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Transcript of Electric
The narrow visible part of the electromagnetic spectrum corresponds to the wavelengths near the maximum of the S un's radiation curve. In interactions with matter, visible light primarily acts to elevate electrons to higher energy levels.
White light may be separated into its spectral colors by dispersion in a prism.
Frequencies: 4 - 7.5 x 1014 Hz
Wavelengths: 750 - 400 nm
Quantum energies: 1.65 - 3.1 eV Electricity
Magnetism Application Electric Shock Lightning Transformers Household Circuits Video Lightning
Models for the charge buildup which leads to lightning discharges suggest a buildup of a strong negative charge layer near the bottom of the cloud and the formation of a positive ground shadow. When the buildup is large enough to produce ionization of the air, a lightning discharge is initiated. This is called the positive dipole structure for the charge buildup. More recent studies indicate a tripolar structure.
With voltages of hundreds of millions of volts and currents in the tens of thousands of amperes, lightning flashes can be very destructive.
Most of the information in this section comes from E. R. Williams' Scientific American article "The Electrification of Thunderstorms" or from Martin Uman's classic book "Lightning". What is Lightning This is certainly a plausible question, and you can answer that it will take the path of least resistance to the Earth. But the classic response is to say "That is like asking where the 800 pound gorilla is going to sit. Anywhere he wants!" That is not to say that there are no physical laws involved, it is just that there are many variables and a degree of chaos in the environment of the charge. In a medium which could be classified as "nonlinear", implying that it doesn't follow simple proportionalities, the path of the lightning strike is not predictable in practice. As an example of this kind of variability, electric discharges can be produced with a Tesla coil. The path of the discharge changes drastically with time, even though the voltage and the nature of the surrounding air are reasonably constant. Where will lightning strike? Dipole Models for Cloud Charging Charging Model Tripolar model, cloud charging In addition to the positive region at the top of a thundercloud and the main negative N-region near the bottom, a smaller positive region called the p-region has been observed at the bottom of the cloud. This positive region is thought to be important in the triggering of the most common cloud-to-ground discharges. According to Williams, the N-region is a thin, pancake shaped layer of thickness less than a kilometer but which may extend several kilometers horizontally. It is typically at a height of about 6 km,and lightning discharges are typically some 3 km long in cloud-to-ground discharges.
The term lightning flash is used to describe the entire discharge, which takes on the order of 0.2 seconds. But a flash is usually made up of several shorter discharges which last less than a millisecond and which repeat rapidly enough that the eye cannot resolve the multiple events. These individual discharges are called strokes. Sometimes the strokes are separated enough in time for the eye to resolve them, and the lightning appears to flicker. Lightning flashes and strokes The following data is from an Atlanta Journal article
About 95 people die from lightning yearly in the U.S.
A single thunderstorm can release 125 million gallons of water (that's the volume of 16 Washington Monuments).
One storm can discharge enough energy to supply the entire U.S. with electricity for 20 minutes
A large Midwestern cumulonimbus can tower 12-15 miles (Mount Everest is 5.5 miles high.)
There are approximately 2,000 thunderstorms at any given moment worldwide.
Lightning has killed 6,000 persons in the United States in the past 34 years. In 1990, 74 persons were killed and 252 more were injured. Florida is the most dangerous state with 14 killed and 27 injured in 1990. In 1987 an Atlas-Centaur rocket was struck by lightning on launch and had to be destroyed. Lightning Destruction
Williams says a typical lightning bolt bridges a potential difference (voltage) of several hundred million volts. Lightning Voltages Lightning Current
Williams says that a typical lightning bolt may transfer 1020 electrons in a fraction of a second, developing a peak current of up to 10 kiloamperes.
According to Uman, the German scientist Pockels discovered that basalt rock in the vicinity of lightning strikes was magnetized and deduced currents on the order of 10,000 amps in 1897. Ampere's law allows you to deduce the current in a wire from the measurement of the magnetic field at some radius from the wire. Pockels presumably had measured the magnetizing effects of large currents on basalt and was able to scale those experiments to estimate the current associated with the lightning. Based on that principle, magnetic links are widely used for the measurement of the lightning currents. Most measurements have been in the range 5,000 to 20,000 amps but a famous strike just before the Apollo 15 launch in 1971 was measured at 100,000 amperes by magnetic links attached to the umbilical tower. Currents over 200,000 amps have been reported.
One could envision a magnetic detector based on both Ampere's law and Faraday's law which could give you an estimate of lightning current provided you had a measurement of the distance from the detector to the lightning strike point. If you set up a coil of wire in a vertical plane, then the rate of change of magnetic field through the coil would generate a voltage. If you could sum (integrate) the current generated by that voltage, you could calculate the charge transferred in the lightning strike. With several such detectors in an area, you could model the location as well as the charge associated with the strike.
Most commonly, the lightning current ceases in about a millisecond for a given stroke, but sometimes there is a continuing current on the order of 100 amps following one or more of the strokes. This is called "hot lightning" and it is the cause of lightning fires according to Uman. The temperatures of lightning are 15,000-60,000°F for both "cold" and "hot" lightning - it is the continuing current that starts some 10,000 fires per year in the U.S. in the estimation of Uman. Lightning Power
Williams says a moderate thunderstorm generates several hundred megawatts of electrical power. DC Electric Power
The electric power in watts associated with a complete electric circuit or a circuit component represents the rate at which energy is converted from the electrical energy of the moving charges to some other form, e.g., heat, mechanical energy, or energy stored in electric fields or magnetic fields. For a resistor in a D C Circuit the power is given by the product of applied voltage and the electric current: P = VI The details of the units are as follows: Cloud Charge Transfer Uman "The usual cloud-to- ground discharge probably begins as a local discharge between the small pocket of positive charge at the base of the cloud (the p region) and the primary region of negative charge (the N region) above it. This local discharge frees electrons in the N-region that previously had been attached to water or ice particles. These electrons overrun the p-region, neutralize its small positive charge, and then continue on their trip to the ground. " This description is based upon the tripolar model of charge buildup. Following a charge transfer event in the lower part of the cloud, the released electrons proceed to the ground. Uman "The vehicle by which these electrons move from the cloud to the ground is called a stepped leader... it moves to the ground in rapid, luminous steps that are about fifty yards long. Each step occurs in less than one-millionth of a second, and the time between each step is about one fifty-millionth of a second. The stepped leader, moving at a velocity of about 200 miles per hour, takes about one-hundredth of a second to travel from the cloud to earth."
" Measured photographically, the stepped leader is between one and ten yards in diameter. It is thought, however, that most of the current flows down a narrow conducting core that is less than one inch in diameter, and that the large, photographically obvserved diameter is due to a luminous electrical corona surrounding the conducting core." When the stepped leader approaches the ground, carrying some five coulombs of charge, a large positive charge is induced below it and an upward-moving discharge some 30-50 meters long comes up to meet it. The position of the upward-moving discharge actually determines what ground point the lightning will hit, so lightning rods are used to initiate this discharge to offer some control over the strike. When contact is made with the stepped leader, a violent, high-current discharge travels to ground. This highly luminous discharge then travels back up the leader in the return stroke. After the stepped leader and the upward-moving discharge initiate the first violent, luminous discharge near the Earth, "this high luminosity (and the high current) then moves up the leader channel and out its branches at somewhere between one-half and one-tenth the speed of light. This movement from ground to cloud is called the return stroke and is actually the dazzling display we recognize as lightning. The eye is not fast enough to resolve the movement of the return stroke, and so it seems as if all points of the channel become bright simultaneously. " Uman After the return stroke, the lightning flash may be ended, but "most flashes contain three or four strokes - some as many as twenty or thirty." If enough charge is available in the cloud to produce another stroke, a continuous leader called a dart leader moves down the return stroke channel from the previous stroke, depositing negative charge along its length. "Dart leaders generally deposit less charge than stepped leaders do, and, as a result, subsequent strokes generally lower less charge to ground and have smaller currents than first strokes. " Uman Lightning Time Sequence AM Radio Band
The Amplitude Modulated (AM) radio carrier frequencies are in the frequency range 535-1605 kHz. The frequencies 30-535 kHz are used for maritime communication and navigation and for aircraft navigation. Carrier frequencies of 540 to 1600 kHz are assigned at 10 kHz intervals.
Frequencies: 500-1500 kHz
Wavelengths: 600 - 200 m
Quantum energies: 2 - 6 x 10-9 eV AM and FM Radio Frequencies
The Amplitude Modulated (AM radio) carrier frequencies are in the frequency range 535-1605 kHz. Carrier frequencies of 540 to 1600 kHz are assigned at 10 kHz intervals.
The FM radio band is from 88 to 108 MHz between VHF television Channels 6 and 7. The FM stations are assigned center frequencies at 200 kHz separation starting at 88.1 MHz, for a maximum of 100 stations. These FM stations have a 75 kHz maximum deviation from the center frequency, which leaves 25 kHz upper and lower "gaurd bands" to minimize interaction with the adjacent frequency band. Radio Frequency Bands
Because of the division of the FM band for the transmission of FM stereo, the frequency limit for music transmission is at 15 kHz. This allows high fidelity signal transmission. The operational bandwidth is limited to 150 kHz, with 25 kHz on each side of that for gaurd bands. Actually FM stereo covers 106 kHz of that. AM radio is limited to 5000 Hz maximum frequency by the width of the AM bands. Music transmission by AM radio is limited in fidelity, and adjacent stations tend to interfere with each other. The frequencies from the top end of the AM band to the bottom of the VHF television band are generally called the "short wave" range, a historical term. They are part of the general range referred to as "radio frequencies" or RF. The range from 1605 kHz to 54 MHz has multiple communication uses.
1,605 kHz - 30 MHz Amateur radio, government radio, international shortwave broadcast, fixed and mobile communications.
30-50 MHz Government and non-government, fixed and mobile. Includes police, fire, forestry, highway, and railroad services.
50-54 MHz Amateur
The RF frequency range around 40-50 MHz is important as the proton resonance frequency range used in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). TV and FM Radio Band
The carrier frequencies for VHF television Channels 2-4 cover the frequency range 54 to 72 MHz. There is a band from 72-76 MHz which is reserved for government and non-government services, including a standard aeronautical beacon at 75 MHz. VHF TV channels 5 and 6 are between 76 and 88 MHz. The FM radio band is from 88 to 108 MHz between VHF television Channels 6 and 7.Above the FM is a range 108-122 MHz for aeronautical navigation including localizers, radio ranging and airport control. From 122 to 174 MHz is another general service band for both government and non-government signals. It includes fixed and mobile units and amateur broadcast. Channels 7 through 13 span the frequency range 174-216 MHz. 216-470 MHz includes a number of fixed and mobile communication modes, including some aeronautical navigation and citizens radio. 470-890 MHz includes UHF television channels 14 to 83. Frequencies 890-3000 MHz include a variety of aeronautical and amateur uses, studio-transmitter relays, etc. There are radar bands 1,300-1,600 MHz.
The FM stations are assigned center frequencies at 200 kHz separation starting at 88.1 MHz, for a maximum of 100 stations. These FM stations have a 75 kHz maximum deviation from the center frequency, which leaves 25 kHz upper and lower "gaurd bands" to minimize interaction with the adjacent frequency band. Television channels have 5 MHz separation.
The frequency range for mobile cellular telephones is listed as 824.040 - 848.970 MHz. L-Band for Satellite Communication
The range 390-1550 MHz in the ultrahigh radio frequency range is designated as the L-Band and is used for a variety of satellite communication purposes.
For example, the Global Positioning System uses two carrier frequencies in this band for broadcasting navigation data. Microwaves, Radar
While there are some radar bands from 1,300 to 1,600 MHz, most microwave applications fall in the range 3,000 to 30,000 MHz (3-30 GHz). Current microwave ovens operate at a nominal frequency of 2450 MHz, a band assigned by the FCC. There are also some amateur and radio navigation uses of the 3-30 GHz range. In interactions with matter, microwave radiation primarily acts to produce molecular rotation and torsion, and microwave absorption manifests itself by heat. Molecular structure information can be obtained from the analysis of molecular rotational spectra, the most precise way to determine bond lengths and angles of molecules. Microwave radiation is also used in electron spin resonance spectroscopy.
For microwave ovens and some radar applications, the microwaves are produced by magnetrons.
Of great astrophysical significance is the 3K background radiation in the universe, which is in the microwave region. It has recently been mapped with great precision by the WMAP probe.
Frequencies: 1.6-30 GHz
Wavelengths: 187 - 10 mm
Quantum energies: 0.66 x 10-5 - 0.12 x 10-3 eV Millimeter Waves, Telemetry
The range 30-300 GHz is used for a variety of experimental, government and amateur purposes in communication.
Frequencies: 30-300 GHz
Wavelengths: 10 - 1 mm
Quantum energies: 0.12 x 10-3 - 0.12 x 10-2 eV Infrared
The term "infrared" refers to a broad range of frequencies, beginning at the top end of those frequencies used for communication and extending up the the low frequency (red) end of the visible spectrum. The wavelength range is from about 1 millimeter down to 750 nm. The range adjacent to the visible spectrum is called the "near infrared" and the longer wavelength part is called "far infrared".
In interactions with matter, infrared primarily acts to set molecules into vibration. Infrared spectrometers are widely used to study the vibrational spectra of molecules.
Infrared does not penetrate the atmosphere well, but astronomy in the infrared is carried out with the Spitzer Space Telescope.
Frequencies: .003 - 4 x 1014 Hz
Wavelengths: 1 mm - 750 nm
Quantum energies: 0.0012 - 1.65 eV Ultraviolet
The region just below the visible in wavelength is called the near ultraviolet. It is absorbed very strongly by most solid substances, and even absorbed appreciably by air. The shorter wavelengths reach the ionization energy for many molecules, so the far ultraviolet has some of the dange rs attendent to other ionizing radiation. The tissue effects of ultraviolet include sunburn, but can have some therapeutic effects as well. The sun is a strong source of ultraviolet radiation, but atmospheric absorption eliminates most of the shorter wavelengths. The eyes are quite susceptible to damage from ultraviolet radiation. Welders must wear protective eye shields because of the uv content of welding arcs can inflame the eyes. Snow-blindness is another example of uv inflamation; the snow reflects uv while most other substances absorb it strongly.
Frequencies: 7.5 x 1014 - 3 x 1016 Hz
Wavelengths: 400 nm - 10 nm
Quantum energies: 3.1 - 124 eV Gamma-Rays
In interactions with matter, gamma rays are ionizing radiation and produce physiological effects which are not observed with any exposure of non-ionizing radiation, such as the risk of mutations or cancer in tissue.
Frequencies: typically >1020 Hz
Wavelengths: typically < 10-12 m
Quantum energies: typically >1 MeV Electric motors involve rotating coils of wire which are driven by the magnetic force exerted by a magnetic field on an electric current. They transform electrical energy into mechanical energy. How Does an Electric Motor Work? What is Electric motors ? DC Motor Operation AC Motor As in the DC motor case, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More common is the induction motor, where electric current is induced in the rotating coils rather than supplied to them directly.
One of the drawbacks of this kind of AC motor is the high current which must flow through the rotating contacts. Sparking and heating at those contacts can waste energy and shorten the lifetime of the motor. In common AC motors the magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils which produce the magnetic field are sometimes referred to as the "stator", while the coils and the solid core which rotates is called the "armature". In an AC motor the magnetic field is sinusoidally varying, just as the current in the coil varies. What is Electric Shock The primary variable for determining the severity of electric shock is the electric current which passes through the body. This current is of course dependent upon the voltage and the resistance of the path it follows through the body. An approximate general framework for shock effects is as follows: One instructive example of the nature of voltage is the fact that a bird can sit on a high-voltage wire without harm, since both of its feet are at the same voltage. You can also see that the bird is not "grounded" -- you will not be shocked by touching a high voltage if there is no path for the current to reach the Earth or a different voltage point. Typically if you touch a 120 volt circuit with one hand, you can escape serious shock if you have insulating shoes which prevent a low-resistance path to ground. This fact has led to the common "hand-in-the-pocket" practice for engineers and electrical workers. If you keep one hand in your pocket when touching a circuit which might provide a shock, you are less likely to have the kind of path to ground which will result in a serious shock. Will the bird on the high voltage wire be shocked? Electric current flow is proportional to voltage difference according to Ohm's law, and both the bird's feet are at the same voltage. Since current flow is necessary for electric shock, the bird is quite safe unless it simultaneously touches another wire with a different voltage.
Want a scary job? Maintenance on high voltage transmission lines is sometimes done with the voltage "live" by working from a platform on a helicopter, sitting on a metal platform! The helicopter must make sure it doesn't touch neighboring wires which are at a different voltage. Shock Physiological Effects Shock Physiological Effects Current Involved in Electric Shock The electric current in amperes is the most important physiological varible which determines the severity of an electric shock. However, this current is in turn determined by the driving voltage and the resistance of the path which the current follows through the body. One difficulty in establishing the conditions for electrical safety is that a voltage which produces only a mild tingling sensation under one circumstance can be a lethal shock hazard under other conditions.
Will the 120 volt common household voltage produce a
dangerous shock? It depends!
If your body resistance is 100,000 ohms, then the current which would flow would be: But if you have just played a couple of sets of tennis, are sweaty and barefoot, then your resistance to ground might be as low as 1000 ohms. Then the current would be: Transformer and Faraday's Law Faraday's Law
Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field. Transformer
A transformer makes use of Faraday's law and the ferromagnetic properties of an iron core to efficiently raise or lower AC voltages. It of course cannot increase power so that if the voltage is raised, the current is proportionally lowered and vice versa. What is Transformer?
An iron core has the effect of multiplying greatly the magnetic field of a solenoid compared to the air core solenoid on the left. Iron Core Solenoid Electromagnet
Electromagnets are usually in the form of iron core solenoids. The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the smaller driving magnetic field produced by the current in the solenoid. The effect is the multiplication of the magnetic field by factors of tens to even thousands. The solenoid field relationship is
and k is the relative permeability of the iron, shows the magnifying effect of the iron core. Circuit Equations:Transformer
The application of the voltage law to both primary and secondary circuits of a transformer gives:
The transformer is the most common application of the concept of mutual inductance. In the transformer, the effect of the mutual inductance is to cause the primary ciruit to take more power from the electrical supply in response to an increased load on the secondary. For example, if the load resistance in the secondary is reduced, then the power required will increase, forcing the primary side of the transformer to draw more current to supply the additional need. Electric Distribution The electric energy obtained in the electric generation process must be transported to the end user by electric conductors without large resistive power losses in the distribution process. A key part of the strategy for doing so involves using transformers to increase the voltage to hundreds of thousands of volts to minimize loss to heat in the transmission wires.
The three high voltage conductors shown on the utility pole at right indicate that the power distribution is "three-phase", with each conductor 120 degrees in phase away from each of the others. If each section of the large insulators can withstand a working voltage of 10,000 volts, these conductors may be operating at something like 150,000 volts. Household Wiring The standard U.S. household wiring design has two 120 volt "hot" wires and a neutral which is at ground potential. The two 120 volt wires are obtained by grounding the centertap of the transformer supplying the house so that when one hot wire is swinging positive with respect to ground, the other is swinging negative. This versatile design allows the use of either hot wire to supply the standard 120 volt household circuits. For higher power applications like clothes dryers, electric ranges, air conditioners, etc. , both hot wires can be used to produce a 240 volt circuit. Electric Circuits Viltage Law DC Circuit The voltage changes around any closed loop must sum to zero. No matter what path you take through an electric circuit, if you return to your starting point you must measure the same voltage, constraining the net change around the loop to be zero. Since voltage is electric potential energy per unit charge, the voltage law can be seen to be a consequence of conservation of energy. Current Law The electric current in amperes that flows into any junction in an electric circuit is equal to the current which flows out. This can be seen to be just a statement of conservation of charge. Since you do not lose any charge during the flow process around the circuit, the total current in any cross-section of the circuit is the same. Along with the voltage law, this law is a powerful tool for the analysis of electric circuits. Ohm's Law DC Circuit Examples
The basic tools for solving D C circuit problems are Ohm's Law, the power relationship, the voltage law, and the current law. The following configurations are typical; details may be examined by clicking on the diagram for the desired Alternating current (AC) circuits explained using time and phasor animations. Impedance, phase relations, resonance and RMS quantities. A resource page from Physclips: a multi-level, multimedia introduction to physics. AC circuits Circuit Elements Resistance Capacitors Inductors Electric Charge Magnetic Field Lorentz Force Law Electronics The unit of electric charge is the Coulomb (abbreviated C). Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them. Charge is quantized as a multiple of the electron or proton charge: Coulomb's Law
Like charges repel, unlike charges attract.
The electric force acting on a point charge q1 as a result of the presence of a second point charge q2 is given by Coulomb's Law: Electric Field
Electric field is defined as the electric force per unit charge. The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge. Electric Current
Electric current is the rate of charge flow past a given point in an electric circuit, measured in Coulombs/second which is named Amperes. In most DC electric circuits, it can be assumed that the resistance to current flow is a constant so that the current in the circuit is related to voltage and resistance by Ohm's law. The standard abbreviations for the units are 1 A = 1C/s. Maxwell's Equations
Integral form in the absence of magnetic or polarizable media: Electrical and Electronic
Components Basic Circuit Elements Diodes Diodes make use of the nature of a p-n junction to achieve different behaviors for different current directions. Other components may be included in the diode package to achieve specific purposes. Transistors Logic Circuit Test Equipment The Oscilloscope Signal Generator Electric Motors Electromagnetic Spectrum