Chapter 02. Voltage and Current. Atomic Theory Review. Atomic Theory Review. Atomic Theory Review. Electrical Charge.

Transcription

1 Chapter 02 Voltage and Current Atom Atomic Theory Review 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 C-C Source: Tsai Circuit Analysis: Theory and Practice Delmar Cengage Learning 2 Atomic Theory Review 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 Atomic Theory Review 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 2n 2 Copper atom 3 4 Electrical Charge Objects become charged when they have an excess or deficiency of electrons Occurs when electrons are stripped from parent atoms, free electrons For example: Spinning clothes in dryer Rubbing ebonite rod with fur Unit of charge is the coulomb (C) 1 C = electrons (or protons) The charge on one electron (or proton) =1 / C or C 5 Example: Charge An initially neutral body has 1.7μC of negative charge removed. Later, electrons are added. What is the body s final charge? 6 1

2 Coulomb s Law French scientist Charles Coulomb: Studied force between charges Coulomb s law F = k Q 1 Q 2 /r 2 [newtons, N] where k = 9x10 9 (based on MKS) Q 1 and Q 2 are charges in coulombs r is center to center spacing in meters 7 Free Electrons Amount of energy required to escape: Depends on number of electrons in valence shell Few valence electrons Weak attraction Small amount of energy needed Example: copper material Heat can provide enough energy to escape Valence electrons wander from valence shell of one atom to valence shell of another Material remains electrically neutral Presence of large number of free electrons Indicates material is a good conductor 8 Ions Electrically neutral atom Equal number of electrons and protons Ion Atom with an excess or deficit of electrons Positive ion has lost an electron Negative ion has gained an electron Conductors Conductors: Materials through which charges move easily Metals are good conductors because they have few loosely bound valence electrons Excellent conductors Silver Copper Gold Aluminum 9 10 Insulators Materials that do not conduct because their valence shells are full or almost full Glass, porcelain (or china), plastic, and rubber are good insulators High voltage will cause an insulator to break down and conduct Semiconductors Half-filled valence shells (4 electrons) Neither good conductors nor good insulators Silicon (Si) and Germanium (Ge) Primary materials in semiconductor devices Used to make transistors, diodes, and integrated circuits (ICs)

3 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 Difference in potential energy Voltage between two points Voltage One volt if it requires one joule of energy to move one coulomb of charge from one point to another 1 joule V = Work/Charge = W /Q 1 volt 1 coulomb Voltage is always measured between two points Potential energy (Work), W = QV Example: Voltage If it takes 35 J of energy to move a charge of 5 C from one point to another, what is the voltage between two points? DC Voltage Sources Batteries Alkaline Carbon-Zinc Lithium Nickel- Cadmium Lead-Acid Symbols of Voltage Sources Battery Capacity Specified in amp-hours Life: Capacity/current drain Affected by Discharge rates, operating schedules, temperatures, and other factors

4 Series / Parallel Batteries Other Voltage Sources Electronic Power Supplies Solar Cells DC Generators 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 Unit of Current Unit of current is ampere (A) One ampere (1 A) Current in a circuit when one coulomb of charge passes a given point in one second Current = Charge / time I = Q /t Example: Current If 840 coulombs of charge pass through a conducted wire during a time interval of 2 minutes, what is the current? Current direction Direct Current (DC) Electron current flow From the negative terminal of a battery to the positive terminal Conventional current flow (used in this course) Flow from positive to negative

5 Alternating Current (AC) Alternating current (AC) changes direction cyclically. Most common ac source Commercial ac power system supplies power to your home Electronic Power Supplies Electronic systems require dc for operation Portable units use batteries Commercial ac power lines used with built-in power supply AC adapter converts ac to dc for powering circuits and is used in laptop computers, modems, etc. Measure Voltage and current Digital multimeter (DMM) Analog multimeter (VOM) Terminal Designations Multimeter terminals Marked VΩ, A, and COM Terminal VΩ Used to measure voltage and resistance Terminal A Used to measure current COM: common terminal for measurements Voltage-Meter Symbol Measure the voltage: Place voltmeter leads across components Color coded test leads are industry standard Red lead is connected to the VΩ socket Black lead is connected to COM If leads are reversed, you will read the opposite polarity Amper-Meter Symbol Measure the current: Measurable current must pass through meter Open circuit and insert meter Positive current if it enters the A terminal Negative current if it enters the COM terminal 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

6 How to Measure Current Measurable current must pass through meter Open circuit and insert meter Connect with correct polarity Switches Single-pole, single-throw (SPST) Single-pole, double-throw (SPDT) Double-pole, single-throw (DPST) Switches Push-button - normally open (NO) or normally closed (NC) Fuses and Circuit Breakers 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 Exercise 1 1. What unit is for the charge? 2. What the charge is on one electron? 3. Please show the priority of excellent conductors, gold, copper, aluminum, and silver. 4. What two semiconductors are used for electronic devices? 5. Please show the definition of voltage. 6. Please show the definition of current Kernel abilities 1. What is the charge of an electron? Please give an example. 2. What are the differences between conductor, semiconductor, and insulator. Please give their representative materials? 3. What are differences between DC and AC voltages and currents? Please give their examples. 4. How to measure DC and AC voltages and currents? Please give their examples. 5. What the maximum limited current is safe for human?

7 Problem 42: Show their reading Problem 45: What is wrong here? Exercise 2 using Multisim 1. Please show the schematic with Multisim to use DMM to measure AC 110V, DC 12V, AC current, and DC current. 2. Please show the schematic with Multisim to use Oscilloscope to measure AC 110V, DC 12V, AC current, and DC current. 3. Please show the two-way switch control of a light, i.e., two-level down-up-stair floors 4. Please extend the two-way switch control of a light to three-level down-up-stair floors. 39 7