EEE1012Introduction to Electrical &

Electronics EngineeringChapter 1: Fundamental Laws of Electricity

by Muhazam Mustapha, July 2010

Learning Outcome

• By the end of this chapter, students are expected to understand the most basic laws of electricity, i.e. Ohm’s and Kirchhoff’s Law, the units involved and the related symbols

• The ONLY way to score in this course is to do a lot of exercises

Chapter Content

• Brief history of electricity

• Units and Symbols

• Ohm’s Law

• Kirchhoff’s Law

• Resistance and source combination, and voltage and current division

• Analysis of single loop and single node-pair circuit

Brief History of Electricity

Source of Electricity

• Electricity is the direct phenomena due to the net displacement of the sub-atomic particle: ELECTRON

• If the displaced electron is freely moving: ELECTRODYNAMIC phenomenon

• If the displaced electron is tied-up into material: ELECTROSTATIC phenomenon

• What we want to learn in this course is electrodynamic phenomenon

Source of Electricity

• Electron displacement can be achieved by:– Chemical reaction (batteries)– Mechanical interaction (electrostatic)– Magnetic influence (generator)– Nuclear reaction (atomic batteries)

• Physical connection that gives a net electron movement in one close loop is called circuit

• Circuits consist of: POWER supply and LOAD

Discoveries of Electricity

• The earliest known use of electricity was by the Mesopotamians: BAGHDAD BATTERY [http://en.wikipedia.org/wiki/Baghdad_Battery]

• The original connection between lightning and electricity was made by the Muslim Arabian scientists [http://en.wikipedia.org/wiki/Electricity]

– But the first recorded experiment was made by Benjamin Franklin

• The connection between mechanical movement, magnetic field and electricity was made by Michael Faraday

Units and Symbols

Electrical Units

• Electric charge: Coulomb, C– Amount of electric charge in material

• Electric current: Ampere, A [C/s]– Rate of charge movement per second

• Voltage: Volt, V [J/C]– Electrical tension (potential) created when 1 C of

charge is displaced using 1 Joule of energy

• Electric power: Watt, W [VA]– Dissipated power when 1 A of current flows with 1 V

of electrical potential

Electrical Units

• Resistance: Ohm, Ω [V/A]– Opposition to electric flow in material when 1 A is

flowing with 1 V of electrical potential

• Conductance: Siemens, σ or Mho,– Reciprocal of resistance

• Capacitance: Farad, F– Capacitor that can sustain 1 C of charge when 1 V of

potential is given

• Inductance: Henry, H– Inductor that can sustain 1 Wb[*] of magnetic flux

when 1 A current is flowing

Ω

Circuit Symbols

Capacitor

Load

Current Supply Inductor

Voltage Supply

Resistor

Ohm’s and Kirchhoff’s Law

Ohm’s Law

• Electrical Law relating Current, Potential and Resistance

V = IR

Kirchhoff’s Voltage Law

• In closed loop circuit, the total voltage supply is equal to the total voltage drop

V6

V1

V2 V3

V4

V5

V1 + V2 + V3

= V4 + V5 + V6

Kirchhoff’s Current Law

• At a circuit junction (node), the total incoming current is equal to the total out-going current

I3

I1 + I2 + I5

= V3 + V4I4

I2

I1I5

Circuit Simplification

Component in Series

• Components are connected head-to-tail• Series current supplies are not legal

arrangement without considering internal conductance, unless they are the same values

• Voltage supplies are combined by summing up

VT V2

V3

VT = V1 + V2 + V3

V1

Components in Series

• Voltage drops ratio across resistors are equal to the resistance ratio

• Currents are the same through all components• Resistors are combined by summing up

T

3

T

3

T

2

T

2

T

1

T

1

R

R

V

V,

R

R

V

V,

R

R

V

V

V1 V2 V3

I1 I2 I3

R1 R2 R3

VT

RT

321T R R R R 321 I I I

Components in Parallel

• Components are connected head-to-head, tail-to-tail

• Parallel voltage supplies are not legal arrangement without considering internal resistance, unless they are the same values

• Current supplies are combined by summing up

IT = I1 + I2 + I3

IT

I1 I2 I3

Components in Parallel

• Current ratio through resistors are equal to the conductance ratio

• Voltage are the same across all components• Resistors are combined by combining

conductance

T

3

T

3

T

2

T

2

T

1

T

1

G

G

I

I,

G

G

I

I,

G

G

I

I

V1 V2 V3

I1 I2 I3

R1 R2 R3RT

321T R

1

R

1

R

1

R

1

321 V V V

IT

Bridge Circuit

• Couldn’t be resolved to series or parallel• Analysis can be done as mesh or nodal analysis

(Chapter 2)

Exercise

Calculate all V, I and R for all resistors:

10V

1Ω

1Ω

6Ω

2Ω

2Ω

1Ω

1Ω

4Ω 2Ω

Some Special Notations

Ground & Power• Power can just be shown as a bubble at the top

– This simplifies the circuit and shows voltage more clearly

• Ground is a COMMON point whose voltage is assumed to be at a reference point 0V

20V

20V

same point

POWER

GROUND (0V)

Relative Potentials• Voltages with single

subscripts are relative to ground

• Voltages with double subscripts are of the first subscript as seen from the second:Vab = Va − Vb

Va

Vb

Vab

Parallel Operator

R1R2

R1 + R2

=R1 R2=RT

R1 R2