TOPICS ON ELECTRICITY AND MAGNETISM

ELECTRICITY

Electricity is a form of energy found in all matter. It consists of a movement of electrons - components of atoms - from one place to another. Free electrons, that is to say, electrons separated from their parent atoms, take the first opportunity of rejoining an atom that is short of its proper number. The amount of electricity is measured by the number of electrons in the charge and as each one of them repels its neighbour vigorously, the charge has a lively nature which is described as its potential.

We can produce a charge of electricity by rubbing a stick of sealing wax in our coat sleeve. The wax is charged with negative particles ready to return to an association to the positive particles. If we bring the end of the wax stick near some small bits of tissue paper, they will fly up and stick to the wax. This occurs because the charge of negative particles is strong enough to attract the positive particles, or protons, that form the paper; so the bits are held until all the electrons reach those various atoms and final­ly they drop.

From this easy experiment, we infer one of the principles that govern the science of electricity: "Like particles repel; unlike particles attract each other".

In the experiment given we see that friction - as well as induction - is the first important source of static electricity, so called because of its property to keep still.

The second form in which electricity exists is the current or flowing state produced by chemical decomposition in a battery or by moving a conductor across the lines of force of a magnetic field as in a dynamo or by differences of temperature between the junctions of a thermoelement.

Statical electricity is, then, the electricity of stationary charges however produced. Dynamic, voltaic or current electricity deals with moving charges which give origin to the phenomena of the electric current whether generated by friction, by induction, by means of a voltaic battery or dynamo, or in any other way.

In 1780, Luigi Galvani observed that when the charge from a friction machine was applied to the leg of a dead frog, the leg twitched violently. Further more, with a brass wire driven into a spinal marrow and formed into a stand to support the frog above an iron plate, the legs twitched whenever the feet touched the iron plate. He imagined that the charge flowing along nerves caused the frog's muscles to contract and he was right. The flow of charge was called electric current.

Following Galvani's researches, Volta invented the pile bearing his name, which is equivalent to a number of electrochemical cells in series. He made a pile of disks consisting of copper, zinc, moist paper, copper, zinc, moist paper, copper and so on in that order, with which he was able to produce effects like those the friction machines produce. Nicholson and Carlisle in 1800 were able to deposit copper from a copper sulfate solution on a copper wire. Wollaston showed that similar electrolitic results could be produced with charges from a friction machine.

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MAGNETISM

Magnetism is the science that treats of the conditions and laws of magnetic force, that is to say, of the force by which attraction and repulsion are exerted by the poles of a magnet.

The science of magnetism began with William Gilbert who, in the year 1600 published his treatise On the Mag­net and on the Great Magnet of the Earth.

It is a peculiar fact that the earth behaves like an enormous magnet with poles near the geographical poles. It is this fact that makes the mariner's compass possible.

The angle between the direction to which a freely suspended magnet points and the true geographic north is called angle of declination or magnetic variation.

Every magnet is surrounded by a space within which its influence may be felt and this constitutes what we know as magnetic field. When a ferromagnetic substance has a permanent magnetic field and magnetic moment associated with it we call it permanent magnet. If a coil of insulated wire is wound around a bar of iron and a current of electricity is run through the coil, the iron is immediately magnetised. On changing the direction of the current the poles of the magnet change too. If we cut off the current, the magnetism will go at once. This is the electro-magnet, a much more powerful form than the permanent magnet. Electromagnets are very largely used in electric motors and generators, in electric bells, buzzers and chimes, in telephones, in relays, in dynamo­meters and as magnets for lifting iron and steel. Magnets of great power are made for industrial aims.

MAGNETS

A magnet is a body capable of attracting certain substances and of either attracting or repelling another magnet.

As we have seen previously , there are two different kinds of magnets: permanent and electromagnets. The former are in essence self-contained sources of stored energy transferred to the magnet in the course of getting magnetized by an external agency, while the latter accumulate energy in a soft-iron core only when an electric current passes through a wire wound around it.

Permanent magnets are independent of any outside power source as they are being used, though sometimes they should be strengthened or re-energized. At the beginning of the XX century they were comparatively feeble and easily demagnetized carbon-steel, tungsten-steel and chrome-steel magnets, but nowadays we employ far more powerful and retentive alloys of aluminium, nickel, cobalt and iron. Lightweight magnets known as ferrites can be magnetized to almost the same strength as the pre­vious ones.

The discovery of ferrites dates back to the early part of last century but the subject has come into prominence only recently. Actually, lodestone is a natural ferrite, that contains no free ferro­magnetic components such as uncombined iron, nickel or cobalt, but is composed instead, of a mixture of iron oxides. They possess an electric resistance usually higher than that of a metal; on account of this they are suitable for being used in transformer cores operated at high frequencies. Other advantages of ferrites involve low weight as well as low cost, factors that contribute largely to their use in computers, radio and television.

As they are submitted only to temperature limitations and eliminate the complexities sometimes associated with electrical circuits, permanent magnets can be used in any location. They are employed in loudspeakers, toys, games, refrigerator-door latches, openers, kitchen utensil hold­ers, some types of gauges, and oil filters, memory elements in old computers and in different sorts of cores in high-frequency radio and television transformers.

As regards electromagnets, we can say that the over­all magnetism of an electromagnet stops when the current in its surrounding coil is interrupted and the previously stored energy is dissipated as heat or returned to the source.

Besides the possibility of being much stronger than the most powerful permanent magnets, they offer the convenience and control that the others lack.

Due to their flexibility, electromagnets have all the uses mentioned in the previous lesson plus some others such as magnetic amplifiers, magnetic lenses in electron microscopes, deflecting coils in TV picture tubes and so on.

There are magnets made for industrial purposes. For example, there is a device in use in eye hospitals for removing small splinters of iron or steel from the eyes of workmen injured in factory work. In this device, the magnetism is concentrated into a core with a sharp conical point. When the patient's eye is brought close to it, the splinter is gently drawn out. Another type of electromagnet is used in steelworks for lifting heavy steel plates and girders.

Magnets were known to the ancient world. The name derives from Magnesia, a province in Asia Minor where an iron-ore with strong magnetic properties is found. The Chinese were the first to discover that when a bar of this ore was hung by a strand of silk it assumed a position along a line pointing north and south. For centuries men were satisfied to know this about the magnet. Later on, it was discovered that there was more than a connection between electricity and magnetism: in fact, they were part of the same science.

MATERIAL USED IN ELECTRICITY

The materials used in wires, cables, switches, fuses and everything concerning the electric industry, are cho­sen to suit their specific requirements. Thus, copper, which is of high conductivity, and aluminium (annealed) are employed for general purposes such as single wires or stranded cables, while for resistance units and tele­graphic lines, annealed iron is usually preferred as a conductor.

In the past years silk and cotton were used as insulating materials for wire, coils and armatures, while rubber was chiefly used for cables and bell-wires, later being replaced by synthetic rubbers. Porcelain - as well as plastics - is a good insulator for fuse-carriers and switch-bases. Other materials employed for insulating purposes are cotton, paper and some special kinds of varnishes.

We must bear in mind that cables are prepared according to the peculiar needs of industries as, for example, the trailing cables used on lifts or in mines. These wires and cables are specially insulated for resisting heat, using asbestos or glass.

In the manufacture of switches we have to add that the base may be made of wood, or better, of some insulating substance such as fibre or slate. For the metal parts, sheet brass or sheet copper is used.

Lead, or some alloy which melts at a low temperature, is a suitable material for the making of fuses.

The electrical resistance of a metal or alloy is a function ot temperature, decreasing as the temperature falls and tending to zero at the absolute zero, that is the lowest temperature theoretically possible. It is found that for certain metals and alloys as for example lead and tin, the resistance changes abruptly, becoming very small at a temperature in the neighbourhood of a few degrees above absolute zero. This phenomenon is termed super-conductivity and the temperature at which it sets in is the transition temperature.

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