Section 2 understanding electricity

І

The word electric comes from the Greek word for amber, electron. Amber is a fossil resin, yellowish in colour. It is found on the southern shores of the Baltic Sea.

Thousands of years ago, man discovered that when amber is rubbed with fur or cloth it will attract small light objects, for example, particles of dust. It does this in the same way that a magnet will attract pieces of iron. This discovery was, perhaps, the first step made towards man’s understanding of electricity.

Today we know that this attraction of small particles or objects is caused by an electrical charge, which is given to the amber by the friction, or rubbing together, of the materials. This is known as static electricity.

An electric current is the flow of electrons. To understand what an electron is, we must first think of matter and how it is constructed, or made up.

The smallest piece of any substance is called an atom. An atom is made up of a proton, which has a positive electrical charge, and a number of electrons, which have a negative charge. When the two charges are equal, the atom is said to be neutral.

Until an atom is charged, it will remain in its original form. It is held together because opposite charges are attracted to each other. Similar charges of electricity will repel. These are the two fundamental laws of electricity.

Electricity is made in many ways.

One way is by battery, as used in a transistor radio. A simple battery has a positive terminal, or pole, connected to a carbon rod. The terminal may be coloured or marked with a cross. The carbon rod is placed in a chemical paste inside a zinc container. Attached to the zinc container is the negative terminal, coloured dark or marked with a short horizontal line. The action of the chemical causes the poles to be charged.

A simple battery converts chemical energy into electrical energy. Condensers, or batteries can store small amounts of electrical power. But it’s not possible to store the great quantities that we use every day in our homes and in industry. The generating stations must be operating continuously to cope with this demand. There are certain times during the day, when this demand for power is very high and other times when it may be very low. These are known as peak and off-peak periods. In order to operate the power stations more efficiently they are all connected to a grid system, which is a network of high voltage cables to ensure a more even distribution of power all over the country.

A great natural source of electricity is lightning. The dazzling flash of light we see during a thunderstorm is caused by the discharge of electricity from one cloud to another. Sometimes the discharge may be from a cloud to earth. In this case the electricity may pass through a tree or a house. When this happens we say that the house is struck by lightning. To avoid the risk of damage to buildings a lightning conductor is used. It is made from a metal rod placed high on the building, which is connected by a thick conductor wire to a metal plate buried in the earth. If lightning strikes the building, the charge is conducted safely to earth.

Electricity and magnetism are closely connected. Each may be used to produce the other.

Around the poles of a magnet is an area called a magnetic field. In this area the magnet exerts its attraction. The directions of this attraction and the shape of the magnetic field are shown by lines of force. These lines of force can be seen if a small magnet is placed under a sheet of paper onto that iron filings are sprinkled.

II

When an electric current flows through a conductor, a magnetic field is set up around it. The conductor becomes a magnet. If a conductor is made to rotate in a magnetic field, a pressure of electricity, called a voltage, is produced in it. This is the principle on which a dynamo produces electricity.

Electricity flowing through a conductor may be likened to water flowing through a pipe. The amount of electricity that flows is measured in units called amperes. The voltage is measured in volts. Just as a thin pipe will not allow as much water to flow through it as a large one, so thin conductor will not allow as much electricity to flow as a thick one. The restriction to the flow caused by the conductor is known as its resistance. A unit of resistance is called an ohm. The resistance of a conductor will also vary according to the material of which it is made. Copper has a much lower resistance than other common metals. It is a very good conductor.

Current, voltage and resistance bear a definite relationship to one another. Ohm’s law tells us that current equals voltage divided by resistance. The current and the voltage multiplied together determine the power, the rate at which the electrical energy is used up by being converted to other forms of energy. A unit of power is watt.

To simplify the figures used for great power, abbreviations are used. One thousand watts is a kilowatt and one million watts is a megawatt. Very sensitive instruments can measure a milliwatt, which is a thousandth part of a watt. A millionth part of a watt is a microwatt. Electrical appliances are described according to the amount of power they consume; we talk of a 60 watt lamp, a 2 kilowatt fire.

In some pieces of electrical apparatus it is sometimes necessary to store an electrical charge, to build it up and hold it for some time. This is done by means of a capacitor, sometimes called a condenser.’ A simple capacitor is made from two metal plates, called electrodes,’ which are separated by an insulating material such as air, paper, or mica, called the dielectric.

III

The unit of capacitance is called a farad. This is a relatively large unit. Many capacitors used in electronic apparatus are rated in microfarads, millionth part of a farad. It is sometimes necessary to use a capacitor whose value can be varied. An example of this is the tuning control on a radio receiver, when it is turned the value of the capacitor is changed.

The most common way of making electricity is by the conversion of mechanical energy. A dynamo is a machine that makes electricity when it is made to rotate at high speed. A turbine that is rotated by steam pressure often drives it. This machinery is then called a turbo-generator. A building which houses a turbo-generator is called a power or generating station. A station that uses the power of falling water is known as a hydroelectric station. Most of the world’s electricity for domestic and industrial use is made оr generated in such power stations.

Any material that will allow electricity to flow through it is called a conductor. Most metals are good conductors. Any material that will not permit the flow of electricity is known as an insulator.

Conductor wires, or leads, in any piece of electrical apparatus or installation must not be allowed to touch each other, or any other metal part. If they do, a short circuit will result which will cause failure of the apparatus and the possible dangers of fire and electric shock.

Insulating materials are used to cover and protect conductor wires and sometimes to separate them. Common insulating materials include air, oil, glass, porcelain, mica, rubber and plastics. Heavy power cables are sometimes given an extra cover, known as a bonding or shield, made of metal for the further protection of the insulation.

Two different kinds of electricity are used to operate electronic apparatus.

One kind is called direct current, usually abbreviated to DC.

This is the kind of electricity we get from a battery, the flow of current is in one direction only. The other kind is alternating current, usually known as AC. This is the kind of electricity that is generated in power stations for domestic and industrial use. It is called alternating current because the direction of flow changes very quickly. The number of changes of direction, called cycles, in every second is known as the frequency.

IV

The frequency of the alternating current in most parts of Europe is 50 cycles. In North America it’s 60 cycles. Very high frequencies are used in radio communication, sometimes several million cycles.

Electricity can be very dangerous if it is carelessly used. A faulty installation or a defective piece of equipment can cause a serious fire and there is also the danger of an electric shock, with possibly fatal consequences. Many precautions are taken and safety devices are used to minimize these dangers. Insulation, earthing, fuses and automatic cutouts are among the most common safety devices.

Insulating is the converting of a conductor with a non-conducting material to prevent it from touching another conductor. The material used vary according to the nature of the conductor and the condition in which it is used. It may have to withstand extremes of temperature and resist corrosion. The insulation of power leads to portable appliances must be flexible and also very strong.

Any piece of electrical apparatus which has a metal body or frame, especially portable appliances such as electric irons, kettles and power tools, must be earthed as a safety measure. Connecting the metal body or frame directly to earth with thick conductor brings about earthing. In a three-pin power plug the thickest pin is always the earth connection.

If a fault should develop inside the apparatus or if the insulation on a flexible lead should break down, any possibility of a person getting an electric shock by touching the apparatus will be avoided because the current will take the easier path back to earth.

As an additional precaution the handles of all such appliances are made of an insulating material such as rubber or molded plastics.

The most common safety device and one that is used in every household installation is the ordinary fuse. This is simply a piece of wire that is connected in series in the circuit. It is of such a resistance that it will melt and therefore break the circuit if the current flowing in the circuit exceeds a certain amount. When this happens we say the fuse “blows”. It is very important that fuse wire of the correct amperage is used.

It is often necessary to increase or decrease the voltage of the electricity supply. One reason for this is the economic transmission of power from its source, the generating station, to wherever it is required for use, which may be at a great distance. It is cheaper and easier to carry a very high voltage but low current, over long distances. It can be done using thinner overhead conductor wires, with an air gap between them to act as an insulator.

When the overhead cables reach the area where the electricity is required for use, the voltage must then be carried by much heavier conductors, which have to be well insulated and are therefore more costly.

V

A transformer is used to increase or decrease the voltage of an electric power supply. A transformer is a static machine it has no moving parts. It consists of two coils of wire that are wound around a soft iron core. The coils are called windings, one is the primary, or input winding, and the other is the secondary, or output winding.

When a current is passed through the primary winding, a magnetic field is set up around the iron core that induces a voltage in the secondary winding. If the number of turns in the secondary is greater than the number in the primary it is a step-up transformer and the output voltage is greater than the input. A step-down transformer reduces the input voltage.

Some complicated pieces of electrical apparatus, for example radio and television receivers, do not use alternating but direct current. As the domestic supply is alternating current, it is therefore necessary to change it to direct current inside the apparatus. This change, or conversion, is brought about by means of a rectifier, sometimes called a diode.

A simple diode consists of a valve that has only two electrodes; one is the anode and the other the cathode. When it is used to rectify, it will pass current only during each half cycle of the applied alternating current. The result is a pulsating current flowing in one direction, an irregular, or uneven current, but a direct current. It can be made regular and even by means of a smoothing circuit.

A valve is a device that allows a flow in one direction only. In a radio valve this is the flow of electrons from cathode to anode. A simple thermionic valve is called a diode because it has two electrodes: the cathode, which is negative, and the anode, which is positive. It is called a thermionic valve because the cathode is in the from of a filament and when it becomes hot, negative electrones become free and are attracted towards the positive anode.

There are many different kinds of valves in a radio receiver; they are specially designed to perform different functions. A triode is a valve with three electrodes, an anode, a cathode and a control grid. A tetrode has four, and a pentode five electrodes.

A major development in the electronics industry during recent years has been the gradual replacement of thermionic valves by transistors. A transistorized circuit is simpler, smaller, and technically stronger. Because of these advantages the transistor has contributed greatly to rapid developments in other scientific fields, particularly space travel.

VI

A transistor is a semiconductor. This means that it is not entirely a conductor, nor is it an insulator. It is crystalline in structure and has three electrodes, a base, an emitter and a collector.

The main difference between a valve and a transistor is that while a valve amplifies, or gives a voltage gain, a transistor gives an increase in current. Transistors can be made from germanium, selenium, silicon and other substances.

The various components that go to make up a complicated electrical circuit may be connected to the circuit in two ways. If the supply is fed directly through each component in turn, they are said to be wired in series. If, however, the supply is taken to each one independently then they are wired in parallel.

Every component in series in a circuit will add to the total resistance of the circuit and decrease the current flowing through it, but every additional resistance in parallel will reduce the total resistance of a circuit. A high-tension battery is made up of a number of small batteries connected in series. Its voltage is equal to the sum of the voltages of the smaller batteries.

A switch is used in almost every piece of electrical apparatus. Its purpose is to make complete or to break an electrical circuit. When a circuit is switched on it is a closed circuit and current will flow through it. When it is switched off the circuit is broken, it becomes an open circuit and the flow of current is stopped. The main contacts of a switch are called poles. These are connected when the switch is on.

There are very many different kinds of switches ranging from a micro-switch, which is very sensitive, to the large and complicated ones, which are used for high voltages and are sometimes known as circuit breakers. Temperature or pressure control or timing devices can operate switches automatically.

It is sometimes convenient or economical to operate electronic apparatus from a distance. This is known as remote control and it may be brought about by the use of a relay, which is really an electromagnetic switch.

A relay consists of an electro-magnet which, when energized attracts a soft iron armature. The armature is returned to its former position by a spring when the electro-magnet is de-energized and therefore there is no longer any attraction. The movement of the armature opens or closes the contracts of the relay. In this way one electrical circuit can be used to control another.

The telephone uses very little electric current. It consists of two main parts, the microphone and the earphone.

The microphone contains a very thin and flexible metal disc called a diaphragm. It also contains a magnet. When we speak we cause the diaphragm to vibrate. The movements of the diaphragm in the magnetic field set up a varying electrical current. The current is carried along wires to the earphone of the person to whom we are speaking.

The earphone contains a small electro-magnet and a diaphragm similar to the one in the microphone. The varying current is fed into the electro-magnet and causes it to exert a varying attraction on the diaphragm, which vibrates and so reproduces the sound that went into the microphone.

An electric bell is made to work by a “make-and-break” circuit It consists of an electro-magnet whose armature is the hammer of the bell. The circuit includes a pair of contracts, one of which is attracted to the armature. A spring holds the armature in such a position that the contacts are closed.

When the operating button is pushed, the electro-magnet attracts the armature against the tension of the spring, and the hammer strikes the bell. At the same time, the contacts are opened, and so the circuit is broken and the spring returns the armature to its former position. The contacts are now closed again and as long as the button is pushed the armature will vibrate and the bell will continue to ring.