1	Chapter 2 Voltage and Current
2	Atomic Theory Atom Contains a nucleus of protons and neutrons Nucleus is surrounded by a group of orbiting electrons Electrons are negative, protons are positive
3	Atomic Theory Electrically neutral atom Equal number of electrons and protons Ion An atom with an excess or deficit of electrons
4	Atomic Theory Bohr model Electrons orbit the nucleus in discrete orbits called shells Designated by letters K, L, M, N, etc. Only certain numbers of electrons can exist within any given shell
5	Atomic Theory Quantum mechanical model Electrons occupy positions within the atom that are determined statistically
6	Atomic Theory Valence shell Outermost shell of an atom Electrons in this shell are called valence electrons
7	Atomic Theory No element can have more than eight valence electrons Number of valence electrons affects its electrical properties
8	Conductors Materials with a large numbers of free electrons Metals are good conductors because they have few loosely bound valence electrons
9	Conductors Excellent conductors Silver Gold Copper Aluminum
10	Insulators Materials that do not conduct because their valence shells are full or almost full Glass, porcelain, plastic, and rubber are good insulators High voltage will cause an insulator to break down and conduct
11	Semiconductors Half-filled valence shells (4 electrons) Neither good conductors nor good insulators Silicon and germanium Primary materials in semiconductor devices Used to make transistors, diodes, and integrated circuits
12	Electrical Charge Objects become charged when they have an excess or deficiency of electrons An example is static electricity
13	Electrical Charge Unit of charge is the coulomb (C) One coulomb 6.24 × 10¹⁸ electrons (or protons) The charge on one electron (or proton) 1/ 6.24 × 10¹⁸ or 1.6 × 10^-19 C
14	Voltage When two objects have a difference in charges They have a potential difference or voltage between them Unit of voltage is the volt Thunderclouds Millions of volts between them
15	Voltage Difference in potential energy Voltage between two points One volt if it requires one joule of energy to move one coulomb of charge from one point to another
16	Voltage V = Work/Charge Voltage is always measured between two points
17	Current Movement of charge is electric current More electrons per second passing through a circuit, the greater the current Current is rate of flow of charge
18	Current Unit of current is ampere (A) One ampere Current in a circuit when one coulomb of charge passes a given point in one second Current = Charge/time I = Q/t
19	Current Electron current flow Electrons flow from the negative terminal of a battery to the positive terminal Conventional current flow We may also assume currents flow from positive to negative
20	Current Conventional current flow is used in this course Alternating current changes direction cyclically Alternating voltage changes sign cyclically
21	Batteries Alkaline Carbon-Zinc Lithium Nickel-Cadmium Lead-Acid
22	Battery Capacity Specified in amp-hours Life Capacity/current drain Affected by Discharge rates, operating schedules, temperatures, and other factors
23	Other Voltage Sources Electronic Power Supplies Solar Cells DC Generators
24	How to Measure Voltage Place voltmeter leads across components Red lead is positive Black lead is negative If leads are reversed, you will read the opposite polarity
25	How to Measure Current Measurable current must pass through meter Open circuit and insert meter Connect with correct polarity
26	Switches Single-pole, single-throw Single-pole, double-throw Double-pole, single-throw Push-button - normally open or normally closed
27	Fuses and Circuit Breakers Protect equipment or wiring against excessive current Fuses use a metallic element that melts Slow-blow and fast-blow fuses
28	Fuses and Circuit Breakers If current exceeds rated value of a circuit breaker Magnetic field produced by the excessive current operates trips open a switch