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Transcript
Chapter 2 Voltage and Current Atomic Theory • An atom consists of a nucleus of protons and neutrons surrounded by a group of orbiting electrons. • Electrons are negative, protons are positive. • In its normal state, each atom has an equal number of electrons and protons. Atomic Theory • Electrons orbit the nucleus in discrete orbits called shells. • These shells are designated by letters K, L, M, N, etc. • Only certain numbers of electrons can exist within any given shell. Atomic Theory • The outermost shell of an atom is called the valence shell. • The electrons in this shell are called valence electrons. • No element can have more than eight valence electrons. • The number of valence electrons affects its electrical properties. Conductors • Materials that have large numbers of free electrons are called conductors. • Metals are generally good conductors because they have few loosely bound valence electrons. • Silver, gold, copper, and aluminum are excellent conductors. Insulators • Materials that do not conduct because their valence shells are full or almost full are called insulators. • Glass, porcelain, plastic, and rubber are good insulators. • If high enough voltage is applied, an insulator will break down and conduct. Semiconductors • Semiconductors have half-filled valence shells and are neither good conductors nor good insulators. • Silicon and germanium are good semiconductors. • They are used to make transistors, diodes, and integrated circuits. Electrical Charge • Objects become charged when they have an excess or deficiency of electrons. • An example is static electricity. • The unit of charge is the coulomb. • 1 coulomb = 6.24 × 1024 electrons. Voltage • When two objects have a difference in charges, we say they have a potential difference or voltage between them. • The unit of voltage is the volt. • Thunderclouds have hundreds of millions of volts between them. Voltage • A difference in potential energy is defined as voltage. • The voltage between two points is one volt if it requires one joule of energy to move one coulomb of charge from one point to another. • V = Work/Charge • Voltage is defined between points. Current • The movement of charge is called electric current. • The more electrons per second that pass through a circuit, the greater the current. • Current is the rate of flow of charge. Current • The unit of current is the ampere (A). • One ampere is the current in a circuit when one coulomb of charge passes a given point in one second. • Current = Charge/time • I = Q/t Current • If we assume current flows from the positive terminal of a battery, we say it has conventional current flow. • In metals, current actually flows in the negative direction. • Conventional current flow is used in this course. • Alternating current changes direction cyclically. Voltage Sources • • • • Electronic Power Supplies Batteries: AA, AAA, C, D Solar Cells DC Generators How to Measure Voltage • Measure voltage by placing voltmeter leads across the component. • The red lead is the positive lead; the black lead is the negative lead. • If leads are reversed, you will read the opposite polarity. How to Measure Current • The current you wish to measure must pass through the meter. • You must open the circuit and insert the meter. • Connect with correct polarity. Switches • • • • Single-pole, single-throw Single-pole, double-throw Double-pole, single-throw Push-button - Normally open or Normally closed.