1. ECA1212 Introduction to Electrical & Electronics Engineering Chapter 1: Fundamental Laws of Electricity by Muhazam Mustapha, September 2011

2. 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

3. Chapter Content Brief history of electricity Units and Symbols
Ohm’s Law
Kirchhoff’s Law
Resistance and source combination, and voltage and current division

4. Brief History of Electricity
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5. 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
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6. 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
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7. Discoveries of Electricity
The earliest known use of electricity was by the Mesopotamians: BAGHDAD BATTERY [
The original connection between lightning and electricity was made by the Muslim Arabian scientists [
But the first recorded experiment was made by Benjamin Franklin
The connection between mechanical movement, magnetic field and electricity was made by Michael Faraday
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8. Units and Symbols
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9. 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
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10. 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
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11. Circuit Symbols Voltage Supply Resistor Load Current Supply Capacitor
Inductor
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12. Ohm’s and Kirchhoff’s Law
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13. Ohm’s Law
Electrical Law relating Current, Potential and Resistance
V = IR
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14. Kirchhoff’s Voltage Law
In closed loop circuit, the total voltage supply is equal to the total voltage drop V2 V3
V1 + V2 + V3
= V4 + V5 + V6 V1 V4 V6 V5
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15. Kirchhoff’s Current Law
At a circuit junction (node), the total incoming current is equal to the total out-going current
I1 + I2 + I5
= I3 + I4 I3 I2 I4 I1 I5
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16. Circuit Simplification
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17. Component in Series VEQ = V1 + V2 + V3
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 V1
VEQ = V1 + V2 + V3
VEQ V2 V3
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18. 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
REQ R1 R2 R3 I1 I2 I3 V1 V2 V3
VEQ
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19. 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
IEQ
IEQ = I1 + I2 + I3 I1 I2 I3
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20. 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
IEQ I1 I2 I3
REQ V1 R1 V2 R2 V3 R3
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21. Bridge Circuit Couldn’t be resolved to series or parallel
Analysis can be done as mesh or nodal analysis (Chapter 2)
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22. Exercise Calculate all V, I and R for all resistors: 1Ω 2Ω 1Ω 10V 6Ω4Ω 2Ω 1Ω 2Ω 1Ω
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23. Voltage & Current Divider
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24. Voltage Divider
Due to Ohm’s Law, the voltage distributed across series connected resistors is directly proportional to individual resistance value RT R1 R2 R3 V1 V2 V3 VT
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25. Current Divider
Due to Ohm’s Law, the current distributed through parallel connected resistors is inversely proportional to individual resistance value IT I1 I2 I3 R1 R2 R3
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26. Some Special Notations
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27. 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
POWER
20V
same point
GROUND (0V)
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28. 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
Vab Vb
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29. Parallel Operator R1 R2
R1R2 R1 + R2 RT =
R1 R2 =
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