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8/3/2019 EEE3 Lecture 1 - Exam1 - Basic Concepts
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Basic Concepts
Lecture 1
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System of Units
• Systeme Internationale d’Unites (SI)
deci- (d-) - 10-1
deka- (da-) - 101
hecto- (h-) - 102
kilo- (k-) - 103
mega- (M-) - 106
giga- (G-) - 109
tera- (T-) - 1012
atto- (a-) - 10-18
femto- (f-) - 10-15
pico- (p-) - 10-12
nano- (n-) - 10-9
micro- (µ-) - 10-6milli- (m-) - 10-3
centi- (c-) - 10-2
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The Unit of Charge
• Matter is made up of atoms
• Atoms are composed of the
“fundamental” particles: Protons – “positively” charged
Electrons – “negatively” charged
Neutrons – neutral (no charge)
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The Unit of Charge
• Interaction of charges
Electrical charges
cause electrical forces Non-gravitational
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Coulomb
• Two small, identically chargedparticles which are separated by 1meter in a vacuum and repel eachother with a force of 10-7c 2 N possessan identical charge of (either + or –)1 Coulomb (C). c
: velocity of light electrons: -1.60219 x 10-19 C
1 C: 6.24 x 1018 electrons
6
Charge (Q, q)
• Q: time-invariant value of charge
• q or q(t): time-varying value of charge, instantaneous value of charge
• Convention (for this class): Capital letters: time-invariant values
Small letters: time-varying values,instantaneous values
7
Current (I, i)
• Charges in motion
• Defined by magnitude and direction
• Measures the rate at which netcharges move past a given referencepoint in a specified direction
i = dq/dt
Ampere: 1 A = 1 C/s
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Current (I, i)
• We will consider network elements tobe electrically neutral.
No net + or – charge can accumulate.
Current entering = Current leaving
• Direction in computations: assumed
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Current (I, i)
3A is flowing from a to b .
-3A is flowing from b to a .
3 A -3 A
a b a b
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Voltage (V, v)
• Given a general circuit element, anelectrical voltage or a potential differenceis said to exist between its two terminalsor across the element if the passage of charge through the element requires theexpenditure of energy.
• Voltage = work required to move 1 C fromone terminal to the other.
1 V: 1 joule/coulomb
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Voltage (V, v)
• May exist whether current is flowingor not.
• Defined by magnitude and polarity.• Quantified with respect to a
reference point.
• Polarity in computations: assumed.
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Voltage (V, v)
• Voltage across the element is 5 V. (no polarity)
• Voltage drop from a to b is 5 V.
• Voltage drop from b to a is -5 V.
• Voltage rise from a to b is -5 V.
• Voltage rise from b to a is 5 V.
• Point a is at 5V higher potential than point b.
5 V – 5 V
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Voltage Reference Point
• A region or point where thevoltage is arbitrarily taken to bezero – the zero reference voltage.
• House electrical system – earth(ground).
• Electronic equipment – groundplane or negative terminal.
Battery
Groundcable
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Voltage (V, v)
5 V – 5 V
• Single-subscript notation
Defines voltages wrt an absolute reference point
• Double-subscript notation for voltageacross a and b
Vab = Va – Vb = 5 V
Vab = –Vba
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Energy (W, w)
• Work expended in forcing electricalcharges through an element.
1 Joule: 1 Nm
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Power (P, p)
• Rate at which energy is beingtransferred, consumed, or supplied.
p = dw/dt = vi
1 Watt = 1 J/s
1 W = 1 V * 1 A = 1 J/C * 1 C/s
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Power (P, p)
• Polarity of voltage and direction of current determines whether power isbeing supplied or consumed.
Power consumed: current enters terminal withhigher voltage.
Power supplied: current leaves terminal with
higher voltage.
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Power (P, p)
• In both cases, P = 10 W.
(a): consuming 10 W orsupplying -10 W.
(b): supplying 10 W orconsuming -10 W.
(a)
+
–
5 V
2 A
(b)
+
–
5 V
2 A
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Circuit Elements
• Circuit Element - The mathematicalmodel of a physical device used to analyzeits behavior.
• Simple Circuit Element - In ckt analysis,basic unit that cannot be subdivided intoother simple circuit elements.
• General Circuit Element - May becomposed of one or more simple cktelements.
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Circuit Elements
• Active element - capable of delivering power to some externalelement.
• Passive element - alwaysconsumes energy.
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Resistors
• Passive circuit element
• the voltage across it is proportionalto the current passing thru it.
v = k i, where k is a constant
k = resistance = ρ L / A
iv
+
-
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Resistors
v
i
k = R
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Independent Voltage Source
• Active circuit element
• Voltage across it is completely
independent of current passing thru.
iV s
-
+iV
siV
s-
+
ACDCDC
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Independent Voltage Source
v
i
V s
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Independent Current Source
• Active circuit element
• Current passing thru is completelyindependent of voltage across it.
I svs
-
+
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Independent Current Source
v
iI s
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Dependent/Controlled Source
• The source quantity (voltage or current) isdetermined by a voltage or current (notnecessarily the same) in some other
location in the system under consideration.
I s
vs
-
+
is
V s
-
+
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Dependent/Controlled Source
Current-ControlledCurrent Source
(CCCS)
Voltage-ControlledCurrent Source
(VCCS)
CurrentSource
Current-ControlledVoltage Source
(CCVS)
Voltage-ControlledVoltage Source
(VCVS)
VoltageSource
Current iscontrollingquantity.
Voltage iscontrollingquantity.
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Dependent/Controlled Source
• i d varies as v x .
• A is constant, unit is amperes/volt.
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Network
• Network – interconnection of two ormore simple circuit elements
• Electric Circuit – network with atleast one closed path.
• Active Network – contains at leastone active element.
• Passive Network – contains noactive elements.
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Network
• Node – the point of connection for circuitelements
• Branch – composed of one simple
element and the node at its end• Path – a collection of branches;
movement through the network whereinno node is encountered more than once.
• Loop – a closed path, starting node =ending node.
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Example