Electric Circuit Theory

Transcription

1 Electric Circuit Theory Nam Ki Min

2 Chapter 1 Circuit Variables Nam Ki Min nkmin@korea.ac.kr

3 1.1 Electrical Engineering 3 Electrical Engineering Electrical engineering is concerned with the generation, transmission and utilization of electrical energy(signal) and with the transmitting and processing of information. Electrical engineers are involved in the analysis, design and production of electric power, radio, radar, television, computing, telecommunication, control and information systems. The typical curriculum of an undergraduate electrical engineering student includes the subjects listed in Table. Network theory Circuit analysis Electromagnetics Solid-state electronics Electric machines Electric power systems Digital logic circuits Computer systems Communication systems Electro-optics Instrumentation systems Control systems Signal processing system Electrical engineering disciplines

4 1.1 Electrical Engineering 4 Five Major Classifications of Electrical Systems Communication system Computer system Control system Power system Signal processing systems Power system

5 1.1 Electrical Engineering 5 Electrical engineering systems in the automobile

6 1.1 Electrical Engineering 6 Electric Circuits An electric circuit is an interconnection of electrical elements. A simple electric circuit is shown in Figure. It consists of three basic components: a battery, a lamp, and connecting wires. An electric circuit is an mathematical model that approximates the behavior of an actual electrical system. Electric circuits are used in numerous electrical systems to accomplish different tasks. The Size Scale of Circuits Microns Centimeters Meters Kilometers

7 1.1 Electrical Engineering 7 Electric circuit theory is the most important course for the Electrical Engineering Student. Electric circuit theory and electromagnetic theory are the two fundamental theories. All the other subjects(branches) in your electrical engineering rely on the concepts of these theories. Therefore, the basic electric circuit theory course is the most important course for an electrical engineering student, and always an excellent starting point for a beginning student in electrical engineering education. CIE 201 is boring, but absolutely essential!!! CIEN 309,310 Instrumentation CIEN 223, 224 Instrumentation Lab CIEN 201,202 CIEN 311, 312 Electronic Circuits CIEN 207 Physical electronics CIEN 304 Electronic Circuits Lab CIEN 301 Control Eng. CIEN Control Eng. Lab CIEN 231,232 Electromagnetics

8 1.2 The International System of Units(SI) 8 The SI units are based on seven defined quantities: Length Mass Time Electric current Thermodynamic temperature Amount of substance Luminous intensity Basic or fundamental units Derived Units The SI derived units are obtained from some physical law defining that unit and the seven SI base units. Area A = a b m 2 Force F = ma kg m s 2

9 1.2 The International System of Units(SI) 9 Prefixes In many cases, the SI unit is either too small or too large to use conveniently. Standard prefixes corresponding to powers of 10 are then applied to the basic unit. yocto y zepto z It is important to note that the kilogram is the only SI unit with a prefix as part of its name and symbol. Because multiple prefixes may not be used, in the case of the kilogram the prefix names of Table 1.3 are used with the unit name "gram" and the prefix symbols are used with the unit symbol "g." With this exception, any SI prefix may be used with any SI unit, including the degree Celsius and its symbol C. Example: 10-6 kg = 1 mg (one milligram), but not 10-6 kg = 1 µ kg (one microkilogram) peta P exa E zetta Z yotta Y 10 24

10 1.3 Circuit Analysis: An Overview 10 A circuit is said to be solved when the voltage across and the current in every element have been determined.

11 1.4 Voltage and Current 11 Electric Charge The fundamental electric quantity Charge is an electrical property of the atomic particles of which matter consists. Each atom consists of electrons, protons, and neutrons. The nucleus is positively charged and has the protons and neutrons. Electrons are negatively charged and in discrete shells. The presence of equal numbers of protons and electrons leaves an atom neutrally charged. The concept of electric charge is the underlying principle for explaining all electrical phenomena. The following points should be noted about electric charge: The charge is bipolar. - Positive change: proton, +ion, hole - Negative charge: electron, -ion Electrons and protons are often referred to as elementary charge: e = C(Coulomb) Charge quantization: the electric charge exists in discrete quantities, which are integral multiples of the elementary charge. The law of conservation of charge 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. - ion electron gain atom electron loss + ion

12 1.4 Voltage and Current 12 Electric Charge Electrical effects are attributed to both the separation of charge and charges in motion. Electric force(voltage) +q +q Like charges repel The separation of change creates an electric force(voltage). +q -q Unlike charges repel Electric current The motion of charge creates an electric current. +q v

13 1.4 Voltage and Current 13 Definition of Voltage Voltage is electric potential energy per unit charge, measured in joules per coulomb (= volts). Voltage (or potential difference) is the energy required to move a unit charge(one coulomb of charge) from one point to the other. Mathematically, v = dw dq V 1 J 1 C = 1 Volt = 1 V +Q catapult dw Voltage is responsible for establishing current. +dq Potential energy: Energy which results from position or configuration. An object may have the capacity for doing work as a result of its position in a gravitational field (gravitational potential energy), an electric field (electric potential energy), or a magnetic field (magnetic potential energy). It may have elastic potential energy as a result of a stretched spring or other elastic deformation.

14 1.4 Voltage and Current 14 Definition of Current Electric current is the amount of charge that flows past a point in a unit of time (t), measured in amperes (A). Mathematically, i i = dq dt C s = A A Current flow in an electric conductor If current I = 1 A in a wire, then 1 coulomb of charge flows past any point every second.

15 1.5 The Ideal Basic Circuit Element 15 Definition of Ideal Basic Circuit Element An ideal circuit element is a mathematical model of an actual electrical component, like a battery or light bulb: Physical representation Circuit(symbolic) representation An ideal basic circuit element has three attributes: It has only two terminals, which are points of connection to other circuit components. It is described mathematically in terms of current and/or voltage. It cannot be subdivided into other elements. The word ideal imply that a basic circuit element does not exist as a realizable physical component. The word basic imply that the circuit element cannot be further reduced or subdivided into other elements. Thus the basic circuit element s form the building blocks for constructing circuits, but they themselves cannot be modeled with any other type of element.

16 1.5 The Ideal Basic Circuit Element 16 Passive Sign Convention The assignment of the reference polarity for voltage and the reference direction for current are entirely arbitrary. However, once you have assigned the references, you must write all subsequent equations to agree with the chosen references. The most widely used sign conventions applied to these references is called passive sign convention. Whenever the reference direction for the current in an element is in the direction of the reference voltage drop across the element, use a positive sign in any expression that relates the voltage to the current.

17 1.6 Power and Energy 17 Power and energy calculations are important in circuit analysis For practical purposes, we need to know how much power an electric device can handle. We all know from experience that a 100-watt bulb gives more light than a 60-watt bulb. We also know that when we pay our bills to the electric utility companies, we are paying for the electric energy consumed over a certain period of time. Therefore, although current and voltage are the two basic variables in an electric circuit, they are not sufficient by themselves. Definition of Energy Energy is the capacity(ability) to do work, measured in joules ( J). One joule is the work done when a force of one newton is applied through a distance of one meter. Mathematically, work can be expressed by the following equation. work w = F d cosθ To do work(w), forces(f) must cause displacements(d) The work done on the briefcase by the generator is negative, removing energy from the briefcase

18 1.6 Power and Energy 18 Definition of Power Power is the time rate of expending or absorbing energy, measured in watts (W). Mathematically, p = dw dt J s = W p:power in watts(w) w:energy in joules(j) t: time in second(s) Electrical Energy The kilowatt-hour (kwh) is a much larger unit of energy than the joule. There are 3.6 x 10 6 J in a kwh. The kwh is convenient for electrical appliances.

19 1.6 Power and Energy 19 Watt s law The electrical power associated with a basic circuit element is simply equal to the product of the voltage across the element and the current flowing through it. p = dw dt = dw dq dq dt = vi Passive Sign Convention It is important to realize that, just like voltage, power is a signed quantity, and that it is necessary to make a distinction between positive and negative power. Passive sign convention: (a),(d) p = +vi or vi > 0: The current enters through the positive polarity of the voltage(through the positive terminal of an element) It implies that the element is absorbing power or power is being delivered to the circuit inside the box. (b),(c) p = -vi or vi < 0: The current enters through the negative polarity of the voltage(through the negative terminal of an element) It implies that the element is releasing or supplying power or power is being extracted from the circuit inside the box.