Atoms and electrons
3.1.1 This lesson discusses the copper media used in networking. Since all matter is composed of atoms, this page begins with a detailed explanation of atoms and electrons.
All matter is composed of atoms. The Periodic Table of Elements lists all known types of atoms and their properties. The atom is comprised of three basic particles:
• Electrons – Particles with a negative charge that orbit the nucleus
• Protons – Particles with a positive charge
• Neutrons – Neutral particles with no charge
The protons and neutrons are combined together in a small group called a nucleus.
To better understand the electrical properties of different elements, locate helium (He) on the periodic table. Helium has an atomic number of 2, which means that helium has two protons and two electrons. It has an atomic weight of 4. If the atomic number of 2 is subtracted from the atomic weight of 4, the result shows that helium also has two neutrons.
The Danish physicist, Niels Bohr, developed a simplified model to illustrate the atom. This illustration shows the model for a helium atom. If the protons and neutrons of an atom were the size of adult soccer balls in the middle of a soccer field, the only thing smaller than the balls would be the electrons. The electrons would be the size of cherries that would be in orbit near the outer-most seats of the stadium. The overall volume of this atom would be about the size of the stadium. The nucleus would be the size of the soccer balls.
Coulomb's Electric Force Law states that opposite charges react to each other with a force that causes them to be attracted to each other. Like charges react to each other with a force that causes them to repel each other. In the case of opposite and like charges, the force increases as the charges move closer to each other. The force is inversely proportional to the square of the separation distance. When particles get extremely close together, nuclear force overrides the repulsive electrical force and keeps the nucleus together. That is why a nucleus does not fly apart.
Examine the Bohr model of the helium atom. If Coulomb's law is true and the Bohr model describes helium atoms as stable, then there must be other laws of nature at work. Review both theories to see how they conflict with each other:
• Coulomb's law – Opposite charges attract and like charges repel.
• The Bohr model – Protons have positive charges and electrons have negative charges. There is more than one proton in the nucleus.
Electrons stay in orbit, even though the protons attract the electrons. The electrons have just enough velocity to keep orbiting and not be pulled into the nucleus, just like the moon around the Earth.
Protons do not fly apart from each other because of a nuclear force that is associated with neutrons. The nuclear force is an incredibly strong force that acts as a kind of glue to hold the protons together.
Electrons are bound to their orbit around the nucleus by a weaker force than nuclear force. Electrons in certain atoms, such as metals, can be pulled free from the atom and made to flow. This sea of electrons, loosely bound to the atoms, is what makes electricity possible. Electricity is a free flow of electrons.
Loosened electrons that do not move and have a negative charge are called static electricity. If these static electrons have an opportunity to jump to a conductor, this can lead to electrostatic discharge (ESD). Conductors will be discussed later in this module.
ESD is usually harmless to people. However, ESD can create serious problems for sensitive electronic equipment. A static discharge can randomly damage computer chips, data, or both. The logical circuitry of computer chips is extremely sensitive to ESD. Students should take safety precautions before they work inside computers, routers, and similar devices.
Atoms, or groups of atoms called molecules, can be referred to as materials. Materials are classified into three groups based on how easily free electrons flow through them.
The basis for all electronic devices is the knowledge of how insulators, conductors, and semiconductors control the flow of electrons and work together.
The Lab Activity reviews the proper way to handle a multimeter.
The next page introduces voltage.
Voltage
3.1.2 Voltage is sometimes referred to as electromotive force (EMF). EMF is related to an electrical force, or pressure, that occurs when electrons and protons are separated. The force that is created pushes toward the opposite charge and away from the like charge. This process occurs in a battery, where chemical action causes electrons to be freed from the negative terminal of the battery. The electrons then travel to the opposite, or positive, terminal through an external circuit. The electrons do not travel through the battery. Remember that the flow of electricity is really the flow of electrons. Voltage can also be created in three other ways. The first is by friction, or static electricity. The second way is by magnetism, or an electric generator. The last way that voltage can be created is by light, or a solar cell.
Voltage is represented by the letter V, and sometimes by the letter E, for electromotive force. The unit of measurement for voltage is volt (V). A volt is defined as the amount of work, per unit charge, that is needed to separate the charges.
The next page describes resistance and impedance.
This page explains the concepts of resistance and impedance.
3.1.3 The materials through which current flows vary in their resistance to the movement of the electrons. The materials that offer very little or no resistance are called conductors. Those materials that do not allow the current to flow, or severely restrict its flow, are called insulators. The amount of resistance depends on the chemical composition of the materials.
All materials that conduct electricity have a measure of resistance to the flow of electrons through them. These materials also have other effects called capacitance and inductance that relate to the flow of electrons. Impedance includes resistance, capacitance, and inductance and is similar to the concept of resistance.
Attenuation is important in relation to networks. Attenuation refers to the resistance to the flow of electrons and explains why a signal becomes degraded as it travels along the conduit.
The letter R represents resistance. The unit of measurement for resistance is the ohm (Ω). The symbol comes from the Greek letter omega.
Electrical insulators are materials that are most resistant to the flow of electrons through them. Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber, and helium gas. These materials have very stable chemical structures and the electrons are tightly bound within the atoms.
Electrical conductors are materials that allow electrons to flow through them easily. The outermost electrons are bound very loosely to the nucleus and are easily freed. At room temperature, these materials have a large number of free electrons that can provide conduction. The introduction of voltage causes the free electrons to move, which results in a current flow.
The periodic table categorizes some groups of atoms in the form of columns. The atoms in each column belong to particular chemical families. Although they may have different numbers of protons, neutrons, and electrons, their outermost electrons have similar orbits and interactions with other atoms and molecules. The best conductors are metals such as copper (Cu), silver (Ag), and gold (Au). These metals have electrons that are easily freed. Other conductors include solder, which is a mixture of lead (Pb) and tin (Sn), and water with ions. An ion is an atom that has a different number of electrons than the number of protons in the nucleus. The human body is made of approximately 70 percent water with ions, which means that it is a conductor.
Semiconductors are materials that allow the amount of electricity they conduct to be precisely controlled. These materials are listed together in one column of the periodic chart. Examples include carbon (C), germanium (Ge), and the alloy gallium arsenide (GaAs). Silicon (Si) is the most important semiconductor because it makes the best microscopic-sized electronic circuits.
Silicon is very common and can be found in sand, glass, and many types of rocks. The region around San Jose, California is known as Silicon Valley because the computer industry, which depends on silicon microchips, started in that area.
The next page describes electrical current
3.1.4 Electrical current is the flow of charges created when electrons move. In electrical circuits, the current is caused by a flow of free electrons. When voltage is applied and there is a path for the current, electrons move from the negative terminal along the path to the positive terminal. The negative terminal repels the electrons and the positive terminal attracts the electrons. The letter I represents current. The unit of measurement for current is Ampere (A). An ampere is defined as the number of charges per second that pass by a point along a path.
Current can be thought of as the amount or volume of electron traffic that flows. Voltage can be thought of as the speed of the electron traffic. The combination of amperage and voltage equals wattage. Electrical devices such as light bulbs, motors, and computer power supplies are rated in terms of watts. Wattage indicates how much power a device consumes or produces.
It is the current or amperage in an electrical circuit that really does the work. For example, static electricity has such a high voltage that it can jump a gap of an inch or more. However, it has very low amperage and as a result can create a shock but not permanent injury. The starter motor in an automobile operates at a relatively low 12 volts but requires very high amperage to generate enough energy to turn over the engine. Lightning has very high voltage and high amperage and can cause severe damage or injury.
The next page discusses circuits.
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