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Chapter 1: Introduction

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Title: Chapter 1: Introduction


1
Lecture 1
  • Introduction

2
The SI System of Units
  • Length - meter (m)
  • Mass - kilogram (kg)
  • Time - second (s)
  • Electric Current - ampere (A)
  • Temperature - kelvin (K)

3
Power of Ten Notation
  • Used to handle very large and very small numbers.
  • 35 000 3.5 104 or 35 103
  • 458 000 4.58 105 or 458 103
  • 0.000 042 4.2 10-5 or 42 10-6

4
Powers of Ten
  • To multiply numbers in power of 10 notation,
    multiply their base numbers, then add their
    exponents.
  • To divide numbers in power of 10 notation, divide
    their base numbers, then subtract their exponents
    (top - bottom).

5
Power of Ten Notation
  • To add or subtract, first adjust all numbers to
    the same power of ten.
  • It does not matter what exponent you choose, as
    long as they are all the same.

6
Power of Ten Notation
  • Raising a number to a power is a form of
    multiplication.
  • (4 103)2 (4 103)(4 103)
  • 16 106
  • Fractional powers represent roots.

7
Prefixes
  • Scientific Notation
  • 24 700 2.4 104
  • 0.000 046 4.6 10-5
  • Engineering Notation - uses only powers which are
    factors of 3
  • 24 700 24.7 103
  • 0.000 046 46 10-6

8
Prefixes
  • Metric Prefixes are used for convenience.

9
Significant Digits and Numerical Accuracy
  • The number of digits in a number that carry
    actual information are called significant digits.
  • It is a common error to show more digits of
    accuracy than are warranted.
  • The number of significant digits in a result due
    to multiplication or division is the same as the
    number of significant digits in the number with
    the least number of significant digits.

10
Circuit Diagrams
  • Electric circuits are constructed using
    components.
  • To represent these circuits on paper, diagrams
    are used.
  • Three types are used pictorial, block, and
    schematic.

11
Schematic circuit symbols
12
Pictorial Diagrams
  • Help visualize circuits by showing components as
    they actually appear.

13
Block Diagrams
  • Circuit is broken into blocks, each representing
    a portion of the circuit.

14
Schematic Diagrams
15
Circuit Analysis Using Computers
  • Prepackaged Simulation Software SPICE.
  • Math Software Mathcad, MATLAB
  • Programming Languages BASIC, C, FORTRAN

16
  • Voltage and Current

17
Atomic Theory
  • An atom consists of a nucleus of protons and
    neutrons surrounded by a group of orbiting
    electrons.
  • Electrons are negative, protons are positive.
  • In its normal state, each atom has an equal
    number of electrons and protons.

18
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19
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.

20
Atomic Theory
  • The outermost shell of an atom is called the
    valence shell.
  • The electrons in this shell are called valence
    electrons.
  • No element can have more than eight valence
    electrons.
  • The number of valence electrons affects its
    electrical properties.

21
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.

22
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.

23
Semiconductors
  • Semiconductors have 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.

24
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.

25
Voltage
  • When two objects have a difference in charges, we
    say they have a potential difference or voltage
    between them.
  • The unit of voltage is the volt.
  • Thunderclouds have hundreds of millions of volts
    between them.

26
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.

27
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 flow of charge.

28
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

29
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.

30
Batteries
  • Alkaline
  • Carbon-Zinc
  • Lithium
  • Nickel-Cadmium
  • Lead-Acid
  • Primary batteries cannot be recharged, secondary
    can

31
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.

32
Other Voltage Sources
  • Electronic Power Supplies
  • Solar Cells
  • Thermocouples
  • DC Generators
  • AC generators

33
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.

34
Voltage and current measurement
35
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.

36
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.

37
  • Resistance

38
Resistance of Conductors
  • Resistance of material is dependent on several
    factors
  • Type of Material
  • Length of the Conductor
  • Cross-sectional area
  • Temperature

39
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.

40
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.

41
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.

42
Resistance Formula
  • At a given temperature,
  • This formula can be used with both circular and
    rectangular conductors.

43
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.

44
Resistivity at 20ºC (?m)
  • Silver 1.645x10-8
  • Copper 1.723x10-8
  • Aluminum 2.825x10-8
  • Carbon 3500x10-8
  • Wood 108-1014
  • Teflon 1016

45
Temperature Effects
  • The rate of change of resistance with temperature
    is called the temperature coefficient (?).
  • 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.

46
Temperature effect on resistance
47
Temperature coefficients ? (ºC)-1 at 20ºC
  • Silver 0.0038
  • Copper 0.00393
  • Aluminum 0.00391
  • Tungsten 0.00450
  • Carbon 0.0005
  • Teflon 1016

48
Fixed Resistors
  • Resistances essentially constant.
  • Rated by amount of resistance, measured in ohms.
  • Also rated by power ratings, measured in watts.

49
Fixed Resistors
  • Different types of resistors are used for
    different applications.
  • Molded carbon composition
  • Carbon film
  • Metal film
  • Metal Oxide
  • Wire-Wound
  • Integrated circuit packages

50
Variable Resistors
  • Used to adjust volume, set level of lighting,
    adjust temperature.
  • Have three terminals.
  • Center terminal connected to wiper arm.
  • Potentiometers
  • Rheostats

51
Color Code
  • Colored bands on a resistor provide a code for
    determining the value of resistance, tolerance,
    and sometimes the reliability.

52
Measuring Resistance
  • Remove all power sources to the circuit.
  • Component must be isolated from rest of the
    circuit.
  • Connect probes across the component.
  • No need to worry about polarity.
  • Useful to determine shorts and opens.

53
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54
Thermistors
  • A two-terminal transducer in which the resistance
    changes with change in temperature.
  • Applications include electronic thermometers and
    thermostatic control circuits for furnaces.
  • Have negative temperature coefficients.

55
Photoconductive Cells
  • Two-terminal transducers which have a resistance
    determined by the amount of light falling on
    them.
  • May be used to measure light intensity or to
    control lighting.
  • Used as part of security systems.

56
Diodes
  • Semiconductor device that conducts in one
    direction only.
  • In forward direction, has very little resistance.
  • In reverse direction, resistance is very high -
    essentially an open circuit.

57
Varistors
  • Resistor which is sensitive to voltage.
  • Have a very high resistance when the voltage is
    below the breakdown value.
  • Have a very low resistance when the voltage is
    above the breakdown value.
  • Used in surge protectors.

58
Conductance and conductivity
  • The measure of a materials ability to allow the
    flow of charge.
  • Conductance is the reciprocal of resistance.
  • G 1/R
  • Unit is siemens S.
  • Conductivity ?1/?
  • Unit is siemens/meter S/m.

59
Superconductors
  • At very low temperatures, resistance of some
    materials goes to almost zero.
  • This temperature is called the critical
    temperature.
  • Meissner Effect - When a superconductor is cooled
    below its critical temperature, magnetic fields
    may surround but not enter the superconductor.

60
  • Ohms Law, Power,
  • and Energy

61
Ohms Law
  • The current in a resistive circuit is directly
    proportional to its applied voltage and inversely
    proportional to its resistance.
  • I E/R I V/R
  • For a fixed resistance, doubling the voltage
    doubles the current.
  • For a fixed voltage, doubling the resistance
    halves the current.

62
Ohms Law
  • Ohms Law may also be expressed as
    E IR and R E/I
  • Express all quantities in base units of volts,
    ohms, and amps or utilize the relationship
    between prefixes.

63
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65
Ohms Law in Graphical Form
  • The relationship between current and voltage is
    linear.

66
Open Circuits
  • Current can only exist where there is a
    conductive path.
  • When there is no conductive path we refer to this
    as an open circuit.
  • If I 0, then Ohms Law gives R E/I E/0 ?
    infinity
  • An open circuit has infinite resistance.

67
Short circuit
  • If resistance R 0 exists between two points we
    refer to this as a short-circuit
  • If R 0, then Ohms law gives I E/0 ?
    infinity
  • Never short-circuit a voltage source, infinitely
    large current will destroy the circuit, injuries
    can result
  • We often assume that the internal resistance of
    an ammeter is zero never connect it across a
    voltage source.

68
Voltage Symbols
  • For voltage sources electromotive force emf, use
    uppercase E.
  • For load voltages, use uppercase V.
  • Since V IR, these voltages are sometimes
    referred to as IR or voltage drops.

69
Voltage Polarities
  • The polarity of voltages across resistors is of
    extreme importance in circuit analysis.
  • Place the plus sign at the tail of the current
    arrow.

70
Current Direction
  • We normally show current out of the plus terminal
    of a source.
  • If the actual current is in the direction of its
    reference arrow, it will have a positive value.
  • If the actual current is opposite to its
    reference arrow, it will have a negative value.

71
Current Direction
  • The following are two representations of the same
    current

72
Power
  • The greater the power rating of a light, the more
    light energy it can produce each second.
  • The greater the power rating of a heater, the
    more heat energy it can produce.
  • The greater the power rating of a motor, the more
    mechanical work it can do per second.
  • Power is related to energy, which is the capacity
    to do work.

73
Power
  • Power is the rate of doing work.
  • Power Work/time
  • Power is measured in watts.
  • One watt one joule per second

74
Power in Electrical Systems
  • From V W/Q and I Q/t, we get
  • P VI
  • From Ohms Law, we can also find that
  • P I2R and P V2/R
  • Power is always in watts, no matter which
    equation is used.

75
Power in Electrical Systems
  • We should be able to use any of the power
    equations to solve for V, I, or R if P is given.
  • For example

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77
Power Rating of Resistors
  • Resistors must be able to safely dissipate their
    heat without damage.
  • Common power ratings of resistors are 1/8, 1/4,
    1/2, 1, or 2 Watts.
  • A safety margin of two times the expected power
    is customary.
  • An overheated resistor is often the symptom of a
    problem rather than its cause.

78
Energy
  • Energy Power time
  • Units are joules watt-seconds, watt-hours, or
    more commonly, kilowatt-hours.
  • Energy use is measured in kilowatt-hours by the
    power company.
  • In SI dominated areas megajoule is used instead
    of kWh
  • 1 kWh 3,600 kWs 3,600,000 Ws 3.6 MJ
  • For multiple loads, the total energy is the sum
    of the energy of the individual loads.

79
Energy
  • Cost Energy cost per unit or
  • Cost Power time cost per unit
  • To find the cost of running a 2000-watt heater
    for 12 hours if electric energy costs 0.08 per
    kilowatt-hour
  • Cost 2kW 12 Hr A0.1425 A3.42

80
Law of Conservation of Energy
  • Energy can neither be created nor destroyed, but
    can be converted from one form to another.
  • Examples Electric energy into heat Mechanical
    energy into electric energy
  • In energy conversions, some energy may be given
    as heat (called loss in electrical engineering),
    giving lower efficiency.

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82
Efficiency
  • Poor efficiency in energy transfers results in
    wasted energy.
  • An inefficient piece of equipment generates more
    heat this heat must be removed.
  • Heat removal requires the use of fans and heat
    sinks.

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84
Electric motor
  • Converts electrical energy from the input into
    mechanical energy at the output
  • The rating on the name plate always gives the
    mechanical output power, ie 2 kW
  • Note that sometime horse power is also used as a
    unit of power 1 hp 746 watts
  • If the efficiency of that motor above is 80 the
    electrical input power will be 2 kW/0.80 2.5 kW

85
Efficiency
  • Efficiency will always be less than 100.
  • Efficiencies vary greatly power transformers may
    have efficiencies of 98, some signal amplifiers
    have efficiencies below 50
  • To find the total efficiency of a system
  • ?Total ?1 ?2 ?3
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