| Voltage and Current |
| Atom | ||
| Contains a nucleus of protons and neutrons | ||
| Nucleus is surrounded by a group of orbiting electrons | ||
| Electrons are negative, protons are positive | ||
| Electrically neutral atom | ||
| Equal number of electrons and protons | ||
| Ion | ||
| An atom with an excess or deficit of electrons | ||
| 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 | ||
| Quantum mechanical model | ||
| Electrons occupy positions within the atom that are determined statistically | ||
| Valence shell | ||
| Outermost shell of an atom | ||
| Electrons in this shell are called valence electrons | ||
| No element can have more than eight valence electrons | |
| Number of valence electrons affects its electrical properties |
| Materials with a large numbers of free electrons | ||
| Metals are good conductors because they have few loosely bound valence electrons | ||
| Excellent conductors | ||
| Silver | ||
| Gold | ||
| Copper | ||
| Aluminum | ||
| 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 | ||
| 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 | ||
| Objects become charged when they have an excess or deficiency of electrons | |
| An example is static electricity |
| 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 | ||
| 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 | ||
| 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 | ||
| V = Work/Charge | |
| Voltage is always measured between two points |
| 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 |
| 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 | ||
| 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 | ||
| Conventional current flow is used in this course | |
| Alternating current changes direction cyclically | |
| Alternating voltage changes sign cyclically |
| Alkaline | |
| Carbon-Zinc | |
| Lithium | |
| Nickel-Cadmium | |
| Lead-Acid |
| Specified in amp-hours | ||
| Life | ||
| Capacity/current drain | ||
| Affected by | ||
| Discharge rates, operating schedules, temperatures, and other factors | ||
| Electronic Power Supplies | |
| Solar Cells | |
| DC Generators |
| Place voltmeter leads across components | |
| Red lead is positive | |
| Black lead is negative | |
| If leads are reversed, you will read the opposite polarity |
| Measurable current must pass through meter | |
| Open circuit and insert meter | |
| Connect with correct polarity |
| Single-pole, single-throw | |
| Single-pole, double-throw | |
| Double-pole, single-throw | |
| Push-button - normally open or normally closed | |
| Protect equipment or wiring against excessive current | |
| Fuses use a metallic element that melts | |
| Slow-blow and fast-blow fuses |
| If current exceeds rated value of a circuit breaker | ||
| Magnetic field produced by the excessive current operates trips open a switch | ||