Chapter 1 Introduction to Electricity for CST 162 LAB Slide content from “Circuit Analysis: Theory...
-
date post
19-Dec-2015 -
Category
Documents
-
view
221 -
download
1
Transcript of Chapter 1 Introduction to Electricity for CST 162 LAB Slide content from “Circuit Analysis: Theory...
Chapter 1
Introduction to Electricity
for CST 162 LAB
Slide content from “Circuit Analysis: Theory and Practice” 4th Ed., by Robbins and Miller, 2007
System International (SI) System of Units
• Electric Current– Ampere (A)
• Electric Voltage– Volt (V)
• Electrical Resistance– Ohm (Ω)
Prefixes
• Metric Prefixes are used for convenience
Significant Digits and Numerical Accuracy
• Significant digits– Digits that carry information – It is a common error to show more digits of
accuracy than are warranted.
Circuit Diagrams
• Electric circuits– Use batteries and resistors as components– Circuit diagrams are used on paper
• Three types of circuit diagrams are used– Pictorial, block, and schematic
Pictorial Diagrams
• Help visualize circuits by showing components as they actually appear
Block Diagrams
• Blocks represent portions of a system
Schematic Diagrams
Chapter 2
Voltage and Current
Atomic Theory
• Atom – Contains a nucleus of protons and neutrons– Nucleus is surrounded by a group of orbiting
electrons
• Electrons are negative, protons are positive
Atomic Theory
• Electrically neutral atom– Equal number of electrons and protons
• Ion– An atom with an excess or deficit of
electrons
Conductors
• Materials with a large numbers of free electrons – Metals are good conductors because they
have few loosely bound valence electrons
Conductors
• Excellent conductors – Silver– Gold– Copper– Aluminum
Electrical Charge
• Unit of charge is the coulomb (C)
• One coulomb = 6.24 × 1018 electrons (or protons)
• The charge on one electron (or proton) =1/ 6.24 × 1018 or 1.6 × 10-19 C
Voltage
• When two objects have a difference in charges– They have a “potential difference” or “voltage” between them
• Unit of voltage is the volt• Thunderclouds
– Millions of volts between them
Voltage
• Difference in potential energy
• Voltage between two points =One volt, “if it requires one joule of energy to
move one coulomb of charge from one point to another”
Voltage
• V = Work/Charge
• Voltage is always measured between two points
coulomb
joulevolt
11
1
Current
• Movement of charge is electric current
• More electrons per second passing through a circuit, the greater the current
• Current is rate of flow of charge
Current
• Unit of current is ampere (A)
• One ampere =Current in a circuit when one coulomb of charge
passes a given point in one second
• Current = Charge/time
• I = Q/t
Current
• Electron current flow– Electrons flow from the negative terminal of a
battery to the positive terminal
• Conventional current flow– We may also assume currents flow from
positive to negative
Current
• Conventional current flow is used in this course, and in our field of study
How to Measure Voltage
• Place voltmeter leads across components
• Red lead is positive
• Black lead is negative
• If leads are reversed, you will read the opposite polarity
How to Measure Current
• Measurable current must pass through meter
• Open the circuit (i.e. disconnect wires) and insert the ammeter, so that the current now flows through the meter
• Connect with correct polarity
Chapter 3
Resistance
Resistance of Conductors
• Resistance of material is dependent on several factors:– Type of Material– Length of the Conductor– Cross-sectional area– Temperature
Type of Material
• Atomic differences of materials cause variations in how electron collisions affect resistance
• Differences produce resistivity
Length
• Resistance of a conductor– Directly proportional to its length– If you double the length of the wire, the
resistance will double
• = length– In meters or feet
Area
• Resistance of a conductor– Inversely proportional to cross-sectional
area of the conductor
• If cross-sectional area is doubled– Resistance will be one half as much
Fixed Resistors
• Resistance of a fixed resistor is constant over a wide temperature range
• Rated by amount of resistance– Measured in ohms (Ω)
• Also rated by power– Measured in watts (W)
Fixed Resistors
• Different resistors for different applications– Molded carbon composition– Carbon film– Metal film– Metal Oxide– Wire-Wound– Integrated circuit packages
Variable Resistors
• Resistance may be changed (varied)– Adjust volume, set level of lighting, adjust
temperature
• Have three terminals– Center terminal connected to wiper arm
• Potentiometers (normally abbreviated to just “Pot”)
• Rheostats
Color Code
• Colored bands on a resistor provide a code for determining– Value– Tolerance– Reliability
Reading color codes
Measuring Resistance
• Use an Ohmmeter
• Remove all power sources to circuit
• Isolate component to be measured
• Connect probes across component
• No need to worry about polarity
• Ohmmeter determines shorts and opens in individual components
Chapter 4
Ohm’s Law
and Energy
Ohm’s Law
• Current in a resistive circuit– Directly proportional to its applied voltage – Inversely proportional to its resistance
R
EI
Ohm’s Law
• For a fixed resistance– Doubling voltage doubles the current
• For a fixed voltage– Doubling resistance halves the current
Ohm’s Law
• Also 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
Ohm’s Law in Graphical Form• Linear relationship between current and
voltage• y = mx
– y is the current– x is the voltage– m is the slope
Ohm’s Law in Graphical Form• Slope (m) determined by resistor
conductance
Ohm’s Law in Graphical Form
Open Circuits• Current can only exist where there is a
conductive path
• An “Open circuit” is defined when there is no conductive path
Open Circuits
• If I = 0– Ohm’s Law gives R = E/I = E/0 infinity
• An open circuit has infinite resistance
Voltage Symbols
• Voltage sources– Uppercase E
• Voltage drops– Uppercase V
• V = I*R– “IR” drops
Voltage Polarities
• Polarity of voltage drops across resistors is important in circuit analysis
• Drop is + to – in the direction of conventional current
• To show this, place plus sign at the tail of current arrow
Voltage Polarities
Current Direction
• Current usually proceeds out of the positive terminal of a voltage source
• If the current is actually in this direction, it will be supplying power to the circuit
Current Direction
• If the current is in the opposite direction (going into the positive terminal), it will be absorbing power (like a resistor)
Current Direction
• See two representations of the same current on next slide
• Notice that a negative current actually proceeds in a direction opposite to the current arrow
Current Direction
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
Law of Conservation of Energy
• Energy can neither be created nor destroyed– Converted from one form to another
• Examples: – Electric energy into heat– Mechanical energy into electric energy
Law of Conservation of Energy
• Energy conversions– Some energy may be dissipated as heat,
giving lower efficiency
Chapter 5
Series Circuits
Series Circuits
• Two elements in a series– Connected at a single point– No other current-carrying connections at this
point
• A series circuit is constructed by connecting various elements in series
Series Circuits
• Normally– Current will leave the positive terminal of a
voltage source– Move through the resistor(s)– Return to negative terminal of the source
Series Circuits
• Current is similar to water flowing through a pipe– Current leaving the element must be the
same as the current entering the element
• Same current passes through every element of a series circuit
Series Circuits
• The laws, theorems, and rules that you apply to DC circuits– Also apply to AC circuits
Kirchhoff’s Voltage Law (KVL)
• The algebraic sum of the voltage that rises and drops around a closed loop is equal to zero
• ET - V1 - V2 - V3 - ∙∙∙ - Vn = 0
Kirchhoff’s Voltage Law (KVL)
• Another way of stating KVL is: – Summation of voltage rises is equal to the
summation of voltage drops around a closed loop
V1 + V2 + V3 + ∙∙∙ + Vn = ET
Resistors in Series
• Most complicated circuits can be simplified
• For a series circuit– V1 + V2 + V3 = E
– IR1 + IR2 + IR3 = E
– I(R1 + R2 + R3 )= E
– I(R1 + R2 + R3 )= IRtotal (Note: I’s cancel)
Resistors in Series
• Total resistance in a series circuit is the sum of all the resistor values
Interchanging Series Components
• Order of series components– May be changed without affecting operation of
circuit
• Sources may be interchanged, but their polarities can not be reversed
• After circuits have been redrawn, it may become easier to visualize circuit operation
Circuit Ground
• Ground – Point of reference or a common point in a
circuit for making measurements
• One type of grounding is chassis ground
• In this type of grounding– Common point of circuit is often the metal
chassis of the piece of equipment
Circuit Ground
• Chassis ground – Often connected to Earth Ground
• Earth ground– Physically connected to the earth by a metal
pipe or rod
Circuit Ground
• If a fault occurs within a circuit, the current is redirected to the earth
• Voltages are often measured with respect to ground
Ammeter Loading Effects
• An ammeter is placed in a circuit to make a current measurement– Resistance in the meter will affect the circuit
• Amount of loading is dependent upon the instrument and the circuit
Ammeter Loading Effects
• If resistance of the meter is small compared to the resistance of the circuit, the loading effect will be small
Chapter 6
Parallel Circuits
Parallel Circuits
• House circuits contain parallel circuits• The parallel circuit will continue to operate even
though one component may be “open”• Only the “open” or “defective” component will no
longer continue to operate• A light bulb with a broken filament = “open”
Parallel Circuits
Parallel Circuits
• Elements in parallel– When they have exactly two nodes in common
• Elements between nodes – Any device like resistors, light bulbs, etc.
• Elements connected in parallel– Same voltage across them
Parallel Circuits
Series - Parallel Circuits
• Circuits may contain a combination of series and parallel components
• Being able to recognize the various connections in a network is an important step in analyzing these circuits
Series - Parallel Circuits
Parallel Circuits
• To analyze a particular circuit– First identify the node– Next, label the nodes with a letter or number– Then, identify types of connections
Parallel Circuits
Kirchhoff’s Current Law (KCL)
• The algebraic sum of the currents entering and leaving a node is equal to zero
0I
Kirchhoff’s Current Law (KCL)
• Currents entering the node are taken to be positive, leaving are taken to be negative
• Sum of currents entering a node is equal to the sum of currents leaving the node
outin II
Kirchhoff’s Current Law (KCL)
• An analogy: – When water flows in a pipe, the amount of
water entering a point is equal to the amount leaving that point
Resistors in Parallel
• Voltage across all parallel elements in a circuit will be the same
Resistors in Parallel
• For a circuit with 3 resistors: IT = I1 + I2 + I3
321T
321T
1111
RRRR
R
E
R
E
R
E
R
E
Resistors in Parallel
• Total resistance of resistors in parallel will always be less than resistance of smallest resistor
Equal Resistors in Parallel
• Total resistance of equal resistors in parallel is equal to the resistor value divided by the number of resistors
Two Resistors in Parallel
• For only two resistors connected in parallel, the equivalent resistance may be found by the product of the two values divided by the sum
• Often referred to as “product over the sum” formula
21
21
RR
RRR
T
Three Resistors in Parallel• For three resistors in parallel:
• Rather than memorize this long expression– Use basic equation for resistors in parallel
323121
321
RRRRRR
RRRR
T
Voltmeter Loading Effects
• A voltmeter– Meter movement in series with a current-
limiting resistance
• If resistance is large compared with the resistance across which the voltage is to be measured, the voltmeter will have a very small loading effect
Voltmeter Loading Effects
• If this resistance is more than 10 times the resistance across which the voltage is being measured, the loading effect can generally be ignored.
• However, it is usually much higher.