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Transcript
EEE1012 Introduction to Electrical & Electronics Engineering Chapter 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 nodepair 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 Ω • 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 Circuit Symbols Resistor Current Supply Load Capacitor Voltage Supply Inductor 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 V2 V3 V1 + V2 + V3 = V4 + V5 + V6 V1 V4 V6 V5 Kirchhoff’s Current Law • At a circuit junction (node), the total incoming current is equal to the total out-going current I1 + I2 + I5 = V3 + V4 I3 I2 I4 I1 I5 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 V1 VT V2 V3 VT = V1 + V2 + V3 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 RT R1 I1 R2 I2 V1 R3 I3 V2 VT V3 V1 R 1 V2 R 2 V3 R 3 , , VT R T VT R T VT R T I1 I 2 I3 R T R1 R 2 R 3 Components in Parallel • Components are connected head-to-head, tailto-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 I1 G1 I 2 G 2 I 3 G 3 , , IT G T IT G T IT G T IT RT I1 I2 V1 R1 V2 I3 R2 V3 V1 V2 V3 R3 1 1 1 1 R T R1 R 2 R 3 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: 1Ω 10V 2Ω 6Ω 1Ω 1Ω 4Ω 2Ω 2Ω 1Ω 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 POWER 20V same point GROUND (0V) Relative Potentials Va Vab Vb • 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 Parallel Operator R1 RT = R1 R2 R2 = R1R2 R1 + R2