circuits chapter 1
TRANSCRIPT
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1.1 OVERVIEW OF ELECTRICAL
ENGINEERINGElectrical Engineering:
a) Collect, Store, Process, Transport, and
Present Information
b) Distribute, Store and Convert Energy betweenvarious forms.
Manipulation of Information
Manipulation of Energy.
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Example
Weather PredictionData collected by weather satellites, land based
radar stations, and sensors at numerous weatherstations.
Sensors: Devices that convert physicalmeasurements to electrical signals.
Information is then transmitted by electroniccommunication systems and processed by
computers to yield forecasts.Forecasts are electronically displayed.
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Example 2
Electrical Power PlantsEnergy is converted from various forms to
electrical form.Electrical distribution systems transport the energy
to our homes, factories, businesses.
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Applications of Electrical
Engineering
Many ApplicationsAutomakers use electrical technology to provide
increase functionality at lower costs. Theseinclude:
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Subdivisions of Electrical
Engineering
1) COMMUNICATION SYSTEMS:Transport information in electrical form.
Cell Phones, Satellite TV's, Internet
2) COMPUTER SYSTEMS:Process and store information in digital form.PC's, Computers used in auto's, homes, etc.
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3) CONTROL SYSTEMS:Collect information w/ sensors and use electrical
energy to control a physical process.Heating/Cooling Systems: Thermostat (sensors)
measures current temperature and compares it w/desired value. Control circuits operate the furnace orthe air conditioner to achieve the desired temperature.
Tall buildings have control systems installed to reducethe movement by wind.
Feedback is important!
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5) ELECTRONICS
Study and applications of materials, devices andcircuits used in amplifying and switching electrical
signals.
Transistors: used everywhere electricalinformation or energy is employed.
Cardiac Pacemaker: Senses heart beats, if nobeat, then applies a minute of electrical stimulusto the heart, forcing the heart to beat.
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6) PHOTONICS
New Field: promises to replace conventionalcomputing, signal processing, sensing andcommunications devices.
Previous are all based on manipulating electrons.
PHOTONICS is based on manipulating photons.Light generation by lasers and light emittingdiodes, transmission of light through optical
components.
Readers for DVDs, holograms, optical signalprocessors, Fiber optic communication systems.
Future Optical Computers, HolographicMemories, Medical Devices.
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7) POWER SYSTEMS
Convert energy to and from electrical formTransport energy over long distancesComposed of generators, transformers and
distribution lines, motors, etc.
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8) SIGNAL PROCESSING
Used to extract useful information from electricalsignals derived from sensors
Machine vision for robots.
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WHYSTUDY ELECTRICAL ENGINEERING?
1) To pass the Fundamentals of Engineering(FE) exam as a 1st step in becoming a registered
Professional Engineer.
2) To have a broad knowledge base so you can
lead design projects in your own field
3) Ability to operate and maintain electricalsystems, such as those found in control systems
for manufacturing processes.
4) Ability to communicate with ElectricalEngineering Consultants.
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SECTION 1.2
CIRCUITS, CURRENTS,VOLTAGES
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Figure 1.2 The headlight circuit. (a) The actual physical layout of the circuit. (b) The circuit diagram.
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Chemical forces in the battery cause the electricalcharge (electrons) to flow through the circuit.
Energy is gained from the chemicals in the batteryand delivered to the headlights.
Transported through an excellent electricalconductor(copper wire), that are insulated.Switch is used to control the flow of the current.
When the switch is closed (shorted), current flows,when it is open, the current does not flow.
Headlamps contain tungsten wires they canwithstand high temperatures. They do not
conduct electricity (they resist)
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When electrons collide with the atoms of thetungsten wires, heat is released.
Thus a chemical action in the battery istransformed by the electrons through insulatedwires to the tungsten, where electrons are resisted
and heat occurs.
Tungsten becomes hot so light is emitted.
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FLUID FLOW Analogy
Electrical Circuits are similar to fluid flow systems.
Battery --- PumpCharge ----- FluidConductors ---- Frictionless pipes
Current ---- Rate of Flow of the Fluid
Voltage --- Pressure differentialSwitches --- Valves
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Electrical Circuits:
Electrical circuits consist of various circuitelements (CE) connected in a closed path by
conductors.
Circuit elements can beVoltage sources
ResistancesInductances
Capacitances
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Electrical Current:
Is the time rate of flow of electrical charge through
a conductor or circuit element.
Units: Amperes (A)==> Coulombs / second (c/s)
Charge on an electron: -1.602x10^-19 C
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We take the cross section of a circuit elementperpendicular to the flow of the current. Then we
select a REFERENCE DIRECTION along thedirection of the flow.
The electrical current following in the REF. DIR. Is
given by:
i(t) = dq(t)/dt.
A current of 1 Ampere means that one coulomb ofcharge passes through the cross section each
second.
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To find the charge, we integrate the current.
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REFERENCE DIRECTIONS:
In analyzing electrical circuits, we may not initially
know the actual direction of the current flow.We start by assigning current variables and weselect an arbitrary direction for each current of
interest.
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Direct Current and Alternating Current
Direct Current (DC) --- Constant w/ timeAlternating Current (AC), Varies w/ time,
magnitude and direction may change periodically.
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Double Subscript Notation for Currents
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Ex: A constant current of4A flows through a CE.In 10 Sec, how much net charge passes through
the CE?
Solution:
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Example
q(t) = 0.01 sin(200t), determine i(t).
Solution:
i(t) = d/dt q(t) = d/dt (.01sin(200t))
= 2 cos (200t) A.
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Voltages:
When charges move through a CE, energy istranferred.
In the case of the automobile headlights, energy is
supplied by the chemicals in the battery and isabsorbed by the headlights as heat and light.
Voltage ==> Energy transferred per unit of
charge.
Units: V = J/C
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Switches:
Control the flow of currents in a circuit.When an ideal switch is open, no current will flow
through it and the voltage is determined by theother elements in the circuit.When an ideal switch is closed, (we call it short
circuit), the voltage across it is zero, and the
current through it is determined by the otherelements in the circuit.
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Assigning Arbitrary values for voltages:
When we are analyzing circuits, if we do not knowthe voltage variables, we can start by choosing
reference polarities arbitrarily.
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Figure 1.12 The voltage vab has a referencepolarity that is positive at point a and negative at
point b.
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Figure 1.13 The positive reference forv is at the
head of the arrow.
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Example: Vab
= 20V, A positive charge of 2Cmoves through the CE from b to a.
Find the energy transferred.
+
--
a
b
2C
E = 20 V x -2 C
= 20 (J/C) x -2 C
= -40 J Supplied By the CE
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Section 1.3
Power and Energy
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P = V I Volts x Amperes(J/C) x (C/s)
(J/s) == > Watts !!!
Power = Rate of Energy Transfer
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Positive Reference Configuration:
If P is positive, Energy is being absorbed by the
CE
If P is Negative, Energy is being supplied by theCE.
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Figure 1.14 When current flows through anelement and voltage appears across the element,
energy is transferred. The rate of energy transferis p = vi.
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Energy Calculations:
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Section 1.4
Kirchhoff's Current Law
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Node: A point in an electrical circuit where two ormore Ces are joined together.
Kirchhoff's Current Law (KCL):The net current entering a node is zero.
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i1
+ i2- i
3= 0
i3 - i4= 0i5
+ i6
+ i7
=0
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Note: All points in a circuit that are connected by
simple conducting wires (i.e. no CE), areconsidered to be a single node.
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Section 1.5
Kirchhoff's Voltage Law
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Kirchhoff's Voltage Law (KVL):
The algebraic sum of all the voltages equals zerofor any closed path (loop) in an electrical circuit.
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Parallel Circuits:
Two circuit elements are parallel if both ends ofone element is connected directly to the
corresponding ends of the other element.
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Section 1.6
Introduction to Circuit Elements
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Conductors:
Conduct electricity (copper wires).Voltage across = 0 regardless of current thru it.
When two points are joined by an ideal conductor,we say the points are shorted together.
Ideal Conductor === Short Circuit
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Independent Voltage Sources:
Maintain a specified voltage across its terminals.Voltage is independent of other voltages across
other CE's and the current thru it.
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Dependent Voltage Sources:
Voltage across the source depends on the othervoltages or currents in the circuit.
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Independent Current Sources:
Forces a specified current to flow through itself.This current value is independent of the voltage
across it or the currents flowing through the otherCE's.
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Dependent Current Sources:
Forced current depends on the voltage or thecurrent across some other element in the circuit.
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Resistors and Ohm's Law:
The voltage across an ideal resistor is proportionalto the current through the resistor.
The constant of proportionality is the resistance R.
Ohm's Law: v = I R
Units of R are ohms.
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Ex: Compute the Resistance of a copper wirehaving a diameter of 2.05 mm and a length of
10m.
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