Magnetism

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Categoria:	Inglese
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Data:	18.07.2007
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MAGNETS AND MAGNETISM
The greeks discovered that a certain kind of rock had the power to attract and pick up bits of iron. These rocks are called natural magnets. It was discovered that a magnet that could turn freely, would point one side to the north.
Lodestone, bits of magnetite suspended on a string, were used as crude compasses, for desert travel by Chinese more than 2000 years ago, and by sailors in the early voyages of exploration.
The earth is a larger natural magnet, and its magnetism or force of attraction, turns toward the north.
A piece of iron stroked with a natural magnet became magnetized, forming an artificial magnet. Magnets of soft iron are called temporary magnets, because it becomes and loses its magnetism easily. Magnets of stell alloys hold their magnetism for a long period, and are called permanent magnets. They’re usually made of a bar of steel alloy.
Tungsten, nickel, cobalt are usually used in strong magnets.
Magnetism in a magnet is concentrated at two points, called “poles” of the magnet, one on the north and the other on the south.
The magnetic field consists of invisible lines of force, called “flux lines”, that form an area called “flux pattern”. Flux density is the number of flux lines per square. The points where the flux lines leave or enter the magnet are called poles. The magnetic circuit is the path taken by the magnetic lines of force.
Between north poles, there is a force of repulsion. (also between south poles). But between north pole and south pole, there a force of attraction, because of static charges.
Two magnetic fields placed near each other, will reform a distorted flux pattern.
ELECTROMAGNETISM
When a current flows through a conductor, is generated an elecotromagnetic field.
The relation between the direction of electron flow and the direction of the magnetic lines of force can be determinated by the left hand rule. The thumb(police) points in the direction of the electron flow, and the fingers curl (arricciano) around the conductor in the direction of magnetic force.
RELUCTANCE
Reluctance is the amount of opposition, that all materials set up to the number of magnetic lines forces. Reluctance and magnetic lines are reverse proportional.
The lines of force inside the coil travel through the iron, making it a magnet. Electromagnet is the combination of an iron core and a coil. Soft iron core carries magnetic lines of force more easily than steel core. Soft iron core has the greater permeability.
The total flux of an electromagnet depends on the amount of current flowing through the coil, the number of coil turns, and the material.
MAGNETOMOTIVE FORCE
Mmf, is the force produced by the magnetization in a magnet circuit. It’s measured in amperespire.
CORE SATURATION
It’s a point where an increase of current through the coil doesn’t produce an increase of flux density.
HYSTERESIS
When the current through the coil reverse alternately direction, there is a rapid change in magnetization, creating a Residual magnetism that cause a delay in the increase of the magnetic flux.
BATTERIES
Are electrochemical devices, widely used to supply (fornire) energy for electrical and electronic products. Chemical energy stored in a battery is converted into electric current. Batteries are classified in primary and secondary. Primary batteries may be used only one time. Are widely used in products at low cost. The most common primary batteries are carbon zinc dry cell and the alkaline cell. Secondary batteries may be used repeatedly by recharging it. The most common are lead-acid battery and the nickel cadmium battery.
Battery is a series combination of individual cells assembled in a pack ( multi-cell-batteries)
THE STRUCTURE OF A MODERN DRY CELL
A dry cell consists of a zinc tube (the negative electrode), a protective outer case. Inside there is a carbon rod, the positive electrode fitted(installato) with a metal cap (calotta). The rod is in contact with a mixture of powdered carbon and a depolariser, which are contained in a perforate bag. this allow to come into contact with electrolyte, which contain ammonium chloride, in the form of jelly (gelatina).

VOLTA VS GALVANI
Galvani had discovered that animals contain a type of electricity that make the muscles jump. A frog’s leg only move when two different metals touching them. This type of electricity is much weaker than the powerful shocks given by static electricity machines. The different metals which produce the biggest effect are zinc and silver.
Galvani arranged discs of zinc, silver and leather( cuoio) in salt water in column, repeating thirty times. When he connected a wire to the top and the bottom discs, he obtained an electric current. He invented the voltaic pile or battery.
One of the most used primary cells is the dry cell, or Leclanchè, which has an EMF of 1,5V. It consists of a carbon rod (bacchetta) placed in the centre of the cell forming the positive pole. It’s surrounded by a mixture of carbon powder and manganese dioxide, which acts as depolariser.
They’re contained in a porous gauze, placed in a zinc container. The space between the zinc container and the bag is filled with a paste of ammonium chloride and zinc chloride which acts as the electrolyte.
A plastic cap is sealed (sigillato) at the top of the carbon rod, working as the positive terminal and the zinc container has the function of a negative pole.
SOLAR CELLS
Convert solar energy into electrical energy.
Use sun energy to heat water and convert it into steam(vapore), using its power to generate electricity. The water is usually heated using a system of mirrors to focus the sun’s rays onto water.
Light consists of a streams of very very small particles called photons, which carry energy. When it hit an atom, it can gives its energy to one of electrons in the atom, which become free, and it can move on to another atom, producing the folw of electric current.
A solar cell consists of two layers of semiconductors, the upper is an n-type(have extra electrons), and the under is a p-type( have fewer electrons). When the light hit the upper n-type, this frees some of electrons, which are attracted to the p-type semiconductor, setting an electric current.