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Matter exists in three states; namely, solid, liquid and gaseous.
Solid matter tends to hold its shape when acted upon by a force. For example, if a weight of one pound is suspended from one end of a ten inch length of No. 20 B. & S. gauge steel piano wire, it will be found by careful measurement that the wire has stretched. If the weight is removed another measurement will show that the wire has returned to its original length.
Liquid matter, in general, does not oppose a change in its shape when acted upon by a force. Liquid mercury will run about when spilled on a flat surface or if it is poured into a bottle, it will assume the shape of the bottle.
Gaseous matter tends continually to occupy a greater volume, in other words, to diffuse. Liquid ammonia is a solution of a gas called ammonia in water. If some liquid ammonia is spilled the ammonia gas is liberated and fills the whole room with its distinctive odor.
Many substances can occur in the three states. The state in which a substance is found is dependent upon the temperature and pressure. For example, air can be liquefied by subjecting it to a very low temperature and a very high pressure. In turn, this liquid air can be frozen by a further reduction in temperature.
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The molecule is the smallest integral part of any substance. Thus, a water molecule is the smallest water particle and is totally different from a mercury molecule, a wood molecule or a molecule of any other substance. In other words, the molecule has an individuality. The nature of a given substance is dependent solely upon the molecules of which it is composed.
Each molecule is separate and distinct from all other molecules. It is free to move about and is believed to be in a continual and very rapid state of vibration, or oscillation, at ordinary temperatures. It is thought that one molecule collides with other molecules many millions of times per second, even in air. The rate of vibration and the number of collisions vary with the temperature and the density of the substance.
The molecules are relatively very closely packed together in solids, yet each molecule is able to oscillate about a mean position and is never in permanent contact with other molecules. They cannot, however, travel far. When heat is applied to one end of a metal rod, the molecules at that end vibrate more rapidly and violently and drive the neighboring molecules away, thus increasing the space around themselves. This action is transmitted along the length of the rod and heat is finally felt at the other end. The length of the rod is also increased. This expansion is caused by the space between the molecules having been increased.
The difference between solids, liquids and gases is in the degree of separation of the molecules, the molecules themselves remaining unchanged. There is very little attraction among the molecules in liquids and still less in gases. This is evidenced by the fact that liquids flow and that gases will become diffused or mix with air.
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Some molecules are made up of only one atom, such as the monatomic gas molecules, while others are more complex and contain many atoms of different kinds. For example, two atoms of hydrogen, combined, form hydrogen gas, and the combination of twelve atoms of carbon, twenty-two atoms of hydrogen and eleven atoms of oxygen forms ordinary cane sugar.
It is thought that there are some ninety odd different kinds of atoms and that the almost infinite number of combinations of atoms possible constitute the endless variety of molecules and, therefore, of substances in the world.
Atoms vary in size and weight. The hydrogen atom,is the lightest known. Its weight is taken as unity and the atomic weight of all other atoms is given relatively to that of the hydrogen atom. For example, the atomic weight of lead is 206.4.
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Atoms differ from one another only in the number and arrangement of electrons that constitute them.
The nucleus of the atom is considered as being composed of both positive and negative electrons, but with the positive electrons predominating so that the nucleus is always of a positive nature.
In order to more easily identify them, the positive electrons are called protons and the negative electrons simply electrons.
The electrons and protons of the nucleus are closely bound together but exterior to the nucleus of the atom, there are a number of negative electrons more loosely bound and are therefore more or less free agents that can leave their atomic home with little urging.
The electron, however, does not ordinarily exist in a free or isolated state and normally has a sort of family life, in combination with other electrons, in the atom.
Ordinarily, the atom is electrically neutral, the outer electrons balancing the positive nucleus. If, however, something happens to disturb this balance the loosely bound or foot-loose electrons begin to leave home and electrical activity becomes evident.
Electricity is simply the evidence of electrons in motion and electrons in motion constitute an electric current.
The ease with which electrons are able to be transferred from one atom to another in any material is a measure of the electrical conductivity of that material. If the electrons can readily leave home and pass from one atom to another it is said that the material is a good conductor. On the other hand if the electrons find great difficulty in leaving one atom to pass to another of a material, it is said that the material is a nonconductor or insulator.
Silver, copper, aluminum and most metals are relatively good conductors of electricity.
Some substances do not act as conductors, that is, when they are acted upon by an electrical force there is practically no drift of electrons and, hence, no electrical current flow. Such substances serve a very useful purpose.
A perfect insulator or dielectric is one in which the atoms never lose an electron. Such a substance has an atomic structure believed to be of such a nature that the electrons are imprisoned in cells within the limits of which they can move. Under the influence of an electrical force every electron is impelled in one direction, out of its normal position in its cell, and held out of equilibrium only as long as the electrical force is acting. With every increase or decrease of the electrical force the electron is, respectively, more or less displaced and the tension increased or decreased. As soon as the electrical force is removed, the electrons return to their normal positions in the atomic structure.
A rupture of the insulator or dielectric occurs if the electrons are strained beyond the elastic limit of the atomic structure. The atoms then lose an electron and the insulator becomes a conductor.
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The action of storing up this electrical energy is somewhat similar to that of a gas tank used for the storage of gas.
Any electrical capacitor has three essential parts two of which are usually metal plates separated and insulated by the third part called the dielectric.
The amount of electricity which a capacitor will hold depends on the electrical pressure or voltage applied to the capacitor just as the amount of gas a tank will hold depends upon the pressure.
If the pressure is doubled on the gas tank, twice the amount of gas is forced into the tank, and if the electrical pressure or voltage applied to a capacitor is doubled, twice the amount of electricity will be forced into the capacitor.
Capacitors, regardless of type or kind, are all designated by their electrical size. Obviously, when it is desired to designate the size of some object, some unit of measurement must be employed. In capacitors, this electrical size is designated as capacity.
The capacity of an electrical capacitor is the ratio of the quantity of electricity and the electrical pressure or voltage. In other words the capacity of a capacitor depends on the amount of electricity it will hold at a certain electrical pressure or voltage.
This ratio may be expressed as follows:
By the same token the capacity may be expressed as follows:
Where the capacity is equal to the quantity of electricity divided by the electrical pressure or voltage.
The capacity of a capacitor is dependent upon the size and spacing of the conducting plates and the type of insulating or dielectric medium between the plates.
The dielectric of a capacitor is one of three essential parts. It may be found in solid, liquid or gaseous form or even in combinations of these forms in a given capacitor.
The simplest form of capacitor consists of two electrodes ot conducting plates separated by air. This represents a capacitor having a gaseous dielectric.
Other dielectrics in common use are mica, paper, glass, sulphur, mineral and vegetable oils, waxes and synthetic insulating compounds such as the chlorinated groups.
It is common practice to divide or identify capacitors in accordance with the type of dielectric employed in their structures. For example, there are mica capacitors, air capacitors, oil capacitors and paper capacitors.
There is also another type of capacitor and that is the electrochemical type or electrolytic capacitor. It is to this type that this publication will be almost entirely devoted.
As was previously mentioned, the simplest form of capacitor consists of two metal plates separated by air. The air, of course, in this case, is the dielectric.
Also, as previously mentioned, the capacity of a capacitor is dependent on the size of the plates and the space between them as well as the kind of dielectric medium employed. Knowing these facts, it becomes apparent that there must exist some fixed relationship which would allow for the predetermination of any desired capacity. The most fundamental of such a relationship is expressed as follows:
In other words, the capacity is proportional to the product of the area of one plate multiplied by the dielectric constant, divided by the thickness of the dielectric.
It is important that this fundamental fact be remembered, that doubling the area of the plates of a capacitor doubles the capacity and reducing the thickness of the dielectric by one-half also doubles the capacity of a capacitor.
If, in the simplest form of capacitor, the air space between the plates, be replaced with mica it would be found that the capacity will have increased in value.
Due to this increase in capacity, it is said that mica has a higher dielectric constant than air. Also in order that the mica dielectric can be said to have a certain definite dielectric constant, it has been established that air has a dielectric constant of one.
As has been mentioned, capacitors are designated by their electrical size or capacity and the unit of capacity is the farad.
The farad, unfortunately, represents a capacity so enormous that such a capacitor is rarely if ever produced or used. For practical purposes therefore, a small multiple of the farad is used. This is called the microfarad and is one millionth of a farad. A still smaller multiple is also in common use and this is the micromicrofarad or one millionth of one millionth of a farad. The microfarad is generally designated as MFD or MF and the micro-microfarad is MMFD or MMF.
There is still another designation which is applied to capacitors and that is the voltage rating. In relation to this designation the gas tank analogy will again serve for illustrative purposes.
The pressure upon the gas in a tank cannot be increased indefinitely, for the tank will ultimately yield and break. Similarly there is a limit to the electrical pressure which can be applied to a capacitor, for the dielectric will be broken down or punctured if the limit is exceeded.
The voltage or electrical pressure at which a spark will pass and the dielectric be punctured is called the dielectric strength.
The dielectric strength of a given dielectric is determined by the thickness of the dielectric. In other words, the dielectric strength of a dielectric is determined by the thickness and kind of material.
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The capacity of an electrolytic capacitor is determined by the same factors which apply to any other electrical capacitor. That is, the capacity varies directly in proportion to the area of the conducting surfaces and inversely in proportion to the thickness of the dielectric.
The electrolytic capacitor, however, departs from the more conventional types of electrical capacitors in that only one of its conducting surfaces is a metallic plate, the other conducting surface being a chemical compound or electrolyte. The dielectric employed is a very thin film of oxide of the metal which constitutes the one metallic plate used in the structure.
This oxide, which constitutes the dielectric, possesses remarkable characteristics as an insulator under certain conditions. Under these conditions it is quite common practice to employ field strengths in the dielectric of the order ten million volts per centimeter of thickness. Although this is almost unbelievable, it is due to this fact that electrolytic capacitors can be fabricated which possess high capacity and small physical size.
Electrolytic capacitors are divided into two general types; namely, the wet electrolytic capacitor and the dry electrolytic capacitor. Fundamentally, there is no difference between these two types but physically there is sufficient difference to warrant their being treated separately in this publication.
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