An electrical circuit is any arrangement that permits an electrical current to flow. All circuits contain four elements: a source, a load, a transmission system and a control.
The source provides the electromotive force. This establishes the difference in potential which is necessary to make current flow possible. The source can be any device which supplies electrical energy, for example, a generator or a battery.
The load converts the electrical energy from the source into some other form of energy. For example, a lamp converts electrical energy into light and heat. The load can be any electrical device.
The transmission system conducts the current round the circuit. Any conductor can be part of a transmission system. Most systems consist of wires although the metal frame of a unit may be part of the transmission system. The control regulates the current flow in the circuit.
Communication is the transfer of information from one place to another. The person sending the information is called the sender, the source or the originator. The person receiving the information is called the receiver or the destination. The physical path, that is the wires, fibre optic cable or radio waves over which the information travels is called the transmission line or channel.
Information is physical, for example the shape of a letter or a sound. It must be understood by both the sender and the receiver. If you listen to a foreign radio station, information is being sent and received, but it cannot be called communication unless you understand the language in which it is being sent.
Information is usually sent between humans in the form of sound, light or touch in a way that can be detected by the human senses of hearing, sight and feeling. If the receiver does not understand the patterns being received, he thinks that no meaningful information is being communicated.
Computers are electronic devices that process information. They are capable of communicating with the user, of doing five arithmetic operations, and of making three kinds of decisions. However, they are incapable of thinking. They accept data and instructions as input, and after processing the information, they output the results.
When talking about computers, both hardware and software need to be considered. The former refers to the actual machinery, whereas the latter refers to the programs that control and coordinate the activities of the hardware.
The first computer was built in 1930 but since then computer technology has evolved a great deal. There are three different kinds of computer in use today: the mainframe, the minicomputer, and the microcomputer. However, the dividing line between these has become blurred; a modern micro is often as powerful as a mainframe was ten years ago. All three have one thing in common; they operate quickly and accurately in solving problems.
The earliest method of communication was human speech which progressed through the ages from simple grunts and growls to the modern languages that we know today. The problem with a directly spoken language however is that it is slow and does not travel very far. Even the loudest yells are limited to a few hundred feet. In the early days, drums were used to increase the range but the rate of information transfer remained slow and it required the knowledge of a special code. A modern version of the drum method is the telegraph which was developed in 1844. This was the earliest electrical communication. At the transmission end, a switch or key was closed in a certain pattern to represent a letter of the alphabet. The electrical energy on the wire, that is the signals, were sent in the same pattern of short and long bursts. At the receiving end, this energy was converted into a pattern of sound clicks which were decoded by a telegrapher. The code used by both the transmitter and receiver was the Morse Code.
Electricity is an important ingredient in matter. When we study the structure of matter, we find that all elements are formed from three main components - protons, neutrons, and electrons. Each atom has a central nucleus around which the electrons revolve. Electrons carry a negative charge while the nucleus containing protons carries a positive charge. Neutrons have no electrical charge. Normally an atom has an equal number of electrons and protons. The charges cancel each other out, so that the atom is electrically neutral. A stream of moving electrons is called an electrical current. All materials can be classified into three groups according to how easily they permit an electrical current to flow. Firstly conductors, which provide an easy path for the electrical current. Copper is a good conductor and so it is used for cables. Secondly insulators, which do not easily release electrons. Pvc is used to cover cables because it is a good insulator. Finally semi-conductors, which act as insulators or conductors, depending on the conditions.
There are two kinds of materials used in engineering - metals and non-metals. We can divide metals into ferrous and non - ferrous metals. The former (ferrous) contain iron whereas the latter do not. Cast iron and steel, which are both alloys or mixtures of iron and carbon, are the two most important ferrous metals. Steel contains a smaller proportion of carbon than cast iron. Certain elements can improve the properties of steel and are therefore added to it. For example, chromium may be included to resist corrosion and tungsten to increase hardness. Aluminium, copper and the alloys bronze and brass, are common non-ferrous metals.
Plastics and ceramics are non-metals, however, plastics may be machined like metals. Plastics are classified into two types - thermoplastics and thermosets. Thermoplastics can be shaped and reshaped by heat and pressure but thermosets cannot be reshaped because they undergo chemical changes as they harden.
Ceramics are often employed by engineers when materials which can withstand high temperatures are needed.
Fibre - optic communications, in which electrical signals are converted into pulses of light that are squirted along very thin glass pipes, have several advantages over traditional transmission systems. Firstly, they have a much larger capacity than copper wires. This means that they can carry much more information. Secondly, most fibres are made from silica which is very cheap. Although at the moment production costs are high, once the silica fibres are mass produced their cost will be greatly reduced. Thirdly, silica fibres are light and flexible and take up much less space than the present copper cables. Finally, the quality of transmission is very high. The signal fades less, therefore it does not need frequent amplification and there is little interference or crosstalk.
We can describe a force only by its effects. It cannot be measured directly like a length. A force can start something moving. If we push against a small object it moves. A force can also stop something moving or hinder motion. If we brake a moving car, it slows down and will eventually stop. If we suspend a heavy mass from a copper wire, the wire extends, showing that a force can stretch a body. Forces may also compress, bend or even break an object.
A force can be one of attraction. The force of attraction exerted by the huge mass of the earth is called gravity. If we pick up a stone, then release it, it falls to the ground because of gravitational force. Gravity is an example of a natural force.
Whether a force is naturally or deliberately exerted it cannot exist by itself. Forces must always occur in pairs, never in isolation. When a force acts on a rigid body it is balanced by an equal reaction force which acts in the opposite direction.
Whenever one surface moves over another, a force is set up which resists the movement. This force, called friction, always opposes motion. It exists in every machine. It can be reduced by lubrication but never completely removed.
In general, the force opposing motion is slightly greater before one surface starts moving over another surface than after movement has started. This slightly greater force is called static friction. The force which must be overcome to keep one surface moving over another is known as sliding friction. Static friction is greater than sliding friction. The value of sliding friction depends on the nature of the two surfaces which touch each other. Thus friction between two rough planks can be lessened if they are made smooth. Sliding friction is independent of the area of surface in contact. In theory a small brake pad will exert as much braking force as a large one of greater surface area. In practice a small pad will wear down more quickly and therefore is not used.
Heat is one of several forms of energy. It may be produced deliberately or incidentally as a by-product of the conversion of energy from one form to another. Fuels are converted into heat by burning. The unit of energy used is the joule or megajoule.
Heat energy is transferred by three separate processes called convection, radiation and conduction. The measure of how readily heat flows through a material is called its thermal conductivity. In general, metals conduct heat well and non-metals, such as glass, wood and plastics tend to be poor conductors of heat [or insulators]. Handles of metal cooking pots ars usually made of non-metallic materials, so that they don't burn your hand by conducting too much heat to it.
There are also wide variations within these broad categories. For example, silver is more than ten times as good a conductor of heat as lead, and brick is a much better conductor than wood, which is a much better conductor than cork.
A lot of information or ideas can be represented on one picture. For example, on road signs a picture may tell you not to park. This picture is a code of the information.
Many communication systems, especially electronic ones, change information into a code before sending it. This is done to make it easier for the signals to travel along the wires. In order to do this a coder at the sending side and a decoder at the receiving side are needed.
Even the simplest direct communication involves the coding of information. When one person talks to another, the vocal cords in the voice box of the speaker encode the information from the brain into sound waves that can travel over a distance to the listener. Similarly, the ear of the listener converts the patterns in the sound waves into electrical patterns that his brain can understand. In this way information is coded and decoded into a physical form for transmission over the transmission link or communication channel which in this case is the air between two people.
Metals can be joined by three basic methods: mechanical, thermal, and adhesive. The mechanical and thermal methods can be further divided. The former includes screw threads and rivets, and the latter can be divided into soldering, brazing, and welding.
Screw-fastening is used as a joining process where components have to be assembled and disassembled regularly. Riveting is a method of joining metals permanently. The process consists of drilling the metal plates, inserting the rivet and then closing it by compression force.
Soldering, brazing, and welding are methods of joining metals by heat. Soldering involves the use of lead and tin (or any other easily melted alloy) as an intermediate filler; brazing uses melted brass in the joints; and welding involves heating and hammering together, or the use of oxy-acetylene or arc welding.
Adhesive joining is used when the materials are dissimilar in composition, of difficult shape, or heat sensitive.
Logic gates are electronic switching devices. They contain semiconductors, not mechanical switches, which can be open or closed. They only have two levels of input and output; a high level and a low level. These correspond to the closed and open states of the switches. The high level is represented by 1 and the low level by 0. All information in digital systems is transmitted in terms of these two levels.
A truth table can be made for any logic gate. This table represents the output value of a gate for all possible combinations of inputs.
Common digital devices are OR, AND, NOR and NAND gates, and inverters. An inverter is a device which inverts its input. Therefore, an input of 0 will have an output of 1 and vice-versa. Complex circuits can be made by combining these basic devices.
Friction in machines causes loss of power. Engineers try therefore to reduce friction by good design. They can also use materials with a low coefficient of friction for devices such as bearings. The third method for reducing friction is lubrication.
Although the surface of a block of polished steel may seem perfectly flat, when we examine it under a powerful microscope, we see that it is covered in tiny hills and valleys. When one block slides over another therefore, the two surfaces scrape against each other, breaking off tiny pieces from each surface. However if we lubricate the two surfaces, oil fills the tiny valleys so that the surfaces do not weld together, and one block can move smoothly over another. As the surfaces do not scrape each other, wear on the material is also reduced. Some power, however, will still be lost depending on the thickness of the lubricant used.
Selection of the correct lubricant depends on the speed and temperature range of the machinery which is lubricated.
A number of instruments can be used to measure voltage. The moving-coil multimeter is often used because it is cheap and reasonably accurate. Moreover it is robust and versatile. However if the impedance of the instrument is low, the meter will draw too much current from the circuit. In addition, the frequency range is limited on ac.
The electronic voltmeter is almost ideal since the circuit is not loaded. Furthermore, it has a wide frequency range. The electrostatic meter is the simplest instrument for very high voltages. It responds both to ac and dc. However it is insensitive and has a non-linear scale.
The cathode ray oscilloscope gives more information than any other instrument. It not only measures the voltage but it also gives information about its waveform and frequency. However oscilloscopes are usually large and expensive.
You can only communicate information by radio waves by changing the wave in some way. This change is known as modulation. Stopping and starting the wave as in the dots and dashes of Morse code, is the simplest method of doing this.
Speech and music produce audio frequencies which cannot be transmitted in this way. They can be used however to modulate high-frequency radio waves which then act as carriers of the audio-frequency signal. The carrier wave is demodulated by the receiver and the audio-frequency signal is recovered.
The amplitude, frequency and phase of a wave can be modulated. In AM, the amplitude of the carrier wave is modulated according to the amplitude of the modulating signal, but the frequency of the carrier is kept the same. In FM, the amplitude of the carrier wave is kept constant, but the frequency is varied in proportion to the amplitude of the modulating signal. FM is better than AM because there is less interference. FM broadcasts are in the vhf band (30-300 MHz) because FM stations need greater spacing.
Modulation to select radio transmission and tuning to a station.
In radio and television broadcasting, radio telephony or mobile communications, free space is the common transmission medium and clearly chaos would occur if there were no means of tuning into the transmission.
The selection and separation of many hundreds of transmissions that could easily be present at one time, is only made possible by allocating each station its own carrier of specified frequency and where the information signal is impressed on this carrier using modulation. Without modulation, only one transmitting station could operate in a given area. However, by tuning a receiver to the required station carrier frequency, we can select this station and reject all others.
Modulation for more efficient radiation
In radio communications, electromagnetic waves are radiated into free space to be picked up at a distance by one or more receivers. In order to radiate efficiently, the transmitter antenna should have dimensions of at least one-tenth of a wavelength and preferably much more.
However, many information signals, particularly sound and telegraphy, have frequency components down to 100 Hz or less. In theory this would mean that antennas of the order of 300 km in height would be necessary, which is clearly impractical. By using modulation however, the information signal can be impressed on a high frequency carrier. This enables more practical and efficient antennas to be used for both transmission and reception.
Modulation, in communications, is the impressing of the baseband information signal waveform onto a carrier wave. The frequency of this carrier wave is normally considered higher than the highest frequency components contained in the baseband signal. Modulation is necessary because most baseband signals cannot be efficiently transmitted over a communication channel. The term "communication" channel is used to denote both the path for the transmission and the reception of electrical signals and the specified frequency band occupied by the signal in the transmission. Modulation allows us to match the signal we wish to transmit to the transmission medium.
Most plastics today are produced from oil. They are used widely in engineering because they are cheap and have a resistance to atmospheric corrosion. Their behaviour when heated makes it easy for us to divide them into two main categories. The first group, called thermoplastics, includes well-known plastics such as PVC, nylon and polythene. These can be heated, softened and cooled several times, changing their shape repeatedly. This process is possible because only weak bonding is present between their molecules, and when warm they slide very easily past one another. Pipes, bags and combs are made using this method. The second group, called thermosetting plastics, becomes rigid on heating. Examples are bakelite, melamine and polyurethane. These materials consist of polymer chains which react with one another at points of contact becoming strongly linked together. The intermolecular bonds then prevent movement of the original chains. Thermosetting plastics are used for plugs, foams and tableware.
The term memory usually refers to the internal storage locations of a computer. It is also called real storage or primary memory and is expressed as quantities of K, for example 16K or 152K depending on the storage capacity. Each K is equal to 1,024 bytes and each byte equals 8 bits. A megabyte is equal to 1048576 bytes.
Primary memory is closely associated with the central processing unit because it stores programs and data temporarily, thus making them immediately available for processing by the CPU. In order to do this , speed and random access are necessary. This means that any part of the memory may be read or accessed equally quickly. This is possible because of the system of addresses in the primary memory, where the storage locations are like a series of tiny compartments, each having its own address.
Primary memory is costly therefore it is used transiently which means that the program, or part of it, is kept in the internal storage only while that program is being executed.
A stream of moving electrons from negative to positive is called an electric current. Any material which provides an easy path for this current is called a conductor. Copper is a good conductor and is therefore used in the manufacture of cables. Any material which does not easily release electrons is called an insulator. Rubber and glass are insulators. Materials whose conducting properties are midway between conductors and insulators are called semiconductors. The two major semiconducting elements are germanium and silicon. Germanium is a rare element. Silicon on the other hand, is very common. The silicon atom has four electrons in it's outermost shell. However electron sharing or bonding occurs with other atoms. A cluster of silicon atoms sharing outer electrons forms a regular pattern called a crystal. Silicon can be grown into big crystals. It is then cut into wafers for making electronic parts. Before this can occur however, doping must take place. This means that the pure silicon is contaminated with small amounts of impurities.
A body is in stress when forces are applied to it which cause its size and shape to change. In other words, stress causes distortion. The intensity of stress depends on the size of the force and the cross-sectional area (c.s.a.) of the body which resists the force. That is, stress = applied force divided by c.s.a. of the body. Distortion due to stress is called strain. Different forces will distort bodies in different ways. A tensile force will lengthen a body. One subjected to a compressive force will contract. If a body has a uniform c.s.a., that is, if it has the same c.s.a. throughout its length, we calculate strain as strain = change of length divided by original length. Most materials used in engineering are elastic. A material which has the property of elasticity will return to its original size and shape when the forces producing strain are removed. However, if these forces go beyond a certain limit, called the elastic limit, even an elastic material will not regain its original dimensions.
The purpose of a switching system is to establish a temporary link between callers. In the first simple exchanges, all calls were handled manually by a switchboard operator, who plugged in the lines linking subscribers.
In 1892 Strowger introduced the automatic exchange operated on the step by step system. Many of these exchanges are still operating. Next came the crossbar exchanges, developed in Sweden.They consisted of a series of vertical and horizontally crossed bars, operated electromechanically. Although this system is still found throughout the world, it involves thousands of wear-prone exposed moving parts, which are costly to install and maintain.
In 1948, the invention of the transistor led to the first semi-electronic telephone exchange. Nowadays many exchanges are built around electronic components. The latest digital switching centres have many advantages over old exchanges, reducing the cost, size of equipment, installation time, and increasing reliability and number of customer services.
The first true telecommunications system using electrical signals to carry messages began in the 1840's with machine telegraphy developed by Samuel Morse. The next major step came in 1878 with the invention of the telephone by Bell. This enabled speech to be transported as electrical signals along wires. In 1886 Hertz verified that electrical energy could be radiated and thus proved the existance of electromagnetic waves. This provided the basis for all radio and television broadcasting. In 1901, Marconi established long-distance telegraph communication by transmitting across the Atlantic. With the discoveries of the diode and thermionic valve, advances were made in receiver and transmitter design. Television was established in 1937 and radar [radio detection and ranging] played a vital role in World War 2. The transistor was invented in 1947 and led to the development of microelectronic integrated circuit technology. Today hundreds of satellites orbit the earth providing links for all forms of communication.
On the 10th of March 1876, Alexander Graham Bell "the father of the telephone", relayed the first words by telephone. Nowadays telecommunications systems in general and the telephone in particular, have become indispensable tools for communicating the spoken word in business.
People and organisations are linked nationally and internationally by satellite, radio, undersea cable and land-line which provide telephone, telex, computer and video links. Fibre optic technology and computerised telephone exchanges are leading to even more sophisticated telephone services. Many organisations now operate their own private network systems (PABX) and teleconferencing, whereby a televised meeting of individuals in various different locations can take place, is increasingly popular. In the near future the video-telephone service will be widely available to business and domestic users and will become as common as the car telephone.
The telephone is an instrument which enables us to transmit speech via wires. The body of the telephone contains an induction coil, capacitors, resistors, a regulator which controls the sensitivity of the instrument, and a bell. The handset contains a microphone and a receiver which are enclosed by screwed caps at the ends of the handset. The bell contains a hammer which is operated by a solenoid. The hammer is set between two domes which are electrically mounted. In a traditional subscriber unit, a dial is mounted on the face of the telephone. It consists of a rotatable plate which has ten finger holes in it. A wheel which has ten slots cut in it's edge which match the finger holes, is fitted below the dial. When the plate is turned, the wheel causes spring contacts to open and close a number of times which correspond to the number dialled. This transmits pulses down the line causing selectors, which connect the calling line to the line being called, to operate.
In 1948, the invention of the transistor at Bell Telephone Laboratories, led to a revolution in electronics. Modern transistors are made of silicon. After oxygen, silicon is the most common element on this planet. It is neither a good conductor or insulator. It is a semiconductor.
Transistors are semiconductor devices with three leads. A very small current at one lead can control a much larger current flowing through the other two leads. This means that transistors can be used as amplifiers and switches. The two main families are bipolar and field-effect transistors (fets). Many different kinds of transistors are available for various purposes. The small switching transistors are used to amplify low level signals. Power transistors are used in high power amplifiers. High-frequency transistors operate at radio, television and microwave frequencies.
In the simplest telephone system, speech, in the form of air pressure waves, in other words, mechanical energy, is converted into electrical energy by the telephone microphone. This electrical energy travels along a pair of wires to a receiver, which converts the electrical energy back into speech.
Many different transmission systems are used in the telecommunications industry. Transmission on open-wire lines was the earliest method used for telephone traffic and this method is still used in many countries. Nowadays, however, local networks are built up using aerial or underground cable.
In a traditional system, analogue transmission is used. This means that the current has the same wave form as the variations in air pressure.
More recently, pulse code modulation, has been introduced. This means that the height of the waveform is measured, given a binary code, and transmitted as a train of pulses.
Several quantities like mass, length, time and temperature, are called fundamental or base quantities while others are derived from them. For example length is a base unit. Area is derived from length times width. Velocity equals a length divided by a time. The International System of Units [SI] suggests a set of convenient units and suitable abbreviations for these quantities. These SI units will be referred to throughout your Physics course.
The system uses seven base units. These include the metre [m], kilogram [kg], second [s], ampere [A], kelvin [K], candela [cd], and mole [mol]. All other SI units are derived from these.