Basic Definitions & Basic Laws Dr. Mohamed Refky Amin Electronics and Electrical Communications Engineering Department (EECE) Cairo University [email protected] http://scholar.cu.edu.eg/refky/ Outline of this Lecture • Course Contents, References, Course Plan • Basic Definitions • Topology of Electric Circuit • Basic Laws – Ohm’s Law – Kirchhoff's Voltage Law – Kirchhoff's Current Law • Power and Energy Dr. Mohamed Refky 2 References • Textbook: – “Fundamentals of Electric Circuits”, Alexander and Sadiku, 5th edition • References: – J. W. Nilsson and S. Riedel, Electric Circuits. 8th Edition, Prentice Hall, 2008. – Syed A. Nasar, “Electric Circuits,” Schaum’s Solved Problems Series, Mcgraw-Hill, New York, 1988. – Cunningham Stuller, “Circuit Analysis,” John Wiley & Sons Inc., New York, 1995. Dr. Mohamed Refky 3 Course Plan • Instructor: Prof. Mohamed Fathy Dr. Mohamed Refky • TA: TBA • Grading: – 40% Activity • Quizzers • Assignments • Attendance – 20 % Midterm – 40% Final Exam • Office Hours: TBA Dr. Mohamed Refky 4 Course Contents Week Topic 1 Basic Definitions & Basic Laws 2 3 Methods of solution of DC circuits 4 8 Network Theorems 6 Midterm 7 Introduction to Capacitors and Inductors 8 Sinusoids and phasors 9 10 Sinusoidal steady state analysis 11 12 Three Phase Circuits Dr. Mohamed Refky 5 Introduction Importance of this Course For an electrical engineering student, basic electric circuit theory is an essential to understand several other courses. Many branches of electrical engineering, such as power, electric machines, electronics, and instrumentation, are based on electric circuit theory. Dr. Mohamed Refky 6 Outline of this Lecture • Course Contents, References, Course Plan • Basic Definitions • Topology of Electric Circuit • Basic Laws – Ohm’s Law – Kirchhoff's Voltage Law – Kirchhoff's Current Law • Power and Energy Dr. Mohamed Refky 7 Basic Definitions Introduction In general, electrical engineering is interested in transferring electrical energy from one point to another. Transfer of electrical energy requires an interconnection of electrical component. The interconnection is referred to as an electric circuit, and each component of the circuit is known as a circuit element. Dr. Mohamed Refky 8 Basic Definitions Elements of Electrical Circuits Generally, any electrical circuits has three main components: ▪ Source of power (Battery) ▪ Load (Lamp) ▪ Connecting wires ▪ Switch Electrical circuits are modeled using a simple schematic to represent the circuit components. The circuit load such as lamp, speaker, … is usually modeled as a resistor. Dr. Mohamed Refky 9 Basic Definitions Elements of Electrical Circuits Electrical Circuit Active Elements Passive Elements Voltage Current Resistor Capacitor Coil Source Source Active elements are the components that are capable of generating energy. On the other hand, passive elements are the component that are not capable of generating energy Dr. Mohamed Refky 10 Basic Definitions Elements of Electrical Circuits Electrical Circuit Active Elements Passive Elements Voltage Current Resistor Capacitor Coil Source Source An independent source is the source Independent Dependent whose value is independent of any Source Source signal (voltage/current) in the circuit. Dr. Mohamed Refky 11 Basic Definitions Elements of Electrical Circuits Electrical Circuit Active Elements Passive Elements Voltage Current Resistor Capacitor Coil Source Source A dependent source is the source Independent Dependent whose value depends on another Source Source signal (voltage/current) in the circuit. Dr. Mohamed Refky 12 Basic Definitions Elements of Electrical Circuits Electrical Circuit Active Elements Passive Elements Voltage Current Resistor Capacitor Coil Source Source Voltage dependent Source Independent Dependent Source Source Current dependent Source Dr. Mohamed Refky 13 Basic Definitions Elements of Electrical Circuits Electrical Circuit DC Circuit AC Circuit DC Active Passive AC Active Passive elements element elements element In Direct Current (DC) circuits, currents and voltages are constant with time. While in Alternating Current (AC) circuits, current and voltage are time varying. Dr. Mohamed Refky 14 Basic Definitions Elements of Electrical Circuits Electrical Circuit DC Circuit AC Circuit DC Active Passive AC Active Passive elements element elements element Resistor Resistor Capacitor Coil Dr. Mohamed Refky 15 Basic Definitions Elements of Electrical Circuits Passive element independent Sources Dependent Sources Dr. Mohamed Refky 16 Basic Definitions Basic Quantities Quantity Symbol Unit Charge Time Work Current Voltage Power Dr. Mohamed Refky 17 Basic Definitions Dr. Mohamed Refky 18 Basic Definitions The conservation of charge law states that Charge can neither be created nor destroyed, only transferred Thus the algebraic sum of the electric charges in a system does not change. Dr. Mohamed Refky 19 Basic Definitions Dr. Mohamed Refky 20 Basic Definitions A battery causes current to flow in a resistor. Current flows in a resistor from high voltage to low voltage. Important: the direction of the conventional current is opposite to the direction of the electrons flow. Dr. Mohamed Refky 21 Basic Definitions Dr. Mohamed Refky 22 Outline of this Lecture • Course Contents, References, Course Plan • Basic Definitions • Topology of Electric Circuit • Basic Laws – Ohm’s Law – Kirchhoff's Voltage Law – Kirchhoff's Current Law • Power and Energy Dr. Mohamed Refky 23 Topology of Electric Circuit Electric Circuit An electric circuit consists of Branches Nodes Loops Dr. Mohamed Refky 24 Topology of Electric Circuit Branch Dr. Mohamed Refky 25 Topology of Electric Circuit Node A node is a common terminal between two or more elements. • Simple node is a common terminal inside a branch 1,4,5,6 • Non-simple node is a common terminal of more than two branches 2, 3, 0 Dr. Mohamed Refky 26 Topology of Electric Circuit Loop A Loop is a set of branches making a closed path. • Independent loops 12501, 23052, 34603 • Non-independent loops 12301, 1234601, 2346052 Dr. Mohamed Refky 27 Topology of Electric Circuit Branch, Nodes and Loops Dr. Mohamed Refky 28 Outline of this Lecture • Course Contents, References, Course Plan • Basic Definitions • Topology of Electric Circuit • Basic Laws – Ohm’s Law – Kirchhoff's Voltage Law – Kirchhoff's Current Law • Power and Energy Dr. Mohamed Refky 29 Basic Laws Ohm’s low Dr. Mohamed Refky 30 Basic Laws Kirchhoff's Current Law (KCL) Kirchhoff's Current Law (KCL) states that The algebraic sum of the currents entering any node is zero To do an algebraic sum, algebraic sign corresponding to a reference direction must be assigned to every current at the node Dr. Mohamed Refky 31 Basic Laws Kirchhoff's Current Law (KCL) Kirchhoff's Current Law (KCL) states that The algebraic sum of the currents entering any node is zero To do an algebraic sum, algebraic sign corresponding to a reference direction must be assigned to every current at the node Dr. Mohamed Refky 32 Basic Laws Kirchhoff's Current Law (KCL) Kirchhoff's Current Law (KCL) states that Dr. Mohamed Refky 33 Basic Laws Kirchhoff's Current Law (KCL) KCL is usually applied only for non-simple node. Dr. Mohamed Refky 34 Basic Laws Kirchhoff's Voltage Law (KVL) Dr. Mohamed Refky 35 Basic Laws Kirchhoff's Voltage Law (KVL) Dr. Mohamed Refky 36 Basic Laws Kirchhoff's Voltage Law (KVL) For loop 2: For loop 3: Dr. Mohamed Refky 37 Basic Laws Series Connection Dr. Mohamed Refky 38 Basic Laws Series Connection Dr. Mohamed Refky 39 Basic Laws Series Connection The series combination of resistors results in increasing the total resistance Dr. Mohamed Refky 40 Basic Laws Parallel Connection Dr. Mohamed Refky 41 Basic Laws Parallel Connection Dr. Mohamed Refky 42 Basic Laws Parallel Connection Dr. Mohamed Refky 43 Basic Laws Parallel Connection Dr. Mohamed Refky 44 Basic Laws Parallel Connection Dr. Mohamed Refky 45 Basic Laws Voltage Divider When resistors are connected in series, the total voltage across these resistors is divided between them with a ratio that depends on the values of theses resistors. Dr. Mohamed Refky 46 Basic Laws Voltage Divider When resistors are connected in series, the total voltage across these resistors is divided between them with a ratio that depends on the values of theses resistors. Dr. Mohamed Refky 47 Basic Laws Current Divider When resistors are connected in parallel, the total current is divide between these resistor with a ratio that depends on the values of theses resistors. Dr. Mohamed Refky 48 Basic Laws Current Divider When resistors are connected in parallel, the total current is divide between these resistor with a ratio that depends on the values of theses resistors. Dr. Mohamed Refky 49 Outline of this Lecture • Course Contents, References, Course Plan • Basic Definitions • Topology of Electric Circuit • Basic Laws – Ohm’s Law – Kirchhoff's Voltage Law – Kirchhoff's Current Law • Power and Energy Dr. Mohamed Refky 50 Power and Energy Definition Dr. Mohamed Refky 51 Power and Energy Power Calculations Passive Sign Convention Power is Power is delivered absorbed Power is Power is delivered absorbed Dr. Mohamed Refky 52 Power and Energy Power Calculations Passive Sign Convention Dr. Mohamed Refky 53 Power and Energy Power Calculations Passive Sign Convention For a source, power is either delivered or absorbed Power is delivered Power is absorbed Dr. Mohamed Refky 54 Power and Energy Power Balance The total delivered power to the circuit equal to the total power dissipated Dr. Mohamed Refky 55 Power and Energy Power Balance The total delivered power to the circuit equal to the total power dissipated Dr. Mohamed Refky 56.