Electricity

Materie:Appunti
Categoria:Inglese

Voto:

2 (2)
Download:85
Data:18.07.2007
Numero di pagine:5
Formato di file:.doc (Microsoft Word)
Download   Anteprima
electricity_1.zip (Dimensione: 7.54 Kb)
trucheck.it_electricity.doc     32.5 Kb
readme.txt     59 Bytes



Testo

WHAT IS ELECTRICITY
All matter is composed of microscopic particles called ATOMS. In the centre of each atom there is a NUCLEUS made up of PROTONS( positive electric charge) and NEUTRONS( neutral e.c.). They’re much heavier than ELECTRONS, negative electric charges that spin around the nucleus, and they form most of the atom’s mass. The number of electrons in an atom is exactly equal to the number of protons. The p.e.c. of the protons is balanced by the n.e.c. of the electrons and the atom is electrically NEUTRAL.
Electrons orbit at different distances from the nucleus and the farthest electron can sometimes become detached (staccarsi) from the atom and change into FREE ELECTRONS. If an atom loses an electron it then has more protons than electrons and it’s called “POSITIVE ION”. If a free electron meets a neutral atom, it may go into the outer orbit around the nucleus. An atom in this state is called “NEGATIVE ION”. The movement through material of free electrons passing from atom to atom results as electric current.

STATIC ELECTRICITY is a particular type of electricity that isn’t composed by the flow of electrons and can’t flow to the wires. It is composed by the attraction of opposite charges. It flows in the surface of the objects.
1)The friction between the cloth and the balloon causes electrons to transfer from pullover to the balloon. The pullover then has an overall positive charge because it has more protons than electrons. The ballon takes on a negative charge because it has extra electrons. Positive charge on the pullover attracts the negative charge on the balloon, therefore ( perciò) sticks ( s attacca) to the pullover.
2)The result of static electricity is LIGHTNING. In a thunderstorm winds blowing through raindrops (pioggia) causing ELECTRIC CHARGE. On the top of the cloud there is POSITIVELY CHARGED, and in the bottom NEGATIVELY CHARGED. The electric charge may jump between the different parts of the cloud causing LIGHTNING (SPARK). If the cloud is LOW, the N.C. at the bottom of cloud, induce P.C. on the surface of the ground. This is followed by a flash of lightning, where electricity from the cloud is discharged through the air.

BENJAMIN FRANKLIN in 1752 demonstrated that lightning is caused by electricity. He carried out ( effettuò) an experiment: flying a kite(aquiline) in a thunderstorm. He concluded that electricity in the cloud caused the spark and lightning a gigantic spark. It led the invention of the lightning conductor.

In 1785 Charles-Augustin de Coulomb invented a law for measuring the forces between magnets and between electric charges.The unit for measuring an amount of electricity is named coulomb. One coulomb is the amount of electricity that flows past a point when a current of one ampere flows for one second.

ALESSANDRO VOLTA, in the late 1790s, demonstrated that in a wet (bagnato) material which is contact with two different metals occurs ( avviene) a chemical action. That produces an electric current. Volta put togheter pairs (coppia) of metal discs,separated by cloth soaked (bagnato) in salt (salata) water, one made of silver or copper, and the other of zinc. Each individual produced a little electricalmotive force, but with a pile of them the E.F. was increased. This was the first battery ( Volta’s pile). The unit of E.F. is the volt.

George ohm, in 1827, devised a mathematical law to describe the relationship between current, voltage and resistance ( ohm’s law). A larger voltage can hold a larger current through a resistence.

HANS OERSTED, Danish physicist, in 1820, was the first person to show that when he passed an electric current through a wire, it produces a magnetism.

ANDRE’ AMPERE, during the 1820’s devised the mathematical relationship between currents and magnetic fields. It made possible to measure the amount of electric current flowing through a circuit. Current is measured in amperes. One ampere is the flow of about 6 thousand of million electrons per second.

Michael Faraday, in 1831 discovered that moving a magnet near a coil (spirale) of wire produces an electric current in the wire. This led ( portò) to the invention of Electric motor and E. generator. Farad is the unit used to measure electrical capacitance. If one coulomb of charge gives the capacitor a potential differences of one volt, the capacitance of capacitor is one.

NIKOLA TESLA, in 1887 devised an electric generating and distribution system that transmitted alternating current.
Johnstone Stoney developed the theory that electric current was caused by the movement of extremely small, electrically charged. In 1891, this particles was called ELECTRONS.

JOSEPH JOHN THOMSON, in 1897, proved the existence of electrons and showed that all atoms contain them. This is the beginning of the development of the electronic.

APPLICATIONS OF STATIC ELECTRICITY

ELECTROSTATIC PRECIPITATORS
They’re widely used to clean air in factories, offices, hospital and homes. They can remove particles of dust and smoke, like bacteria.
It consists of a filter, an ionizer, a collector, and a power pack, which provides direct-current electricity. A fan blows the contaminated air through the filter, which removes large particles of dirt. The air then enters to the ionizer which contains a small number of electrically charged wires. Any dust particles in the air receive a positive electric charge and are ionized. Then they pass to the collector, which consists of a series of metal plates, some of which carry a negative charge. The positively charged dust particles are attracted to the negatively charged metal plates because opposite charges attract one another. The dust particles stick to the plates which can then be removed for cleaning at regular intervals.

PHOTOCOPIERS
They work by using static electricity. The photocopy machine has an aluminium drum covered with a layer of light-sensitive material ( selenium). This material is charged with static electricity. Light reflected from the document or illustration to be copied passes through a lens. When the light strike the light-sensitive surface, it forms a positively charged invisible image corresponding to the dark areas of the original. The remainder of the surface loses its charge. The P.E. charges of the image attract the negatively charged particles of black powder (polvere), the toner, producing a visible image on the drum. A sheet of copy paper is passed close to the drum, and a positive electric charge under the sheets attracts the negatively charged ink particles, resulting in the transfer of the image to the copy paper. Finally heated rollers melt the toner, so that it sticks to the paper and forms a permanent image.

CONDUCTORS AND INSULATORS
Electric current is the flow of electric charge through a material. If the electricity moves easily through a substance, the material is called a conductor; which contain electrons that are free to move through the material. Metals are good conductors because they contain a large number of free electrons. Some liquids and gases are also conductors.
Insulators resist to the movement of electric charge because they’are not free to move around the atoms. Glass, rubber, plastic, dry wood and ordinary dry air are good insulators. Insulators are important for electrical safety.
Resistance is the material’s opposition to the passage of electric charges through it. There is resistance when electrons moving in the material, collide (si scontrano) with atoms and give up energy, which is converted into heat. A good conductor ha low resistance.
Insulators have high resistance, like wood or glass. Resistance depends not only on the tupe of material, but also on its size and shapes(forma).
Superconductors offer no resistance to the flow of electric charges. Electrons have no losing energy. Superconductors work at a very cold temperature.

SEMICONDUCTORS
Silicon conducts electric charge better than insulators but not as well as conductors. Doping it, adding small amounts of certain impurities, semiconductors change their electrical properties. Adding arsenic or phosphorus, which have more lectrons per atom than silicon, are produced extra free electrons, that can move and form electric current. These materials are called n-type semiconductors. Adding aluminium, borom or gallium, which have fewer electrons per atom than silicon, are produced holes, that can also form electric current. These materials are called p-type semiconductors.

Esempio