BBC - GCSE Bitesize: Direct current and alternating current Direct current and alternating current You should know the differences between direct current (d.c.) and alternating current (a.c.) electrical supplies. Direct current Direct current If the current [current: Moving electric charges, for example, electrons moving through a metal wire. ] flows in only one direction it is called direct current, or d.c. Batteries and cells supply d.c. electricity, with a typical battery supplying maybe 1.5V. The diagram shows an oscilloscope screen displaying the signal from a d.c. supply. Alternating current Alternating current If the current constantly changes direction, it is called alternating current, or a.c.. Mains electricity is an a.c. supply, with the UK mains supply being about 230V. It has a frequency [frequency: The number of repetitions per second of a wave. The unit of frequency is the hertz, 'Hz'. ] of 50Hz (50 hertz), which means it changes direction, and back again, 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an a.c. supply.

Alternating current - higher Alternating current The potential difference of the live terminal varies between a large positive value and a large negative value. However, the neutral terminal is at a potential difference close to earth, which is zero. The diagram shows an oscilloscope screen displaying the signals from the mains supply. The red trace is the live terminal and the blue trace the neutral terminal. Note that, although the mean voltage of the mains supply is about 230V, the peak voltage is higher. Now try a Test Bite - foundation Page: Alternating Current (AC) Electricity - Physics Lessons: School for Champions by Ron Kurtus Alternating Current (AC) Electricity by Ron Kurtus (revised 13 February 2016) Alternating current (AC) electricity is the type of electricity commonly used in homes and businesses throughout the world. While direct current (DC) electricity flows in one direction through a wire, AC electricity alternates its direction in a back-and-forth motion. The direction alternates between 50 and 60 times per second, depending on the electrical system of the country. AC electricity is created by an AC electric generator, which determines the frequency.

What is special about AC electricity is that the voltage can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications. Note: We usually say AC electricity instead of simply saying AC, since that is also the abbreviation for air conditioning. You need to be exact in science to avoid any misunderstandings. Questions you may have include: What is the difference between AC and DC electricity? Why do we use AC instead of DC? What are advantages of AC electricity? This lesson will answer those questions. Useful tool: Units Conversion Difference between AC and DC electricity Electrons have negative (−) electrical charges. Since opposite charges attract, they will move toward an area consisting of positive (+) charges. This movement is made easier in an electrical conductor, such as a metal wire. Electrons move direct with DC electricity With DC electricity, connecting a wire from the negative (−) terminal of a battery to the positive (+) terminal will cause the negative charged electrons to rush through the wire toward the positive charged side.

The same thing happens with a DC generator, where the motion of coiled wire through a magnetic field pushes electrons out of one terminal and attracts electrons to the other terminal. Electrons alternate directions in AC electricity With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a short while and then reverses its direction when the generator armature is in a different position. This illustration gives an idea of how the electrons move through a wire in AC electricity. Of course, both ends of the wire extend to the AC generator or source of power. AC movement of electrons in wire The charge at the ends of the wire alternates between negative (−) and positive (+). If the charge is negative (−), that pushes the negatively charged electrons away from that terminal. If the charge is positive (+), the electrons are attracted in that direction. Rate of change AC electricity alternates back-and-forth in direction 50 or 60 times per second, according to the electrical system in the country. This is called the frequency and is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz).

(See Worldwide AC Voltages and Frequencies for more information.) Light bulbs Many electrical devices—like light bulbs—only require that the electrons move. They don't care if the electrons flow through the wire or simply move back-and-forth. Thus a light bulb can be used with either AC or DC electricity. AC is periodic motion The regular back-and-forth motion of the electrons in a wire when powered by AC electricity is periodic motion, similar to that of a pendulum. (See Periodic Motion and Pendulum for more information.) Because of this periodic motion of the electrons, the voltage and current follow a sine waveform, alternating between positive (+) and negative (−), as measured with a voltmeter or multimeter. Waveform varies between positive and negative as it travels in time The rate that the voltage or current peaks pass a given point is the frequency of the AC electricity. Advantages of AC electricity There are distinct advantages of AC over DC electricity. The ability to readily transform voltages is the main reason we use AC instead of DC in our homes.

Transforming voltages The major advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages. Since high voltages are more efficient for sending electricity great distances, AC electricity has an advantage over DC. This is because the high voltages from the power station can be easily reduced to a safer voltage for use in the house. Changing voltages is done by the use of a transformer. This device uses properties of AC electromagnets to change the voltages. (See AC Transformers for more information.) Tuning circuits AC electricity also allows for the use of a capacitor and inductor within an electrical or electronic circuit. These devices can affect the way the alternating current passes through a circuit. They are only effective with AC electricity. A combination of a capacitor, inductor and resistor is used as a tuner in radios and televisions. Without those devices, tuning to different stations would be very difficult. Summary We commonly use AC electricity to power our television, lights and computers. In AC electricity, the current alternates in direction. AC electricity was proven to be better for supplying electricity than DC, primarily because the voltages can be transformed.

AC also allows for other devices to be used, opening a wide range of applications. Electrify society by applying your knowledge of science Resources and references Electronics Club - AC, DC and Electrical Signals AC, DC and Electrical Signals Also See: Diodes | Power Supplies AC means Alternating Current and DC means Direct Current . AC and DC are also used when referring to voltages and electrical signals which are not currents! For example: a 12V AC power supply has an alternating voltage (which will make an alternating current flow). An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit. Alternating Current (AC) Alternating Current (AC) flows one way, then the other way, continually reversing direction. An AC voltage is continually changing between positive (+) and negative (-). The rate of changing direction is called the frequency of the AC and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second. Mains electricity in the UK has a frequency of 50Hz. See below for more details of signal properties .

An AC supply is suitable for powering some devices such as lamps and heaters but almost all electronic circuits require a steady DC supply (see below). AC from a power supply This shape is called a sine wave. This triangular signal is AC because it changes between positive (+) and negative (-). Direct Current (DC) always flows in the same direction, but it may increase and decrease. A DC voltage is always positive (or always negative), but it may increase and decrease. Electronic circuits normally require a steady DC supply which is constant at one value or a smooth DC supply which has only a small variation called ripple. Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits. Power supplies contain a transformer which converts the mains AC supply to a safe low voltage AC. Then the AC is converted to DC by a bridge rectifier but the output is varying DC which is unsuitable for electronic circuits. Some power supplies include a capacitor to provide smooth DC which is suitable for less-sensitive electronic circuits, including most of the projects on this website. Lamps, heaters and motors will work with any DC supply. Please see the power supplies page for further information. Power supplies are also covered by the Electronics in Meccano website.

Steady DC from a battery or regulated power supply, this is ideal for electronic circuits. Smooth DC from a smoothed power supply, this is suitable for some electronics. from a power supply without smoothing, this is not suitable for electronics. Properties of electrical signals An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit. The voltage-time graph below shows various properties of an electrical signal. In addition to the properties labelled on the graph, there is frequency which is the number of cycles per second. The diagram shows a sine wave but the properties apply to any signal with a constantly repeating shape. Amplitude is the maximum voltage reached by the signal. It is measured in volts, V. Peak voltage is another name for amplitude. Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace it is usual to measure peak-peak voltage. Time period is the time taken for the signal to complete one cycle. It is measured in seconds (s) but time periods tend to be short so milliseconds (ms) and microseconds (µs) are often used.

1ms = 0.001s and 1µs = 0.000001s. Frequency is the number of cycles per second. It is measured in hertz (Hz), but frequencies tend to be high so kilohertz (kHz) and megahertz (MHz) are often used. 1kHz = 1000Hz and 1MHz = 1000000Hz. Frequency and time period Frequency and time period are the inverse of each other: frequency = 1 frequency Mains electricity in the UK has a frequency of 50Hz so it has a time period of 1/50 = 0.02s = 20ms. Root Mean Square (RMS) Values The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. Clearly for most of the time it is less than the peak voltage, so this is not a good measure of its real effect.

Instead we use the root mean square voltage (VRMS) which is 0.7 of the peak voltage (Vpeak): VRMS = 0.7 × Vpeak Vpeak = 1.4 × VRMS These equations also apply to current. It is important to note that these equations are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 factors are different values for other shapes. The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC value which gives the same effect. For example a lamp connected to a 6V RMS AC supply will light with the same brightness when connected to a steady 6V DC supply. However, the lamp will be dimmer if connected to a 6V peak AC supply because the RMS value of this is only 4.2V (it is equivalent to a steady 4.2V DC). You may find it helps to think of the RMS value as a sort of average, but please remember that it is NOT really the average! In fact the average voltage (or current) of a typical AC signal is zero because the positive and negative parts exactly cancel out.

What do AC meters show, is it the RMS or peak voltage? AC voltmeters and ammeters show the RMS value of the voltage or current. What does '6V AC' really mean, is it the RMS or peak voltage? If the peak value is meant it should be clearly stated, otherwise assume it is the RMS value. In everyday use AC voltages (and currents) are always given as RMS values because this allows a sensible comparison to be made with steady DC voltages (and currents), such as from a battery. For example a '6V AC supply' means 6V RMS, the peak voltage is 8.4V. The UK mains supply is 230V AC, this means 230V RMS so the peak voltage of the mains is about 320V. So what does root mean square (RMS) really mean? First square all the values, then find the average (mean) of these square values over a complete cycle, and find the square root of this average. That is the RMS value. Confused? Ignore the maths (it looks more complicated than it really is), just accept that RMS values for voltage and current are a much more useful quantity than peak values.

Next Page: Oscilloscopes (CROs) | Study Privacy Policy & Cookies This website does not collect personal information. If you send an email your email address and any personal information will be used only to respond to your message, it will not be given to anyone else. This website displays advertisements. If you click on these the advertiser may know that you came from this site and I may be rewarded. No personal information is passed to advertisers. Cookies: this website does not use cookies itself but it does use Google AdSense which uses cookies to serve advertisements based on your use of websites (including this one) as explained by Google . To learn how to delete and control cookies from your browser please visit AboutCookies.org . electronicsclub.info © John Hewes 2017 Severe Weather 101: Lightning FAQ Severe Weather 101 Frequently Asked Questions About Lightning What is lightning? Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground.

When the opposite charges builds up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. The flash of lightning temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again. Lightning can occur between opposite charges within the thunderstorm cloud (intra-cloud lightning) or between opposite charges in the cloud and on the ground (cloud-to-ground lightning). Lightning is one of the oldest observed natural phenomena on earth. It can be seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms. What are cloud flashes? A cloud flash is lightning that occurs inside the cloud, travels from one part of a cloud to another, or from the cloud to the air. What is a “stepped leader?” A stepped leader is a stream of weakly charged particles that flows from the cloud – it moves towards the ground, starting and stopping, and sometimes branching, trying to find the path of least resistance. Is it possible to have thunder without lightning? No, it is not possible to have thunder without lightning. Thunder is a direct result of lightning.

However, it IS possible that you might see lightning and not hear the thunder because it was too far away. Sometimes this is called “heat lightning” because it occurs most often in the summer. Is lightning always produced by a thunderstorm? Thunderstorms always have lightning (thunder is caused by lightning, and you can't have a thunderstorm without thunder), but you can have lightning without a thunderstorm. Lightning can also be seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, and in heavy snowstorms. What causes thunder? Thunder is caused by lightning. The bright light of the lightning flash caused by the return stroke mentioned above represents a great deal of energy. This energy heats the air in the channel to above 50,000 degrees F in only a few millionths of a second! The air that is now heated to such a high temperature had no time to expand, so it is now at a very high pressure. The high pressure air then expands outward into the surrounding air compressing it and causing a disturbance that propagates in all directions away from the stroke. The disturbance is a shock wave for the first 10 yards, after which it becomes an ordinary sound wave, or thunder.

Thunder can seem like it goes on and on because each point along the channel produces a shock wave and sound wave. What is dry lightning? Dry lightning is lightning that occurs without rain nearby. The NOAA Storm Prediction Center routinely forecasts dry lightning because this kind is more likely to cause forest fires. What is a “bolt from the blue”? A “Bolt from the Blue” is a cloud-to-ground flash which typically comes out of the back side of the thunderstorm cloud, travels a relatively large distance in clear air away from the storm cloud, and then angles down and strikes the ground. These lightning flashes have been documented to travel more than 25 miles away from the thunderstorm cloud. They can be especially dangerous because they appear to come from clear blue sky. A helmeted bicyclist experienced a lightning strike to the head under fair weather conditions with a cloudless sky. It was determined that the bolt probably originated in a thunderstorm that was about 16km away and obscured by mountains. Lightning strikes the ground approximately 25 million times each year in the U.S. According to the NWS, the chance of an individual in the U.S. being killed or injured during a given year is one in 240,000.

Assuming an average life-span of 80 years, a person's odds over their lifetime becomes one in 3000. Assuming the average person has ten family members and others with whom they are close, then the chances are one in 300 that a lightning strike will closely affect a person during their lifetime. Does lightning always strike the tallest object? Never say always! Lightning USUALLY strikes the tallest object. It makes sense that the tallest object is most attractive, because it is the easiest path for the lightning to take. What type of electricity is lightning? Lightning is an electrostatic discharge accompanied by the emission of visible light and other forms of electromagnetic radiation. How many volts and watts are in lightning? Lightning can have 100 million to 1 billion volts, and contains billions of watts. Why are positive lightning bolts deemed more dangerous than the more common negatively charged bolts? Positive lightning is often considered more dangerous because its electrical field is stronger (forming at the top of the storm), the flash duration is typically longer, and its peak charge can be much greater than a negative strike.

Plus, positively charged lightning can occur near the edge of a cloud or strike more than 10 miles away – when people aren't aware of the danger. Does lightning strike from the sky down, or the ground up? The answer is both. Cloud-to-ground lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast - in about one-millionth of a second - so the human eye doesn't see the actual formation of the stroke. How hot can lightning make the air? Energy from lightning heats the air anywhere from 18,000 degrees Fahrenheit to up to 60,000 degrees Fahrenheit. What causes lightning to be colored rather than the usual white or blue?

Lightning can appear to be many different colors depending on what the light travels through to get to your eyes. In snowstorms, where it is somewhat rare, pink and green are often described as colors of lightning. Haze, dust, moisture, raindrops and any other particles in the atmosphere will affect the color by absorbing or diffracting a portion of the white light of lightning. How does the Earth benefit from lightning? The earth benefits from lightning in several ways. First, lightning helps the Earth maintain electrical balance. The Earth is recharged by thunderstorms. The Earth's surface and the atmosphere conduct electricity easily—the Earth is charged negatively and the atmosphere, positively. There is always a steady current of electrons flowing upwards from the entire surface of the Earth. Thunderstorms help transfer the negative charges back to Earth (lightning is generally negatively charged). Without thunderstorms and lightning, the earth-atmosphere electrical balance would disappear in 5 minutes. Lightning also makes ozone-producing chemicals. What happens to the ground when lightning strikes it? What tends to happen when lightning strikes ground is that it fuses dirt and clays in to silicas.

The result is often a glassy rock (called a fulgurite) in the shape of a convoluted tube. Fulgurite has been found all over the world, but is relatively rare. The color depends on the minerals in the sand that was struck.The shape in the ground is the shape of the path the lightning current followed in the ground. There is often damage to grasses along this path too. Lightning traveling down a tree trunk turns water to steam. If it gets under the bark into the surface moisture of the wood, the rapidly expanding steam can blast pieces of bark from the tree, and the wood along the path is often killed. Can lightning strike the same place twice? Lightning does hit the same spot (or almost the same spot) more than once, contrary to folk wisdom. It could be simply a statistical fluke (i.e., with all the lightning that occurs, eventually lightning will strike somewhere near a previous lightning strike within a short period of time). It could also be that something about the site makes it somewhat more likely to be struck. Typically, when lightning strikes something on the ground, the object that is struck sends a faint channel upward that joins the downward developing flash and creates the connection to the ground.

Taller objects are more likely than shorter objects to produce the upward channel. But it is also possible that something that locally affects the ability of the ground to conduct electricity (such as the salt or moisture content of the ground at the time, the presence or absence of rock, standing water, pipes or other metal objects in the ground), the terrain shape, the shape of leaves or twigs, or something else might make a particular location more likely than another nearby location to be struck. When and where does lightning most frequently strike? Lightning comes from a parent cumulonimbus cloud. These thunderstorm clouds are formed wherever there is enough upward motion, instability in the vertical, and moisture to produce a deep cloud that reaches up to levels somewhat colder than freezing. These conditions are most often met in summer. In general, the US mainland has a decreasing amount of lightning toward the northwest. Over the entire year, the highest frequency of cloud-to-ground lightning is in Florida between Tampa and Orlando. This is due to the presence, on many days during the year, of a large moisture content in the atmosphere at low levels (below 5,000 feet), as well as high surface temperatures that produce strong sea breezes along the Florida coasts.

The western mountains of the US also produce strong upward motions and contribute to frequent cloud-to-ground lightning. There are also high frequencies along the Gulf of Mexico coast, the Atlantic coast in the southeast US, and inland from the Gulf. Regions along the Pacific west coast have the least cloud-to-ground lightning. Does lightning happen during the winter? Lightning occurs less frequently in the winter because there is not as much instability and moisture in the atmosphere as there is in the summer. These two ingredients work together to make convective storms that can produce lightning. Without instability and moisture, strong thunderstorms are unlikely. During the winter, the land surface is cooler because there is not as much heating by the sun to warm it up. Without warm surface temperatures, the near-surface air wouldn't rise in the atmosphere very far. Thus, the kinds of deep (8-15 km deep) thunderstorms that develop in the summertime wouldn't develop. Warm air holds more water vapor. And, when water vapor condenses into liquid water cloud drops, latent heat is released which fuels the thunderstorm. So, warm, moist air near the surface (and the proper conditions aloft to give you lots of instability) can result in deep convection, which may produce lightning discharges.

Clouds become electrified when strong updrafts (fueled by the instability and moisture) bring supercooled liquid water drops and ice crystals at temperatures less than freezing (0 deg C) together. In this environment, interactions between the ice crystals and supercooled water droplets produce electric charges. The exact mechanisms by which this charging happens remain unknown. The electrical charges build up until they are strong enough to overcome the resistance of the surrounding air. The breakdown of the electric fields produced by these charges is the lightning bolt. What is thundersnow? Although thunderstorms are less common in the winter, sometimes lightning can occur within snowstorms. Called thundersnow, relatively strong instability and abundant moisture may be found above the surface, such as above a warm front, rather than at the surface where it may be below freezing. Thundersnow is sometimes observed downstream of the Great Salt Lake and the Great Lakes during lake-effect snowstorms, too. How many flashes a year are there? Over the contiguous 48 states, an average of 20,000,000 cloud-to-ground flashes have been detected every year since the lightning detection network covered all of the continental US in 1989.

In addition, about half of all flashes have more than one ground strike point, so at least 30 million points on the ground are struck on the average each year in the US. Besides cloud-to-ground flashes, there are roughly 5 to 10 times as many cloud flashes as there are ground flashes. Cloud-to-ground lightning can kill or injure people by direct or indirect means. The lightning current can branch off to a person from a tree, fence, pole, or other tall object. It is not known if all people are killed who are directly struck by the flash itself. In addition, flashes may conduct their current through the ground to a person after the flash strikes a nearby tree, antenna, or other tall object. The current also may travel through power or telephone lines, or plumbing pipes to a person who is in contact with an electric appliance, telephone, or plumbing fixture. Similarly, objects can be directly struck and this impact may result in an explosion, burn, or total destruction. Or, the damage may be indirect when the current passes through or near it. Sometimes, current may enter a building and transfer through wires or plumbing and damage everything in its path.

Similarly, in urban areas, it may strike a pole or tree and the current then travels to several nearby houses and other structures and enter them through wiring or plumbing. How can I stay safe from lightning? Home Home ELECTRICITY 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. 3 3 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 one coulomb (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 double the 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 Direct Current Electric Motor Animation 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.

Source: Modern Physical Science Understanding How Transformers Work For Hot Wire Foam Cutter Power Supplies How Transformers Work There are many sizes, shapes and configurations of transformers from tiny to gigantic like those used in power transmission. Some come with stubbed out wires, others with screw or spade terminals, some made for mounting in PC boards, others for being screwed or bolted down. Transformers are composed of a laminated iron core with one or more windings of wire. They are called transformers because they transform voltage and current from one level to another. An alternating current flowing through one coil of wire, the primary, induces a voltage in one or more other coils of wire, the secondary coils. It is the changing voltage of AC current that induces voltage in the other coils through the changing magnetic field. DC voltage such as from a battery or DC power supply will not work in a transformer. Only AC makes a transformer work. The magnetic field flows through the iron core. The faster the voltage changes, the higher the frequency. The lower the frequency, the more iron is required in the core for the efficient transfer of power. In the USA, the line frequency is 60 Hertz with a nominal voltage of 110 volts.

Other countries use 50 Hertz, 220 volts. Transformers made for 50 Hertz must be a little heavier than ones made for 60 Hertz because they must have more iron in the core. Line voltage can vary a little and usually runs between 110 volts and 120 volts or between 220 and 240 volts depending on country or power connections. A house in the USA has 220 volts coming in but is split to two legs of 110V by grounding the center tap (see configuration section below) The ratio of input voltage to output voltage is equal to the ratio of turns of wire around the core on the input side to the output side. A coil of wire on the input side is called the primary and on the output side is called the secondary. There can be multiple primary and secondary coils. The current ratio is opposite the voltage ratio. When the output voltage is lower than the input voltage, the output current will be higher than the input current.

If there are 10 times the number of turns of wire on the primary than the secondary and you put 120 volts on the primary, you will get 12 volts out on the secondary. If you pull 2 amps out from the secondary, you will only be using 0.2 amps or 200 milliamps going into the primary. Transformers can be built so they have the same number of windings on primary and secondary or different numbers of windings on each. If they are the same, the input and output voltage are the same and the transformer is just used for isolation so there is no direct electrical connection (they are only linked through the common magnetic field). If there are more windings on the primary side than the secondary side, then it is a step down transformer. If there are more windings on the seconday side, then it is a step up transformer. A transformer can actually be used in reverse and will work fine.

For example, if you have a step up transformer built for transforming 120 volts to 240 volts, you can also use it for a step down transformer by putting 240 volts into the secondary side and you will get 120 volts on the primary side. Effectively, the secondary becomes the primary and vice versa. Transformer Power Ratings Voltage is measured in volts, current is measured in amps, and the unit of measure for power is watts. Watts is equal to the volts times the amps. There is a little loss of power in a transformer due to the combination of resistance and reactance. Reactance is similar to resistance except it is the resistance to an AC current or more technically, the resistance to change in a change in current due to the change in the field created. This heat is what limits the amount of current or power a transformer can handle. The higher the current, the more heat is produced. When the wires get too hot, the insulation breaks down and shorts with adjacent wires which causes more heat which eventually melts wires and ruins the transformer. A basic transformer has no additional components and so nothing to protect it from overloading.

If you were to connect the two output wires directly together, that will constitute a short circuit and cause far too much current to flow in both the primary and secondary and you will burn out the transformer. In the same way, if you use the transformer to power a hot wire foam cutter and you are using a wire with too little resistance for your foam cutter, you will burn out your transformer if you don't have it protected by a proper value fuse or breaker. You have to make sure that the wire resistance, in other words, the gage or diameter, and the length is correct to limit the amount of current to under the rating of the transformer. The higher the current, the larger the wires need to be that carry that current. When the wires are larger, there is less resistance and so less heat. The power that is changed to heat and lost can be calculated as P=I2R. That means that if you double the current, the power lost to heat increases by four times. If the transformer is a step down transformer, then there will be more current on the output and so the wire in the secondary windings will be heavier than the primary. The reverse is true for a step up transformer.

A transformer may be rated in Amps, Volt-Amps (VA), or Watts (W). For small transformers, VA and Watts are the same thing for all practical purposes. In large industrial transformers, power factors get involved and the two can be different. If the transformer is rated in amps, it usually says X amps at X volts and is rated on the output or secondary side. A 120V transformer with 24V out rated at 2 amps means that you can only safely pull 2 amps from the secondary side. You can find the power rating of the transformer by multiplying the rated amps times the output voltage so 2 X 24 = 48 watts. If the transformer is rated in VA or watts, you can calculate the maximum allowable output current by dividing the VA or watts by the output voltage. So if the transformer is rated at 48 VA with 24 volts output, the allowable output current is 48 / 24 = 2 amps. Transformer Configurations A 120 volt transformer with two wires in and two wires out is very simple.

You hook up the two wires on the primary side, the 120V side, to a wall outlet and your output voltage is on the two wires coming from the secondary side. When a transformer is shown in an electronic circuit, it is shown as a diagram like shown here. The parallel lines represent the laminated iron core, the curved lines represent the primary and secondary windings, the circles represent the terminations whether terminals or short wires. Center Tap A common configuration is a center tap or CT. The secondary side has three wires out. The middle wire on the output side is attached to the secondary coil, usually at the middle. If the winding ratio is 5 to 1, then with 120V input, you get 24 volts output on the two outside wires but if you connect an outside wire and the center wire, you get 12 volts because you are using only half the secondary winding making the connection a 10 to 1 ratio. If the transformer is rated at 2 amps, you still can only use 2 amps output whether you use 12 volts or 24 volts.

Often the center tap is grounded so you then have two 12 volt sources that can be used to make + and - 12V DC after running through a converter (rectifier and filter). Dual Output The dual output configuration is similar to the center tap except that instead of connecting a wire to the center of the coil, the coil is separated into two separate coils with wires with terminals or wires coming out from both ends of both coils so four wires come out of the secondary side instead of three. If the transformer is a 110V input with two 12V outputs, you can connect the two secondary coils in series to get 24 volts out, or you can connect them in parallel to get 12V out. You have to be careful to connect the right ends of the two secondary coils in both the series and in the parallel connections. If you reverse the connections, you will get 0 volts out because the two voltages will cancel each other out. If the transformer is rated at 48VA, then you can use up to 2 amps out for the 24 volt connection which is no different than the center tap or single 24V output configuration.

However, when connected in parallel, you get 12 volts out but double the output current available so you can get 4 amps out. You get the full 48VA output where with the center tap 12V output, you can only get half the rated output or 24VA. This is an advantage in hot wire foam cutters because you have a wider range of wire diameters and lengths depending on whether you connect the outputs in parallel or series. The series and parallel connections are shown below. Dual Input The dual input transformer is often used to make the transformer able to be used in both countries with 120V line voltage and 240V line voltage. The primary is separated into two separate windings with terminals at each end of both windings so there are four wires or terminals on the primary side. To use it with 110 volts input, the two primary windings are connected in parallel as in the left diagram below. Care must be taken to connect the correct ends together. If they are reversed, the fields cancel each other out because the fields generated by each section of the primary are opposite.

Normally, terminals are labeled with numbers or letters and a diagram is provided on the transformer or in an accompanying data sheet showing how the connections must be made for 110V and 220V. If the transformer is to be connected to a 220V supply, then the two coils are connected in series and again, care must be taken to connect the correct terminations together. Parallel connections for 110V and series connections for 220V is shown below. Dual Input and Output And of course, you can have both a dual input and a dual output so you have four wires in and four wires out which gives even more flexibility to the use of the transformer. Some specialized transformers may have several secondary taps or several secondary windings to provide different voltages and they need not be even numbers. A transformer could have a 3V, 5V, 12V, and 24 volt output for example. Autotransformers (Variac) An autotransformer is often referred to as a Variac which is actually one company's trade name for their autotransformer. It has a continuous output voltage from zero to a little over the input value.

It works similar to a potentiometer or rheostat except the change in the voltage is due to the field change rather than resistance. Another difference is that a potentiometer or rheostat is very inefficient because it converts the current flowing through it to heat (Watts = Amps X Volts). As in all transformers, the resistance is low so the amount of heat generated is much less and so much more efficient at transforming voltage An autotransformer has only one winding which serves as both the primary and the secondary winding. Because there is only one winding, there is no electrical isolation between the input and the output but if isolation is not required, then it provides an alternative to multiple winding transformers in some situations. This transformer has the input wires connected to one end of the winding and the other a little ways from the other end. The secondary is connected the same point as the input side that is on the end. The other secondary connection is to a wiper that rides on the top of the windings where the insulation has been removed so the wiper can make contact with the windings at any point on one surface.

The wiper is connected to a knob on the top of the autotransformer so a person can turn the knob to get the voltage they want. Because one primary wire is connected a ways from the end of the winding, the wiper can go past that point and so provide a voltage higher than the input, typically a 110V output can go up to around 130V on the secondary side. Because the autotransformer has only one winding, there is only one wire size and so the maximum input current is also the maximum output current. If a 110 Volt autotransformer is rated at 10 amps, then the maximum output current is 10 amps regardless of the voltage. If it is rated in Watts or VA, then the amps is calculated by dividing the Watts or VA by the rated input voltage. The autotransformer is a good alternative to a step down transformer when the range of desired voltages is on the high end or the whole range of voltages is needed but becomes more expensive if the range is on the low end because you have a lot of unused windings. A step down transformer is more economical.

For hot wire foam cutting, an autotransformer is much more expensive than step down transformers in most applications. If the voltage required is more than 24 volts, then an autotransformer might be considered. Phases and connecting multiple windings For simplicity's sake, I have not mentioned phase but when connecting two or more windings together, the phase becomes very important. AC current is a sine wave and the voltage changes from positive to negative and back in a sinusoidal rhythm many times per second. How often the voltage changes is called frequency and used to be called cycles per second but is now called Hertz (abbreviated Hz). Household current in the USA and some other countries is 60 Hz, in other countries is 50 Hz. When talking about two wave forms such as you have in two windings, the relationship between the two sine waves is the phase. If the sine waves line up, they are in phase, if the positive peak of one wave lines up with the negative peak of the other wave, the two waves are 180ï¿½ out of phase. The phase between one end of a coil and the other are also 180ï¿½ out of phase.

When one end is at the positive peak, the other end will be on the opposite peak. Since there must be a difference in voltage between two points for current to flow, the two ends of the winding must be opposite voltage at any point in time. The phase difference between two windings depends on the direction of windings and how they are connected so in electrical schematics a dot at one end of the winding indicates the beginning of that winding. For simplicity's sake, I have left the dots off the schematics in this article. However, when connecting two coils together, it is very important to connect them correctly. For a series connection you must connect the end of one winding to the start of the other winding (windings for multiple coils are always wound in the same direction). If you connect the start of one winding to the end of the other winding in a series connection, the fields will cancel out and you will get zero output. This will not hurt the transformer but you will get no output voltage. When connecting two windings in parallel, you must connect the start of one winding to the start of the other winding and the two ends of the windings together.

In a parallel connection, connecting the wires in reverse will burn up your transformer if not properly protected (proper current rating) by a fuse or circuit breaker. Be very careful when connecting two coils together. Further reading This has basically just been an overview for a layperson. Although physically a transformer is a fairly simple device with few parts, how it actually works is pretty complicated. I recommend Rod Elliot's excellent articles if you are interested in understanding them better:

Question: How many times per second does mains electricity change from positive to negative ?
Answer: 50