• Electricity – From the word “Elektron”– Greek for “amber”

Electrical systems have two main objectives:

• To gather, store, process, transportinformation & Energy

• To distribute and convert energy

Electrical Engineering Subdivisions

Communication Computer Systems Control Systems Electromagnetics

Electronics Power Systems Signal Processing

Where do we find electrical circuits?• Communications

– Radio, internet, telephone, television• Data Processing

– Desktop computers, servers• Automobiles

– displays, sensors, motors• Home

– lighting, heating, appliances• Weather Stations

– wind speed, precipitation, temperature• Power Plants

– moving magnets, transformers

Human Body is Polymeric System controlled by Electrical System?

Why Study Electrical Engineeringin SNU MSE ?

• To know the Basic Knowledge of Electronics • So you can understand a little about Electronic

System around you • To Operate and Maintain Electrical Systems• To Communicate with Electrical Engineer • Additionally Good Examination Grade

Basic Definitions

Capacitor dtdVCI = 2

21 CVP =

Power

ResistorRVIorIRV ==

InductordtdILV =

Passive Component

Active ComponentDiode

TransistorOP amp

Passive : No Power Gain (Two terminal)Active : Power Gain : Built-in Power Source

Linear

NonLinear

221 LIP =

RVIRVIP

22

21 ===

Circuit Symbols• Battery or Power Supply• Resistor• Capacitor• Switch

More Circuit Symbols

• DC Voltage Supply• AC Voltage Supply• Open Circuit• Closed Circuit• Short Circuit • DC Current Supply• AC Current Supply

Electrical Current • Current - the rate of flow of charge through a

conductor– Conventional Current

• Direction of flow of positive (+) charges

– Electron Current• Opposite to that of conventional current

• Units for Current– Amperes

• Charge of the Electron– e = -1.603 × 10-19 Coulombs

• 1 A = 1 C/s

Effect of Electric Currents on the Body

• 0.001 A can be felt• 0.005 A is painful• 0.010 A causes involuntary muscle contractions• 0.015 A causes loss of muscle control• 0.070 A can be fatal if the current last for more

than 1 second

Current Equation

tqi∆∆

=

Current in Amperes (A)

Charge inCoulombs (C)

Time in Seconds (s)

dt)t(dqi =Calculus:

Figure 1.4

dttdqti )()( = (1.1)

∫ +=t

ttqdttitq

0

)()()( 0(1.2)

)(tq : Charge

)(ti : Currentt0 : some initial time

at which the chargeis known

Types of Electricity

• Static Electricity - no motion of free charges• Current Electricity - motion of free charges

– Direct Current (DC)– Alternating Current (AC)

Voltage Description

Current Description

KIRCHHOFF’S CURRENT LAW (KCL)

• The net current entering a node is zero.• Alternatively, the sum of the currents entering a

node equals the sum of the currents leaving a node.

i1 + i2 - i3 = 0 i3 - i4 = 0 i5 + i6 - i7 = 0

Kirchoff Current Law : Σ Ik=0“The sum of current at a node is always zero”

Parallel Circuit : Constant Voltage

∑∑ ===k kk k Z

ZZV

ZVI 1

Resistor ∑=k

kRR /1 Capacitor ∑=k

kCC

ia - ib + ic - id = 0

Σ Ik=0

Current Direction+ : Inward Direction- : Outward Direction

ia = 4A ib = -2A ic = -8A

ia = ib = ic

Examples of KCL

Voltage • Voltage : A measure of the potential to move electrons.• Sources of Voltage

– Batteries (DC)– Wall Outlets (AC)

V9)t(v =

)t120sin(V110)t(v π=

Unit of voltage : Volts (V) 1V = 1 Joule per coulomb (J/C).

DC

AC

baab vv −=

KIRCHHOFF’S VOLTAGE LAW(KVL)

The algebraic sum of the voltages equals zero for any closed path (loop) in an electrical circuit.

Kirchoff Voltage Law : Σ Vk=0“The sum of voltage drop around a loop is always zero”

Serial Circuit : Constant Current

∑∑ ===k

kk

k ZZIZIZV

Resistor ∑=k

kRR Capacitor ∑=k

kCC /1

Same Current for All Elements Same Volatve for All Elements

A, B C D, E, F

Parallel

Serial

Rearrangement of Circuit

Power and Energy

• Power : the rate of energy conversion• Units : Watts• Equation : p=iv

• Resistors convert electrical energy into heat energy.

p = iv Power delivered by a batteryp = -iv Power dissipated by a resistor

POWER AND ENERGY

)()()( titvtp =

vip =

2Rip =

Rvp

2

=

∫=1

2

)(t

tdttpw

Power

Energy

Introduction to Circuit Element- Conductor- Voltage Source- Current Source- Resistor

Ideal Element :

Conductor : The voltage between ends of an ideal conductor is zero regardless of the current through the conductor

Short Circuit

Note. Open Circuit

Independent Source

Voltage or Current is specified by itself into Circuit.

Constant Voltage Source Constant Current Source

Battery, Home Power Specially Designed Circuit

Dependent Voltage Source

Voltage or Current is determined by others in Circuits

Τransistor, Amplifier, Electrical Generator

Dependent Current Source

Current through Source is designed as 3vx

if vx = 5V Source : 15A

Many type of real world devices such as Electrical machines Transistor, transformer Amplifierdue to Safety & Economics

Current through Source is designed as 2iy

if iy = 5A Source : 10A

Resistor Equations

• Ohm’s Law: v = iR• Resistance:

ALR ρ

=

Resistivity in Ohms-meters (Ω m)

Cross-sectional AreaIn square meters (m2)

Length inMeters (m)

Resistance inOhms (Ω)

ALR ρ

=ρ : resistivity

iRv =R

G 1=Gvi = G : conductance

R : resistance

Ohm’s law

What is an Insulator?

• A material that does not allow electrons to flow freely• Examples:

– Wood– Plastics– Ceramics– Amber

What is a conductor?

• A material that allows free electron movement• Examples:

– Aluminum– Gold– Copper– Most Metals

What is a semiconductor?

• A class of materials whose electron conductivity is between that of a conductor and insulator

• Examples:– Silicon– Germanium

Summary

> 1021

1 × 1012

1 × 1019

10-8 to 1

2.73 × 10-8

3.5 × 10-5

1.72 × 10-8

2.27 × 10-8

1.12 × 10-6

1.63 × 10-8

5.44 × 10-8

Fused quartzGlass (typical)Teflon

Insulators

Silicon (divice grade)depends on impurity concentration

Semiconductors

AluminumCarbon (amorphous)CopperGoldNichromeSilverTungsten

Conductors Resistivity

Resistivity for Various Material at 300K

Mechanical Strain Gauge(Resistor)

Resistor Color Codes

-2 Silver-1 Gold 0 Black1 Brown2 Red3 Orange4 Yellow5 Green6 Blue7 Violet8 Gray9 White

Tolerance 5% Gold10% Silver20% No 4th band

f

p

n

μ

m

k

M

G

Abbreviation

10-15

10-12

10-9

10-6

10-3

103

106

109

Scale factor

femto-

pico-

nano-

micro-

milli-

kilo-

meg- or mega-

giga-

Prefix

INTRODUCTION TO CIRCUITS

Kirchoff Current Law : Σ Ik=0“The sum of current at a node is always zero”Parallel Circuit : Constant Voltage

∑∑ ===k kk k Z

ZZV

ZVI 1

Kirchoff Law

Resistor ∑=k

kRR /1 Capacitor ∑=k

kCC

Kirchoff Voltage Law : Σ Vk=0“The sum of voltage drop around a loop is always zero”

Serial Circuit : Constant Current

∑∑ ===k

kk

k ZZIZIZV

Resistor ∑=k

kRR Capacitor ∑=k

kCC /1

Concept Questions

• How can you change the resistance of a resistor?

• How would you determine the resistivity of pencil lead?

• How would you determine the Number of Electrons in Earth?