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Electrical technology – assignment 4 Properties of electrical materials and Effective use of technology P. 5. Describe the properties and a typical application of a solid and a liquid or gas electrical conductor. A conductor is a material that allows free movement of electrons and therefore allows easy flow of electricity. Most conductors are metals. A common metal used as a conductor in electrical technology is aluminium. It is used quite a lot as it cheap, lightweight and in abundance compared with other electrical conductors.
Aluminium is used in overhead power cables as it is has many properties which make it ideal for this purpose. It is very easy to work Aluminium can be easily fabricated into various forms such as foil, sheets, shapes, rods, tubes, and wires. It also displays excellent malleability and plasticity in bending, cutting, and drawing. Aluminium does not rust as when it is open to the air a layer of aluminium oxide forms and protects it from corrosion. Earlier on transmission lines were made from copper but this was heavier and recently became more expensive.
Conductive Aluminium has relatively low mechanical strength therefore it is either used with steel, or by using an aluminium alloy to help with the mechanical strength that is needed in overhead power lines. Aluminium is a very light metal with a specific weight of 2. 7 g/cm3, about a third that of steel. Aluminium’s Electrical Resistivity at 20°C is at 2. 69 µ?. cm. Aluminiums conductivity is around 60% that of copper (coppers being 59. 6 ? 106 S/m). Argon is a colorless, odorless, tasteless, and non-toxic gas. It is a good example of a gas that can conduct electricity.
It is an inert gas this means that it is none reactive. It is used within bulbs and fluorescent lamps as a substitute for air. If the bulb/lamp is filled with air the filament would oxidise this would cause the bulb to be dimmer this would also be dangerous as the oxygen in air could Cause the filament to burn, so argon is used to prevent any of this happening. Electrical conductivity of argon: 0 mS cm-1 Argon has the ability to be ionized (become electrically conductive) at lower voltages than most other gases.
This is done by heating the gases to a temperature at which it turns to a plasma state, in this state it begins to release electrons which move independently of the atom making it electrically conductive. Argon can be used is also used in fluorescent lamps as a gas which can be conductive when ionized. When it is ionized and electrons are flowing through the argon gas the free electrons bump into gaseous mercury atoms which then get excited and when they return to their normal state the extra energy is given off as light energy (photons) these photons are in the ultraviolet spectrum so we cannot see them.
So a phosphor powder coating is used this will give off visible light when the atoms are exited and return to their normal state. Also used in “Geiger” counters, which measure radiation levels. P. 6. Describe the properties and a typical application of a solid and a liquid or gas electrical insulator. An insulator is a material that blocks the flow of electrical current. An insulator is a poor conductor as it has a high resistance against the flow of electrons. Solid Electrical insulators are commonly used to hold conductors in place such as using porcelain to hold over head cables and to insulate them from the pylon.
Other electrical insulators include rubber, plastic, and mica. Porcelain is a commonly used material for the suspension of high-voltage overhead lines as it is a good insulator, but it is brittle. Porcelain insulators are made from clay, quartz or alumina and feldspar they are covered with a glaze to keep away dirt and are shaped as they are to let water run off them. Where a high durability and mechanical strength is needed a porcelain rich in alumina will be used. It has a dielectric strength of about 4–10 kV/mm. Air contains roughly (by volume) 78. 8% nitrogen, 20. 95% oxygen, 0. 93% argon, 0. 038% carbon dioxide, and trace amounts of other gases. Air is a good example of a gas that is used as an electrical insulator. It has an electrical resistance of around 109? m. Air is a good insulator because normally the atoms are far apart, so it is difficult for electrons to jump from molecule to molecule, also the molecules that make up air have very few free electrons. This means that almost all of the electrons are trapped and cannot be easily moved by a voltage.
Air is used as a dielectric in a wide variety of applications, ranging from electronics to high-voltage high-power electric transmission lines. This is because it is free, abundant and is self repairing after a breakdown. P. 7. Describe the properties and an application of two different magnetic materials commonly used in electrical and electronic engineering. Materials respond differently to the force of a magnetic field. A magnet will strongly attract ferromagnetic materials, weakly attract paramagnetic materials, and weakly repel diamagnetic materials.
Ferromagnetic materials are strongly attracted by a magnetic force. The elements iron, nickel, cobalt are all ferromagnetic. Iron is commonly used as a core in transformers as it can hold a magnetic field well due to it being a ferromagnetic material; it is there to boost the magnetic field of the copper wires. The magnetization due to the current in the coil runs all the way round the iron core. The primary and secondary coils can be wound anywhere on the iron core, because the iron carries the changes in magnetization from one coil to the other.
The core of a transformer is made up of iron laminations insulated from one another using a very thin layer of non conducting material. This is to reduce losses in the form of eddy currents. The magnetic flux in an iron core lags behind the magnetizing force this is known as hysteresis. When a ferromagnetic material is magnetized in one direction, it will not go back to zero magnetization when the magnetizing field is removed. It must be forced back to zero by a field in the opposite direction. Materials that hold there magnetism for longest before being switched back down to zero are the best for making permanent magnets from.
To the right is a basic hysteresis loop. The hysteresis loop is a series of curves that shows the characteristics of a magnetic material. To make an object a magnet you need to encourage all of the magnetic domains within a piece of metal to face in the same direction. An example of this is when you rub a magnet along a pin in the same direction continuously. The domains in the pin will be encouraged to face in the same direction and the pin will become magnetized. The more domains point in the same direction, the stronger the overall field will be.
Alnico magnets are an alloy made from aluminum, nickel and cobalt; it also contains small amounts of other elements including iron and copper. Alnico is produced by casting or sintering processes. Anisotropic alnico magnets and there domains are orientated by heating them up to a high temperature and then cooling them within a strong magnetic field. Heat treatment is needed to get the alnicos magnetic properties. These magnets have a high curie point this a temperature at which the magnet will loose its ferromagnetic properties due to the magnetic domains being exited and becoming unaligned with one another.
It also has a high melting point and its brittleness. This type of magnet is used within electric motors, microphones, sensors, loudspeakers, along with many other types off devices. P. 9. Describe two different applications of electrical technology and, for each of them describe how electrical energy is used to enable them to function. A light bulb is made up of a simple structure. At the base, they have two metal contacts, which connect to the ends of an electrical circuit which supplies it with power.
The metal contacts are attached to two stiff wires, which are attached to a thin metal filament. The filament sits in the middle of the bulb, held up by a glass mount. The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon. When an electrical supply is connected to the bulb free electrons flow from one contact to the other through the wires and filament. As the current flows through the filament it heats up due to the electrons bumping of the atoms that it is made up of and making them vibrate and therefore heat up nd give them a higher energy level temporarily, as the atoms that have just gained the extra energy fall back down to their normal levels they release the extra energy in the form of photons. These photons cannot be seen so need to be heated to around 2,200 degrees C for them to emit a good amount of visible light. The filament in the bulb is made from a long thin strip of tungsten wrapped around in very thin coils and then coiled again, like in the diagram to the right. Tungsten is used to get round the problem of heating to such a high temperature.
Tungsten can be heated up to these temperatures without it melting. Although Tungsten will not melt at these temperatures it will combust, this is where the inert gas comes in this provides an oxygen free environment to prevent the tungsten combusting. The inert gas that is used within the bulb is argon this as well as preventing combustion will help reduce the loss of tungsten increasing the life of the bulb. A loudspeaker is a device that converts an electrical signal given by a CD, tape or DVD’s into sound.
We hear sounds through our eardrums vibrating due to change of air pressures made when an object vibrates. A speaker produces sound waves by rapidly vibrating a flexible cone or diaphragm. Key parts of a speaker. The cone/diaphragm is usually made of paper, plastic or metal. All cone materials have advantages and disadvantages. The key properties designers look for when choosing the material are lightness, stiffness and lack of coloration. This is then connected to a suspension this is made of a flexible material allowing movement of the diaphragm.
The narrow end of the cone is then attached to a voice coil. This is held in place by something called a spider which allows it to move freely back and forth. There is a permanent magnet placed at the bottom of the speaker this magnetic field interacts with the magnetic field created by the voice coil. The frame, or basket, is the structure holding the cone, voice coil and magnet in the proper alignment, here rigidity is important. There are two main types of metal frames stamped and cast.
Stamped frames usually constructed from steel is of lower cost but is of a disadvantage as it may flex if the speaker is at too high a volume. In comparison cast baskets may be more expensive but offer more rigidity and can be cast into more complex shapes. The voice coil is basically an electromagnet; this is placed within a permanent magnets magnetic field. These two magnets then interact with each other the same any other magnets would, being repelled and attracted depending on which poles were facing one another.
The voice coils magnetic poles can be reversed by switching the positive and negative inputs, this can be done many times a second. When the poles of the electromagnet are switched so does the direction of repulsion and attraction between the two magnets. So when this is happening the voice coil is being pushed backwards and forwards rapidly. Whilst the coil moves it pushes and pulls on the speaker’s diaphragm. This then causes the air in front of the speaker to vibrate creating sound waves which we can hear. An electrical signal is used to control the changes in the electromagnet.
There is a wide range of speakers available in different sizes and power each producing different frequencies of sound. The types include Woofers, Tweeters and Midrange. Woofers are the largest and are designed to produce low frequency sounds. Tweeters are designed to produce the highest frequency sounds and are much smaller, and midrange speakers produce the frequencies in the middle of these. The images below show the comparison between Woofers, Tweeters and midrange speakers in comparison to a 1pence piece.