Quantcast electricity. The following is a summary of the important parts in the chapter. In describing the composition of matter, the following terms are important for you to remember: "> Chapter 1 Summary

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SUMMARY

    With the completion of this chapter, you now have gained the necessary information which is the foundation for the further study of electricity. The following is a summary of the important parts in the chapter.

    In describing the composition of matter, the following terms are important for you to remember:

MATTER is defined as anything that occupies space and has weight.

AN ELEMENT is a substance which cannot be reduced to a simpler substance by chemical means.

A COMPOUND is a chemical combination of elements which can be separated by chemical means, but not by physical means. It is created by chemically combining two or more elements.

A MIXTURE is a combination of elements or compounds that can be separated by physical means.

A MOLECULE is the chemical combination of two or more atoms. In a compound, the molecule is the smallest particle that has all the characteristics of the compound.

AN ATOM is the smallest particle of an element that retains the characteristics of that element. An atom is made up of electrons, protons, and neutrons. The number and arrangement of these subatomic particles determine the kind of element.

AN ELECTRON is considered to be a negative charge of electricity.

A PROTON is considered to be a positive charge of electricity.

A NEUTRON is a neutral particle in that it has no electrical charge.

ENERGY in an electron is of two types - kinetic (energy of motion) and potential (energy of position).

ENERGY LEVELS of the electron exist because the electron has mass and motion. The motion gives it kinetic energy and its position gives it potential energy. Energy balance keeps the electron in orbit and should it gain energy it will assume an orbit further from the center of the atom. It will remain at that level for only a fraction of a second before it radiates the excess energy and goes back to a lower orbit.

SHELLS AND SUBSHELLS of electrons are the orbits of the electrons in the atom. Each shell contains a maximum of 2 times its number squared electrons. Shells are lettered K through Q, starting with K, which is the closest to the nucleus. The shell can be split into 4 subshells labeled s, p, d, and f, which can contain 2, 6, 10, and 14 electrons, respectively.

VALENCE OF AN ATOM is determined by the number of electrons in the outermost shell. The shell is referred to as the valence shell, and the electrons within it are valence electrons. An atom with few valence electrons requires little energy to free the valence electrons.

IONIZATION refers to the electrons contained in an atom. An atom with a positive charge has lost some of its electrons, and is called a positive ion. A negatively charged atom is a negative ion.

CONDUCTORS, SEMICONDUCTORS, AND INSULATORS are categorized as such by the number of valence electrons in their atoms. The conductor normally has 3 or less valence electrons and offers little opposition to the flow of electrons (electric current). The insulator contains 5 or more valence electrons and offers high opposition to electron flow. The semiconductor usually has four valence electrons of conductivity and is in the midrange. The best conductors in order of conductance are silver, copper, gold, and aluminum.

CHARGED BODIES affect each other as follows: When two bodies having unequal charges are brought close to each other, they will tend to attract each other in an attempt to equalize their respective charges. When two bodies, both having either positive or negative charges, are brought close together, they tend to repel each other as no equalization can occur. When the charge on one body is high enough with respect to the charge on an adjacent body, an equalizing current will flow between the bodies regardless of the conductivity of the material containing the bodies.

A NEUTRAL BODY may be attracted to either a positively or negatively charged body due to the relative difference in their respective charges.

CHARGED BODIES will attract or repel each other with a force that is directly proportional to the product of their individual charges and inversely proportional to the square of the distance between the bodies.

ELECTROSTATIC LINES of force are a graphic representation of the field around a charged body. These lines are imaginary. Lines from a positively charged body are indicated as flowing out from the body, while lines from a negatively charged body are indicated as flowing into the body.

MAGNETISM is that property of a material which enables it to attract pieces of iron. A material with this property is called a magnet. Any material that is attracted to a magnet can be made into a magnet itself.

FERROMAGNETIC MATERIALS are materials that are easy to magnetize; e.g., iron, steel, and cobalt.

NATURAL MAGNETS, called magnetite, lodestones, or leading stones, were the first magnets to be studied. Most magnets in practical use are artificial or man-made magnets, and are made either by electrical means or by stroking a magnetic material with a magnet.

RELUCTANCE is defined as the opposition of a material to being magnetized.

PERMEABILITY is defined as the ease with which a material accepts magnetism. A material which is easy to magnetize does not hold its magnetism very long, and vice versa.

RETENTIVITY is defined as the ability of a material to retain magnetism.

A MAGNETIC POLE is located at each end of a magnet. The majority of the magnetic force is concentrated at these poles and is approximately equal at both poles.

THE NORTH POLE, or north seeking pole, of a magnet freely suspended on a string always points toward the north geographical pole.

THE LIKE POLES of magnets repel each other, while the unlike poles attract each other.

WEBER'S THEORY OF MAGNETISM assumes that all magnetic material is made up of magnetic molecules which, if lined up in north to south pole order, will be a magnet. If not lined up, the magnetic fields about the molecules will neutralize each other and no magnetic effect will be noted.

THE DOMAIN THEORY OF MAGNETISM states that if the electrons of the atoms in a material spin more in one direction than in the other, the material will become magnetized.

A MAGNETIC FIELD is said to exist in the space surrounding a magnet.

MAGNETIC LINES OF FORCE are imaginary lines used to describe the patterns of the magnetic field about a magnet. These lines are assumed to flow externally from the north pole and into the south pole.

MAGNETIC FLUX is the total number of magnetic lines of force leaving or entering the pole of a magnet.

FLUX DENSITY is the number of flux lines per unit area.

FIELD INTENSITY or the intensity of a magnetic field is directly related to the magnetic force exerted by the field.

THE INTENSITY OF ATTRACTION/REPULSION between magnetic poles may be described by a law almost identical to Coulomb's Law of Charged Bodies, that is, the force between two poles is directly proportional to the product of the pole strengths and inversely proportional to the square of the distance between the poles.

MAGNETIC SHIELDING can be accomplished by placing a soft iron shield around the object to be protected, thus directing the lines of force around the object.

MAGNETS ARE CLASSIFIED BY SHAPE and include the bar magnet, the horseshoe magnet, and the ring magnet. The ring magnet is used in computer memory circuits; the horseshoe magnet in some meter circuits.

ENERGY may be defined as the ability to do work.

THE COULOMB (C) is the basic unit used to indicate an electrical charge . One coulomb is equal to a charge of 6.28 x 1018 electrons. When one coulomb of charge exists between two bodies, the electromotive force (or voltage) is one volt.

VOLTAGE is measured as the difference of potential of two charges of interest.

VOLTAGE MEASUREMENTS may be expressed in the following units: volts (V), kilovolts (kV), millivolts (mV), or microvolts (V).
For example:
1 kV = 1,000 V
1 mV = 0.001 V
1 V = 0.000001 V

METHODS OF PRODUCING A VOLTAGE include:
1. Friction
2. Pressure (piezoelectricity)
3. Heat (thermoelectricity)
4. Light (photoelectricity)
5. Chemical action (battery)
6. Magnetism (electromagnetic induction generator)

ELECTROMAGNETIC INDUCTION GENERATOR To produce voltage by use of magnetism, three conditions must be met: There must be a CONDUCTOR in which the voltage will be produced; there must be a MAGNETIC FIELD in the conductor's vicinity; and there must be relative motion between the field and conductor. When these conditions are met, electrons WITHIN THE CONDUCTOR are propelled in one direction or another, creating an electromotive force, or voltage.

ELECTRON CURRENT is based on the assumption that electron current flow is from negative to positive through a circuit.

AN ELECTRIC CURRENT is a directed movement of electrons in a conductor or circuit.

THE AMPERE is the basic unit used to indicate an electric current. A current of one ampere is said to flow when one coulomb of charge (6.28 x 1018 electrons) passes a given point in one second of time. Current measurements may be expressed in the following units: ampere (A), milliampere (mA), and microampere (A). Current in a circuit increases in direct proportion to the voltage (emf) applied across the circuit.

RESISTANCE is the opposition to current. The ohm is the basic unit of resistance and is represented by the Greek letter omega (W). A conductor is said to have one ohm of resistance when an emf of one volt causes one ampere of current to flow in the conductor. Resistance may be expressed in the following units: ohm (W), kilohm (kW), and megohms (MW). For example, 1,000,000W = 1,000 kW = 1 MW.

THE RESISTANCE OF A MATERIAL is determined by the type, the physical dimensions, and the temperature of the material that is,
1. A good conductor contains an abundance of free electrons.
2. As the cross-sectional area of a given conductor is increased, the resistance will decrease.
3. As the length of a conductor is increased, the resistance will increase.
4. In a material having a positive temperature coefficient, the resistance will increase as the temperature is increased.

THE CONDUCTANCE OF A MATERIAL is the reciprocal of resistance.

THE UNIT OF CONDUCTANCE is the mho and the symbol is V.G or S.

THE RESISTOR is manufactured to provide a specific value of resistance.

THE CARBON RESISTOR is made of carbon, with fillers and binders blended in to control the ohmic value.

THE RESISTANCE OF A WIREWOUND RESISTOR is determined by the metal content of the wire and the wire's length. Wirewound resistors may be tapped so two or more different voltage values may be taken off the same resistor.

THE POTENTIOMETER AND THE RHEOSTAT are variable resistors and can be adjusted to any resistance value within their ohmic range. The rheostat is usually used for relatively high current applications and has two connections; the potentiometer has 3 connections and is a relatively high-resistance, low-current device.

Two examples of potentiometers.

Example of a rheostat.

Table 1-1.Standard Color Code for Resistors

THE WATTAGE RATING OF A RESISTOR is related to the resistor's physical size, that is, the greater the surface area exposed to the air, the larger the rating.

THE STANDARD COLOR CODE for resistors is used to determine the following:

1. Ohmic value
2. Tolerance
3. Reliability level (on some resistors)




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