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Chapter 2 - Voltage, Current & Resistance MECH 1100 Chapter 2 – Voltage, Current and Resistance Topics Atoms Electrical Charge Voltage Current Resistance Basic Circuits 1 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 What are ATOMS? 2 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The two simplest atoms: hydrogen and helium. 4 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Energy levels increase as the distance from the nucleus increases. 5 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The valence shell The outer shell is called the valence shell. Electrons in this shell are involved in chemical reactions and they account for electrical and thermal conductivity in metals. 6 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The valence shell Metals have one, two or three electrons in the valence shell. The atom illustrated here is a sodium atom (Na), with only one electron in its outer shell. Sodium is highly reactive, and easily gives up its single valence electron. For this reason, it is not used in electrical work. Non-metals have either complete or nearly compete outer shells, so they make poor electrical conductors (i.e. they are insulators). + S he ll 1 S he ll 2 S he ll 3 Sodium atom 7 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 8 of 94 Chapter 2 - Voltage, Current & Resistance A neutral Si atom is shown. There are 4 electrons in the valence shell. + Shell 1 MECH 1100 Shell 2 Shell 3 Is Si a conductor, insulator, or semiconductor? 10 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The valence shell 11 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 A neutral Si atom is shown. There are 4 electrons in the valence shell. + Shell 1 Shell 2 Shell 3 Is Si a conductor, insulator, or semiconductor? Semiconductor 12 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The copper atom 13 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 14 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Electrical charge There is a force (F) between electrical charges. Like charges repel; unlike charges attract. • The force is directly proportional to charge. • The force is inversely proportional to square of distance. + + _ + 15 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Electrical charge Coulomb – The unit of charge One Coulomb is the total charge possessed by 6.25 × 1018 electrons. Total Charge šš¢šššš šš ššššš”šššš š= 6.25 × 1018 ššššš”šššš /š¶ 16 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Electrical charge How many coulombs of charge do 88.7 × 1015 electrons represent? šš¢šššš šš ššššš”šššš š= 6.25 × 1018 ššššš”šššš /š¶ 88.7 × 1015 ššššš”šššš š= 6.25 × 1018 ššššš”šššš /š¶ š = 14.91 × 10−3 š¶ = 0.01491š¶ 17 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Voltage Force is required to move a charge against the electric field. + + + + + + + + + e- - When force is applied over a distance, work is done. Work done in moving a charge against the electric field leads to the definition of voltage: Voltage is the work per charge done against the electric field. 18 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The equation for voltage is W Vļ½ Q V - Volts W – Energy in joules Q – Charge in coulombs One volt is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other. 19 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 If 135 J of energy are required to move 15C of Charge, what is the voltage? W Vļ½ Q 135š½ š= = 9š 15š¶ 20 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Symbol for dc voltage sources (battery). Voltage is responsible for establishing current. 21 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Batteries in Parallel Batteries in Series 23 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Batteries An automobile battery is an example of a multiple cell battery. Like all batteries, the automotive battery does not store charge – it stores chemical energy that can be converted to current when an external path is provided to allow the chemical reaction to proceed. Rather than saying “charging” a battery, it is more accurate to say “reversing the chemical reaction” in a battery. 24 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Fuel cells A fuel cell is a device that converts chemical energy into dc voltage directly by combining a fuel (usually hydrogen) with an oxidizing agent (usually oxygen). The hydrogen and oxygen react to form water. The process differs from batteries in that the reactants constantly flow into the cell where they combine and produce electricity. 25 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 A basic power supply. 27 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Electrons flow from negative to positive when a voltage is applied across a conductive or semiconductive material. 29 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Current Current (I in amperes or amps) is the amount of charge (Q in coulombs) that flows past a point in a unit of time (t). The defining equation is: One coulomb equals the Q I ļ½ t W Qļ½ V value of electrical charge in 6.24150965×1018 protons or electrons One ampere is a number of electrons having a total charge of 1 Coulomb moving through a given cross section in 1 s. What is the current if 2 C passes a point in 5 s? 0.4 A 30 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Current sources are not as common as voltage sources Symbol 31 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Conductance Conductance is: the reciprocal of resistance the ease of with which voltage flows. 1 Gļ½ R G – Conductance (S siemens) R – Resistance (ohms Ω) 34 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Resistance Components designed to have a specific amount of resistance are called resistors. Color bands Resistance material (carbon composition) Insulation coating Leads 35 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Resistance/resistor symbol. 36 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Fixed resistors (Value cannot be changed) 37 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Typical fixed resistors 38 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Resistance color-code Color Digit Multiplier Tolerance Black 0 10 0 Brown 1 10 1 1% (five band) Resistance value, first three bands: Red 2 10 2 2% (five band) First band – 1st digit Orange 3 10 3 Yellow 4 10 4 Second band – 2nd digit Green 5 10 5 Blue 6 10 6 Violet 7 10 7 Gray 8 10 8 White 9 10 9 Gold ±5% 10 -1 5% (four band) Silver ± 10% 10 -2 10% (four band) No band ± 20% *Third band – Multiplier (number of zeros following second digit) Fourth band - tolerance * For resistance values less than 10 W, the third band is either gold or silver. Gold is for a multiplier of 0.1 and silver is for a multiplier of 0.01. 41 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Color-code bands on a 4-band resistor. 42 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 What is the resistance and tolerance of each of the four-band resistors? Color Digit Multiplier Tolerance Green, brown, red, gold Black 0 10 0 Brown 1 10 1 1% (five band) Red 2 10 2 2% (five band) Orange 3 10 3 Yellow 4 10 4 Green 5 10 5 Blue 6 10 6 Violet 7 10 7 Gray 8 10 8 White 9 10 9 Gold ±5% 10 -1 5% (four band) Silver ± 10% 10 -2 10% (four band) No band ± 20% Gray, red, yellow, silver 5.1 kW ± 5% 820 kW ±10% Yellow, violet, black, silver 47 W ± 10% Brown, black, gold,gold 1.0 W ± 5% 43 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Three-digit labeling for a resistor. 44 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Alphanumeric Labeling • Two or three digits, and one of the letters R, K, or M are used to identify a resistance value. • The letter is used to indicate the multiplier, and its position is used to indicate decimal point position. • R – multiplier of 1 and no zeros after the digits • K - multiplier of 1000 and 3 zeros after the digits • M - multiplier of 1,000,000 and 6 zeros after the digits 45 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Variable resistors Variable resistors include the potentiometer and rheostat. The center terminal of a variable resistor is connected to the wiper. R Shaft 1 3 2 Wiper Resistive element Variable resistor (potentiometer – divides voltage) R To make a potentiometer into a rheostat, one of the outside terminals is connected to the wiper. Variable resistor (rheostat – controls current) 46 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Symbols for resistance devices with sensitivities to temperature, light, and force. 49 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 A simple electric circuit 50 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 51 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Switches Switches are commonly used to control circuits by either mechanical or electronic means. Different from the book The throw refers to how many circuits are affected by the movable arm of a switch. The pole refers to the number of contacts that are available to form a circuit. 52 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Normally open Pushbutton Normally closed Pushbutton 53 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 54 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 55 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 56 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Closed and open circuits using a SPST switch for control. 57 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 SPDT switch controlling two lamps. 58 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switches • Various sizes • Various shapes • Various uses 59 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switches • Micro switches • Small size • Various ranges of uses 60 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switches • Purpose – To detect the presence or absence of an object – Opens or closes a contact 61 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switches 4 1 2 62 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switches 63 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 GAUGING (MODULE #2) 64 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 65 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switch in process sequence 66 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switches • What can go wrong • • Broken arm • • Internal broken • contact • • Welded contacts • • Arc/pitted contacts • Bad/loose connection • Incorrect adjustment Possible symptoms? No change in signal No signal Continual signal Intermittent signal 67 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Mechanical limit switch • How to check and adjust – Check continuity – Check voltages – Look for changes when switching – Check and readjust as necessary 68 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Fuses, circuit breakers and their symbols. 69 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 WIRE Gauge – Size (AWG or mm) 70 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 WIRE Gauge – Size (AWG or mm) In the AWG scale, the smaller the number the larger the wire diameter and the greater the current carrying capacity. 71 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 72 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Wire resistance Sometimes, the resistance of wires must be accounted for. The equation for wire resistance is: Rļ½ ļ²l A where ļ² (rho) = resistivity in CM-W/ft l = length in feet A = cross sectional area in circular mils (CM) What is the resistance of 400 feet of 22 gage copper wire where the area is 642 CM and the resistivity of copper is 10.37 CM-W/ft and the resistance/1000 feet is 16.14 W/1000 feet. By proportion, the resistance of 400 feet is 0.4 x 16.14 W = 6.46 W By the equation, ļ² l ļØ10.37 W-CM/ft ļ©ļØ 400 ft ļ© Rļ½ ļ½ ļ½ 6.46 W A 642 CM 73 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Ground. (Reference Point for measuring Potential) In U.S. this is typically a bare wire or green insulation or green with yellow stripe insulation. 1 – Overvoltage protection – lightning 2 – Voltage Stabilization- Common reference 3 – Safety – What happens if neutral breaks? 74 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Simple circuit with ground connections. 75 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The electric circuit Switch A basic electric circuit consists of 1) a voltage source 2) a path Battery (2 3) a load. Lamp cells) An example of a basic circuit is a flashlight, which has each of these plus a control element – the switch. Switch Metal reflector Metal strip Spring 76 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 The DMM – Digital Multimeter Volts - AC Volts - DC Ohms Amperes 77 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Analog meters An analog multimeter is also called a VOM (voltohmmilliammeter). No resistance will show as infinite ohms 78 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Measuring current Series Circuit 79 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Measuring voltage or voltage drop Parallel Circuit 80 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Measuring resistance 81 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 82 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Schematic Symbols Review 83 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Key Terms Ampere The unit of electrical current. AWG (American Wire Gauge) A standardization based on wire diameter. Charge An electrical property of matter that exists because of an excess or a deficiency of electrons. Charge can be either + or -. Circuit An interconnection of electronic components designed to produce a desired result. A basic circuit consists of a source, a load, and an interconnecting path. 84 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Key Terms Conductance The ability of a circuit to allow current. The unit is the siemans (S). Coulomb The unit of electrical charge. Current The rate of flow of electrical charge. Electron A basic particle of electrical charge in matter. The electron possesses a negative charge. Ground The common or reference point in a circuit. Ohm (W) The unit of resistance. 85 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Key Terms Potentiometer A three-terminal variable resistor. Resistance The opposition to current. The unit is the ohm (W). Rheostat A two-terminal variable resistor. Siemens The unit of conductance. Volt The unit of voltage or electromotive force. Voltage The amount of energy per charge available to move electrons from one point to another in an electric circuit. 86 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 1. The atomic number is the number of a. protons in the nucleus b. neutrons in the nucleus c. protons plus neutrons in the nucleus d. electrons in the outer shell 87 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 2. Valence electrons are a. in the outer shell b. involved in chemical reactions c. relatively loosely bound d. all of the above 88 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 3. The atomic particle responsible for electrical current in solid metallic conductors is the a. proton b. electron c. neutron d. all of the above 89 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 4. The symbol for charge is a. C b. W c. Q d. W 90 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 5. The definition for voltage is Q a. V ļ½ t b. V ļ½ W t c. V ļ½ W Q d. V ļ½ It 91 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 6. A battery stores a. electrons b. protons c. ions d. chemical energy 92 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 7. The unit of conductance is the a. ohm b. coulomb c. siemen d. ampere 93 of 94 Chapter 2 - Voltage, Current & Resistance MECH 1100 Quiz 10. The circular mil is a unit of a. length b. area c. volume d. resistance 96 of 94