EEE3 Lecture 1 - Exam1 - Basic Concepts

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



2

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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

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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

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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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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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• 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

EEE3 Lecture 1 - Exam1 - Basic Concepts

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