own axis and also as it revolves in the orbit around the nucleus of an atom.
The atoms of most elements are not noticeably magnetized because about equal numbers of electrons spin in opposite directions, thus canceling their magnetic forces. In iron and a few other elements, however, these forces are not canceled and a magnetic field results.
In magnetic material, the atoms group themselves into microscopic regions called domains. Within the domains, the atoms are lined up so that they all point in one direction.
An electromagnet consists of a coil of insulated wire, usually wound around a soft iron core. Its strength depends upon the electrical current animation, number of turns of wire and type of core.
All electrical motors use magnetism to change electric energy into mechanical energy.
