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BMET 4350

BMET 4350. Lecture 2 Components. Circuit Diagrams. Electric circuits are constructed using components. To represent these circuits on paper, diagrams are used. The 4 Basic Circuit Elements. There are 4 basic circuit elements: Energy sources Voltage sources Current sources Resistors

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BMET 4350

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  1. BMET 4350 Lecture 2 Components

  2. Circuit Diagrams • Electric circuits are constructed using components. • To represent these circuits on paper, diagrams are used.

  3. The 4 Basic Circuit Elements There are 4 basic circuit elements: • Energy sources • Voltage sources • Current sources • Resistors • Inductors • Capacitors

  4. Three types of diagrams are used: • pictorial, • block, and • schematic.

  5. Schematic circuit symbols

  6. Pictorial Diagrams • Help visualize circuits by showing components as they actually appear.

  7. Block Diagrams • Circuit is broken into blocks, each representing a portion of the circuit.

  8. Schematic Diagrams

  9. ENTC 4350 COMPONENTS

  10. Voltage and Current

  11. Atomic Theory • An atom consists of a nucleus of protons and neutrons surrounded by a group of orbiting electrons. • Electrons have a negative charge, protons have a positive charge. • In its normal state, each atom has an equal number of electrons and protons.

  12. 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.

  13. Atomic Theory • The outermost shell of an atom is called the valenceshell. • The electrons in this shell are called valenceelectrons. • No element can have more than eight valence electrons. • The number of valence electrons affects its electrical properties.

  14. 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.

  15. 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.

  16. Semiconductors • Semiconductorshave 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.

  17. 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.

  18. Voltage • When two objects have a difference in charges, we say they have a potential differenceorvoltage between them. • The unit of voltage is the volt. • Thunderclouds have hundreds of millions of volts between them.

  19. 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.

  20. A model of a straight wire of length l and cross-sectional area A. • A potential difference of Vb – Va is maintained across the conductor, setting up an electric field E. • This electric field produces a current that is proportional to the potential difference.

  21. 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 flowof charge.

  22. Electric current within a conductor. • (a) Random movement of electron generates no current. • (b) A net flow of electrons generated by an external force. (a) (b)

  23. 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

  24. 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.

  25. Batteries • Alkaline • Carbon-Zinc • Lithium • Nickel-Cadmium • Lead-Acid • Primary batteries cannot be recharged, secondary can

  26. Battery Capacity • The capacity of a battery is specified in amp-hours. • Life = capacity/current drain • Battery with 200Ah – supplies 20A for 10h • The capacity of a battery is affected by discharge rates, operating schedules, temperatures, and other factors.

  27. Other Voltage Sources • Electronic Power Supplies • Solar Cells • Thermocouples • DC Generators • AC generators

  28. 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.

  29. Voltage measurement

  30. 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.

  31. Current measurement Break the circuit

  32. Fuses and Circuit Breakers • Protect equipment or wiring against excessive current. • Fuses use a metallic element which melts. • Slow-blow and fast-blow fuses. • When the current exceeds the rated value of a circuit breaker, the magnetic field produced by the excessive current operates a mechanism that trips open a switch.

  33. Resistance

  34. Resistors Resistors limit electric current in a circuit. Insert figure 1-1

  35. In many cases a light bulb can be modeled with a resistor. Resistors • A resistor is a two terminal circuit element that has a constant ratio of the voltage across its terminals to the current through its terminals. • The value of the ratio of voltage to current is the defining characteristic of the resistor.

  36. In many cases a light bulb can be modeled with a resistor. Resistors • A resistor is a two terminal circuit element that has a constant ratio of the voltage across its terminals to the current through its terminals. • The value of the ratio of voltage to current is the defining characteristic of the resistor.

  37. Resistors – Definition and Units R • A resistor obeys the expression where R is the resistance. • If something obeys this expression, we can think of it, and model it, as a resistor. • This expression is called Ohm’s Law. The unit ([Ohm] or [W]) is named for Ohm, and is equal to a [Volt/Ampere]. • IMPORTANT: use Ohm’s Law only on resistors. It does not hold for sources. iR - + v To a first-order approximation, the body can modeled as a resistor. Our goal will be to avoid applying large voltages across our bodies, because it results in large currents through our body. This is not good.

  38. Schematic Symbol for Resistors The schematic symbols that we use for resistors are shown here. This is intended to indicate that the schematic symbol can be labeled either with a variable, like RX, or a value, with some number, and units. An example might be 390[W]. It could also be labeled with both.

  39. Resistor Polarities • There is no corresponding polarity to a resistor. You can flip it end-for-end, and it will behave the same way.

  40. Getting the Sign Right with Ohm’s Law If the reference current is in the direction of the reference voltage drop (Passive Sign Convention), then…

  41. Resistance of Conductors • Resistance of material is dependent on several factors • Type of Material • Length of the Conductor • Cross-sectional area • Temperature

  42. Type of Material • Differences at the atomic level of various materials will cause variations in how the collisions affect resistance. • These differences are called the resistivity. • We use the symbol . • Units are ohm-meters.

  43. Length • The resistance of a conductor is directly proportional to the length of the conductor. • If you double the length of the wire, the resistance will double. •  = length, in meters.

  44. Area • The resistance of a conductor is inversely proportional to the cross-sectional area of the conductor. • If the cross-sectional area is doubled, the resistance will be one half as much. • A= cross-sectional area, in m2.

  45. Resistance Formula • At a given temperature, • This formula can be used with both circular and rectangular conductors.

  46. Temperature Effects • For most conductors, an increase in temperature causes an increase in resistance. • This increase is relatively linear. • In semiconductors, an increase in temperature results in a decrease in resistance.

  47. Resistivity at 20ºC (m) • Silver 1.645x10-8 • Copper 1.723x10-8 • Aluminum 2.825x10-8 • Carbon 3500x10-8 • Wood 10+8-10+14 • Teflon 10+16

  48. Temperature Effects • The rate of change of resistance with temperature is called the temperaturecoefficient (). • Any material for which the resistance increases as temperature increases is said to have a positive temperature coefficient. If it decreases, it has a negative coefficient.

  49. Temperature effect on resistance

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