ELECTRICITY

Unit – 4

Electricity

Meaning and use:-  Fundamentally, electricity is the by-product of other energy resources.  It is product from energy reserves.  The conventional generation of electricity utilizes coal, oil, gas or nuclear energy to heat water, producing high temp. pressure steam.  The steam flows through an electric turbine generator.

Generator is the machine used to convert mechanical energy into electrical energy.  Generator is comprised of the essential elements of Faraday’s law to produce electrical power.

Faraday’s Law says three things must be present in order to produce electrical current:-

·          Magnetic Field

·         Conductor

·         Relative Motion

·         Conductor cuts lines of magnetic flux; a voltage is induced in the conductor.  Direction and speed is important in this activity.

 

Uses of Electricity

Electricity has invaded our lives and has become vital in almost all aspects of society today. The list of uses will fill a book but here are a few headings:

Transport Trains, buses, trams and cars all use electricity. Many use it as the motive power, meaning that electricity drives the wheels to make the vehicle move. Even gas and diesel powered vehicles use electricity to start the engines, control the engine and power the ancillary devices.

Home Heating, lighting, television, radio, computer, telephones all rely on electricity. Even wireless lights such as solar powered lamps will convert sight to electricity.

Communication As well as providing power for computers, cell phones, fixed phones, electricity is used as the medium for the transmission of signals. Even high speed optical fibers rely on an electrical signal at each end of the line. Without electricity, communication would be reduced to letters, flag waving and lighting fires and shouting at each other. None of the electricity free methods are as flexible as any that we are used to using today

Industry Manufacturing relies on electricity to drive virtually every moving part in a factory. Saws, cutters, conveyor belts, furnaces, chillers - whatever the process, electricity is involved somewhere.

Entertainment The MP3 player, the portable battery powered radio, memory stick are all accepted as part of our everyday lives. All rely on electricity to operate. Whether connected to a mains supply or battery, they all use electricity.

ADVANTAGE AS A TYPE OF ENERGY

Electricity

• Electricity is beneficial because it is clean, cheap, safe and a convenient source of energy.

• An advantageous use of electricity is restarting a person's heart.

• Its the most expensive to use for heating of any kind.

Conductors and Insulators

 

 

 

In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them. "Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor.

Simply stated, most metals are good electrical conductors, most nonmetals are not. Metals are also generally good heat conductors while nonmetals are not.

INSULATORS

Most solid materials are classified as insulators because they offer very large resistance to the flow of electric current. Metals are classified as conductors because their outer electrons are not tightly bound, but in most materials even the outermost electrons are so tightly bound that there is essentially zero electron flow through them with ordinary voltages. Some materials are particularly good insulators and can be characterized by their high resistivities:

	Resistivity (ohm m)
Glass	1012
Mica	9 x 1013
Quartz (fused)	5 x 1016

This is compared to the resistivity of copper:

	Resistivity (ohm m)
Copper	1.7 x 10-8

Circuit Elements

Electric circuits are considered to be made up of localized circuit elements connected by wires which have essentially negligible resistance. The three basic circuit elements are resistors, capacitors, and inductors. Only these passive elements will be considered here; active circuit elements are the subject of electronics.

Electrical conductor

In science and engineering, a conductor is a material which contains movable electric charges. In metallic conductors, such as copper or aluminum, the movable charged particles are electrons (see electrical conduction). Positive charges may also be mobile in the form of atoms in a lattice that are missing electrons (known as holes), or in the form of ions, such as in the electrolyte of a battery.

All conductors contain electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the direct current is proportional to the voltage (as determined by Ohm's law), provided the temperature remains constant and the material remains in the same shape and state.

Most familiar conductors are metallic. Copper is the most common material used for electrical wiring. Silver is the best conductor, but is expensive. Gold is used for high-quality surface-to-surface contacts. However, there are also many non-metallic conductors, including graphite, solutions of salts, and all plasmas. See electrical conduction for more information on the physical mechanism for charge flow in materials.

Non-conducting materials lack mobile charges, and so resist the flow of electric current, generating heat. In fact, all non-superconducting materials offer some resistance and warm up when a current flows. Thus, proper design of an electrical conductor takes into account the temperature that the conductor needs to be able to endure without damage, as well as the quantity of electrical current. The motion of charges also creates an electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. A conductor of a given material and volume (length × cross-sectional area) has no real limit to the current it can carry without being destroyed as long as the heat generated by the resistive loss is removed and the conductor can withstand the radial forces. This effect is especially critical in printed circuits, where conductors are relatively small and close together, and inside an enclosure: the heat produced, if not properly removed, can cause fusing (melting) of the tracks.

Since all non-superconducting conductors have some resistance, and all insulators will carry some current, there is no theoretical dividing line between conductors and insulators. However, there is a large gap between the conductance of materials that will carry a useful current at working voltages and those that will carry a negligible current for the purpose in hand, so the categories of insulator and conductor do have practical utility.

Thermal and electrical conductivity often go together. For instance, most metals are both electrical and thermal conductors. However, some materials are practical electrical conductors without being good thermal conductors.

 Conductor size

In many countries, conductors are measured by their cross section in square millimeters. However, in the United States, conductors are measured by American wire gauge for smaller ones, and circular mils for larger ones.

Conductor materials

Of the metals commonly used for conductors, copper has a high conductivity. Silver is more conductive, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires.

Aluminium has been used as a conductor in housing applications for cost reasons. It is actually more conductive than copper when compared by unit weight, but it has technical problems related to heat and its coefficient of thermal expansion, which tends to loosen connections over time. It is the most common metal used in high-voltage transmission lines, in combination with steel. The surface of anodized aluminium does not conduct electricity.

Conductor voltage

The voltage on a conductor is determined by the connected circuitry and has nothing to do with the conductor itself. Conductors are usually surrounded by and/or supported by insulators and the insulation determines the maximum voltage that can be applied to any given conductor.

Voltage of a conductor "V" is given by

V = IR

where

I is the current, measured in amperes

V is the potential difference measured in volts

R is the resistance measured in ohms

An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate electrical conductors without passing current through themselves. The term is also used more specifically to refer to insulating supports that attach electric power transmission wires to utility poles or pylons.

Some materials such as glass, paper or Teflon are very good electrical insulators. A much larger class of materials, for example rubber-like polymers and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).

Ampere:-

1. A unit of electric current in the meter-kilogram-second system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2 × 10 ^-7 newtons per meter of length.
2. A unit in the International System specified as one International coulomb per second and equal to 0.999835 ampere.

Volt

 

- The volt (symbolized V) is the Standard International (SI) unit of electric potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance. Reduced to SI base units, 1 V = 1 kg times m2 times s-3 times A-1 (kilogram meter squared per second cubed per ampere). voltage can be expressed as an average value over a given time interval, as an instantaneous value at a specific moment in time, or as an effective or root-mean-square (rms) value. Average and instantaneous voltages are assigned a polarity either negative (-) or positive (+) with respect to a zero, or ground, reference potential. The rms voltage is a dimensionless quantity, always represented by a non-negative real number. For a steady source of direct-current (DC) electric potential, such as that from a zinc-carbon or alkaline electrochemical cell, the average and instantaneous voltages are both approximately +1.5 V if the negative terminal is considered the common ground; the rms voltage is 1.5 V. For standard utility alternating current (AC), the average voltage is zero (the polarity constantly reverses); the instantaneous voltage ranges between approximately -165 V and +165 V; the rms voltage is nominally 117 V. Voltages are sometimes expressed in units representing power-of-10 multiples or fractions of one volt. A kilovolt (symbolized kV) is equal to one thousand volts (1 kV = 103 V). A megavolt (symbolized MV) is equal to one million volts (1 MV = 106 V). A millivolt (symbolized mV) is equal to one-thousandth of a volt (1 mV = 10-3 V). A microvolt (symbolized µV) is equal to one-millionth of a volt (1 µV = 10-6 V).

ohm

Resistances and reactances are sometimes expressed in units representing power-of-10 multiples of one ohm. A kilohm is equal to one thousand (10 ³ ) ohms. A megohm is equal to one million (10 ⁶ ) ohms. Fractional prefix multiplier s are seldom used for resistance or reactances; rarely will you hear or read about a milliohm or a microhm. Extremely small resistances and reactances are usually referred to in terms of conductance.

Ohm's Law

 

- Ohm's Law is the mathematical relationship among electric current, resistance, and voltage. The principle is named after the German scientist Georg Simon Ohm. In direct-current (DC) circuits, Ohm's Law is simple andlinear. Suppose a resistance having a value of R ohms carries a current of I amperes. Then the voltage across the resistor is equal to the product IR. There are two corollaries. If a DC power source providing E volts isplaced across a resistance of R ohms, then the current through theresistance is equal to E/R amperes. Also, in a DC circuit, if E volts appear across a component that carries I amperes, then the resistance of that component is equal to E/I ohms. Mathematically, Ohm's Law for DC circuits can be stated asthree equations: E = IR I = E/R R = E/I When making calculations, compatible units must beused. If the units are other than ohms (for resistance), amperes (for current), and volts for voltage), then unit conversions should be made before calculations are done. For example, kilohms should be converted to ohms, and microamperes should be converted to amperes.

 

Difference between AC and DC:-

 

Alternating Current vs. Direct Current

The figure to the right shows the schematic diagram of a very basic DC circuit. It consists of nothing more than a source (a producer of electrical energy) and a load (whatever is to be powered by that electrical energy). The source can be any electrical source: a chemical battery, an electronic power supply, a mechanical generator, or any other possible continuous source of electrical energy. For simplicity, we represent the source in this figure as a battery.

At the same time, the load can be any electrical load: a light bulb, electronic clock or watch, electronic instrument, or anything else that must be driven by a continuous source of electricity. The figure here represents the load as a simple resistor.

Regardless of the specific source and load in this circuit, electrons leave the negative terminal of the source, travel through the circuit in the direction shown by the arrows, and eventually return to the positive terminal of the source. This action continues for as long as a complete electrical circuit exists.

Now consider the same circuit with a single change, as shown in the second figure to the right. This time, the energy source is constantly changing. It begins by building up a voltage which is positive on top and negative on the bottom, and therefore pushes electrons through the circuit in the direction shown by the solid arrows. However, then the source voltage starts to fall off, and eventually reverse polarity. Now current will still flow through the circuit, but this time in the direction shown by the dotted arrows. This cycle repeats itself endlessly, and as a result the current through the circuit reverses direction repeatedly. This is known as an alternating current.

This kind of reversal makes no difference to some kinds of loads. For example, the light bulbs in your home don't care which way current flows through them. When you close the circuit by turning on the light switch, the light turns on without regard for the direction of current flow.

Of course, there are some kinds of loads that require current to flow in only one direction. In such cases, we often need to convert alternating current such as the power provided at your wall socket to direct current for use by the load. There are several ways to accomplish this, and we will explore some of them in later pages in this section.

Properties of Alternating Current

A DC power source, such as a battery, outputs a constant voltage over time, as depicted in the top figure to the right. Of course, once the chemicals in the battery have completed their reaction, the battery will be exhausted and cannot develop any output voltage. But until that happens, the output voltage will remain essentially constant. The same is true for any other source of DC electricity: the output voltage remains constant over time.

By contrast, an AC source of electrical power changes constantly in amplitude and regularly changes polarity, as shown in the second figure to the right. The changes are smooth and regular, endlessly repeating in a succession of identical cycles, and form a sine wave as depicted here.

Because the changes are so regular, alternating voltage and current have a number of properties associated with any such waveform. These basic properties include the following list:

Frequency. One of the most important properties of any regular waveform identifies the number of complete cycles it goes through in a fixed period of time. For standard measurements, the period of time is one second, so the frequency of the wave is commonly measured in cycles per second (cycles/sec) and, in normal usage, is expressed in units of Hertz (Hz). It is represented in mathematical equations by the letter 'f.' In North America (primarily the US and Canada), the AC power system operates at a frequency of 60 Hz. In Europe, including the UK, Ireland, and Scotland, the power system operates at a frequency of 50 Hz. 

• Period. Sometimes we need to know the amount of time required to complete one cycle of the waveform, rather than the number of cycles per second of time. This is logically the reciprocal of frequency. Thus, period is the time duration of one cycle of the waveform, and is measured in seconds/cycle. AC power at 50 Hz will have a period of 1/50 = 0.02 seconds/cycle. A 60 Hz power system has a period of 1/60 = 0.016667 seconds/cycle. These are often expressed as 20 ms/cycle or 16.6667 ms/cycle, where 1 ms is 1 millisecond = 0.001 second (1/1000 of a second). 
• Wavelength. Because an AC wave moves physically as well as changing in time, sometimes we need to know how far it moves in one cycle of the wave, rather than how long that cycle takes to complete. This of course depends on how fast the wave is moving as well. Electrical signals travel through their wires at nearly the speed of light, which is very nearly 3 × 10⁸ meters/second, and is represented mathematically by the letter 'c.' Since we already know the frequency of the wave in Hz, or cycles/second, we can perform the division of c/f to obtain a result in units of meters/cycle, which is what we want. The Greek letter (lambda) is used to represent wavelength in mathematical expressions. Thus,  = c/f. As shown in the figure to the right, wavelength can be measured from any part of one cycle to the equivalent point in the next cycle. Wavelength is very similar to period as discussed above, except that wavelength is measured in distance per cycle where period is measured in time per cycle. 
• Amplitude. Another thing we have to know is just how positive or negative the voltage is, with respect to some selected neutral reference. With DC, this is easy; the voltage is constant at some measurable value. But AC is constantly changing, and yet it still powers a load. Mathematically, the amplitude of a sine wave is the value of that sine wave at its peak. This is the maximum value, positive or negative, that it can attain. However, when we speak of an AC power system, it is more useful to refer to the effective voltage or current.
•  When we deal with AC power, the most important of these properties are frequency and amplitude, since some types of electrically powered equipment must be designed to match the frequency and voltage of the power lines. Period is sometimes a consideration, as we'll discover when we explore electronic power supplies. Wavelength is not generally important in this context, but becomes much more important when we start dealing with signals at considerably higher frequencies.

Why Use Alternating Current?

Since some kinds of loads require DC to power them and others can easily operate on either AC or DC, the question naturally arises, "Why not dispense entirely with AC and just use DC for everything?" This question is augmented by the fact that in some ways AC is harder to handle as well as to use. Nevertheless, there is a very practical reason, which overrides all other considerations for a widely distributed power grid. It all boils down to a question of cost.

DC does get used in some local commercial applications. An excellent example of this is the electric trolley car and trolley bus system used in San Francisco, for public transportation. Trolley cars are electric train cars with power supplied by an overhead wire. Trolley busses are like any other bus, except they are electrically powered and get their power from two overhead wires. In both cases, they operate on 600 volts DC, and the overhead wires span the city.

The drawback is that most of the electrical devices on each car or bus, including all the light bulbs inside, are quite standard and require 110 to 120 volts. At the same time, however, if we were to reduce the system voltage, we would have to increase the amount of current drawn by each car or bus in order to provide the same amount of power to it. (Power is equal to the product of the applied voltage and the resulting current: P = I × E.) But those overhead wires are not perfect conductors; they exhibit some resistance. They will absorb some energy from the electrical current and dissipate it as waste heat, in accordance with Ohm's Law (E = I × R). With a small amount of algebra, we can note that the lost power can be expressed as:

P_lost = I²R

The same reality of Ohm's Law and resistive losses holds true in the country-wide power distribution system. We need to keep the voltage used in homes to a reasonable and relatively safe value, but at the same time we need to minimize resistive losses in the transmission wires, without bankrupting ourselves buying heavy-gauge copper wire. At the same time, we can't use motor-generator pairs all across the country; they would need constant service and would break down far too often.

The answer is to use an AC power system and transformers. (We'll learn far more about transformers in a later page; for now, a transformer is an electrical component that can convert incoming AC power at one voltage to outgoing power at a different voltage, higher or lower, with only very slight losses.) Thus, we can generate electricity at a reasonable voltage for practical AC generators (sometimes called alternators), then use transformers to step that voltage up to very high levels for long-distance transmission, and then use additional transformers to step that high voltage back down for local distribution to individual homes.

When we examined DC circuit theory and discussed DC power losses above, we used capital letters to represent all quantities. This is standard nomenclature; capital letters are used to represent fixed, static values. Thus, we use a capital I to represent DC current and a capital V to represent DC voltage.

By the same token, we use the capital letters R, L, and C to represent the values of circuit components containing resistance, inductance, and capacitance, respectively. These are fixed values that are set at the time the component is manufactured.

On the other hand, AC circuits use constantly-changing voltages and currents. In addition, some circuits contain both AC and DC components, which must often be considered separately. Therefore, we typically use lower-case letters to designate instantaneous values of voltage, current and power. Thus, if you see an equation written as:

P = I × E

you know it refers to DC power, current, and voltage. On the other hand, an equation written as:

p = i × e

refers to the instantaneous power, current, and voltage of some AC signal at some specified instant in time.

There will also be cases where we must refer to an overall AC signal rather than one instantaneous value from it. In such cases, the use of upper- and lower-case letters may be adjusted so we don't have too many of one or the other in a single discussion. We will try to avoid unnecessary confusion by including subscripts in equations, or specifying in the text exactly what we are discussing.

open circuit, closed circuit, short ciruit

Open Ciruit:-

Any circuit which is not complete is considered an open circuit. A complete circuit which is not performing any actual work can still be a closed circuit. For example, a circuit connected to a dead battery may not perform any work, but it is still a closed circuit.

Close Ciruit:-

A circuit is considered to be closed when electricity flows from an energy source to the desired endpoint of the circuit. The open status of the circuit doesn't depend on how it became unclosed, so circuits which are manually disconnected and circuits which have blown fuses, faulty wiring or missing components are all considered open circuits.

short circuit A condition in any circuit of any size in which power jumps from the energy source to either a ground point or to the end of the circuit without actually completing the full circuit. Since this condition shortens the route travelled by the electricity, the condition is known as a short circuit.

 

Short Circuits:- Short circuits in energy transmission systems usually result in an overload which can result in anything from a harmless blown fuse to a lethal electrical fire. Most common short circuits that occur in household electrical service are potentially hazardous which is why fuses and breakers are so critical to safe electrical service. But a short doesn't necessarily indicate a hazard condition..

Calculating and Measuring Power Consumption

The design of an autonomous system always begins with the calculation of how much power is consumed. The easiest way to measure your device is a laboratory power supply that features a voltage and ampere meter. The nominal voltage provided by a lead acid battery typically varies between 11 Volts (empty) and about 14.5 Volt (charging, voltage at charging limit). You can tune the voltage at the laboratory power supply and see how much current the device draws at different voltages. If a laboratory power supply is not available, measurement can be performed by using the supply shipped with the device. Interrupt one cable that goes to the DC input of your device and insert an ampere-meter (or ammeter). Note that a ammeter will burn itself or your power supply if applied between the positive and negative terminal because it behaves like a simple cable between the probes -thus creating a short. Many ammeters have an unfused input, so exercise caution as they can be easily damaged.

The amount of power consumed can be calculated with this formula:

P = U * I

P being Power in Watts, U being voltage in Volts, I being current in Ampere. For example:

6 Watts = 12 Volts * 0.5 Ampere

The result is the rating of the device. If the device of the example is operating for an hour it will simply consume 6 Watt-hours (Wh), respectively 0.5 Ampere-hours (Ah). Thus the device will draw 144 Wh or 12 Ah a day.

To simplify things, I will use the nominal voltage rating of batteries for calculations and not take into account that the voltage provided by the battery varies depending on its state of charge. Batteries are rated at their capacity in Ah -so it is easier to calculate using Ah instead of Wh. A battery from a big truck has typically 170 Ah -thus a 100% charged truck battery would power the device for about 340 hours during a 100% discharging cycle.

Importance of Earthing:

Earthing or grounding is done for safety of equipment and human beings (including all animals and plants).

Equipment Safety:

The outer housing of an electrical equipment is earthed by directly connecting it to a earth grid or earth electrode, thereby providing a low resistance path to ground. In case of a fault involving earth the live part of the equipment gets connected with the low resistance earth path. This produces high earth fault current and the protective devices in the circuit disconnects the circuit from the power source thereby reducing further damage to the equipment.

Neutral of electrical equipment are also earthed for equipment safety. Like, neutral of generators in power plants are earthed through Neutral Grounding Resistor to limit the earth fault current. Three phase transformer's neutral are earthed to provide neutral point to supply single phase loads like lighting and small appliances.

Human Safety:

If a person touches an appliance which has an earth fault in it he will not get an electric shock as his body (standing on the earth) and the equipment's body are at the same potential provided the equipment is earthed properly. Thus proper earthing protects a person from getting electric shock.

Circuit symbols are used in circuit diagrams which show how a circuit is connected together. The actual layout of the components is usually quite different from the circuit diagram. To build a circuit you need a different diagram showing the layout of the parts on stripboard or printed circuit board.