Title: Chapter 1: Introduction
1Lecture 1
2The SI System of Units
- Length - meter (m)
- Mass - kilogram (kg)
- Time - second (s)
- Electric Current - ampere (A)
- Temperature - kelvin (K)
3Power 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
4Powers 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).
5Power 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.
6Power 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.
-
7Prefixes
- 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
8Prefixes
- Metric Prefixes are used for convenience.
9Significant 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.
10Circuit Diagrams
- Electric circuits are constructed using
components. - To represent these circuits on paper, diagrams
are used. - Three types are used pictorial, block, and
schematic.
11Schematic circuit symbols
12Pictorial Diagrams
- Help visualize circuits by showing components as
they actually appear.
13Block Diagrams
- Circuit is broken into blocks, each representing
a portion of the circuit.
14Schematic Diagrams
15Circuit Analysis Using Computers
- Prepackaged Simulation Software SPICE.
- Math Software Mathcad, MATLAB
- Programming Languages BASIC, C, FORTRAN
16 17Atomic 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.
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19Atomic 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.
20Atomic 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.
21Conductors
- 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.
22Insulators
- 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.
23Semiconductors
- 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.
24Electrical 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.
25Voltage
- 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.
26Voltage
- 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.
27Current
- 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.
28Current
- 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
29Current
- 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.
30Batteries
- Alkaline
- Carbon-Zinc
- Lithium
- Nickel-Cadmium
- Lead-Acid
- Primary batteries cannot be recharged, secondary
can
31Battery 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.
32Other Voltage Sources
- Electronic Power Supplies
- Solar Cells
- Thermocouples
- DC Generators
- AC generators
33How 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.
34Voltage and current measurement
35How 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.
36Fuses 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 38Resistance of Conductors
- Resistance of material is dependent on several
factors - Type of Material
- Length of the Conductor
- Cross-sectional area
- Temperature
39Type 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.
40Length
- 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.
41Area
- 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.
42Resistance Formula
- At a given temperature,
- This formula can be used with both circular and
rectangular conductors.
43Temperature 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.
44Resistivity at 20ºC (?m)
- Silver 1.645x10-8
- Copper 1.723x10-8
- Aluminum 2.825x10-8
- Carbon 3500x10-8
- Wood 108-1014
- Teflon 1016
45Temperature 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.
46Temperature effect on resistance
47Temperature coefficients ? (ºC)-1 at 20ºC
- Silver 0.0038
- Copper 0.00393
- Aluminum 0.00391
- Tungsten 0.00450
- Carbon 0.0005
- Teflon 1016
48Fixed Resistors
- Resistances essentially constant.
- Rated by amount of resistance, measured in ohms.
- Also rated by power ratings, measured in watts.
49Fixed Resistors
- Different types of resistors are used for
different applications. - Molded carbon composition
- Carbon film
- Metal film
- Metal Oxide
- Wire-Wound
- Integrated circuit packages
50Variable Resistors
- Used to adjust volume, set level of lighting,
adjust temperature. - Have three terminals.
- Center terminal connected to wiper arm.
- Potentiometers
- Rheostats
51Color Code
- Colored bands on a resistor provide a code for
determining the value of resistance, tolerance,
and sometimes the reliability.
52Measuring 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.
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54Thermistors
- 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.
55Photoconductive 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.
56Diodes
- 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.
57Varistors
- 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.
58Conductance 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.
59Superconductors
- 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
61Ohms 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.
62Ohms 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.
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65Ohms Law in Graphical Form
- The relationship between current and voltage is
linear.
66Open 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.
67Short 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.
68Voltage 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.
69Voltage 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.
70Current 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.
71Current Direction
- The following are two representations of the same
current
72Power
- 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.
73Power
- Power is the rate of doing work.
- Power Work/time
- Power is measured in watts.
- One watt one joule per second
74Power 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.
75Power 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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77Power 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.
78Energy
- 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.
79Energy
- 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
80Law 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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82Efficiency
- 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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84Electric 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
85Efficiency
- 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