E E 2315 Circuits I Lecture 1 - Introduction to Electric Circuits

Overview of Electrical Engineering The electrical engineer combines the physicist’s models of natural phenomena with the mathematician’s tools for manipulating those models to produce systems that meet practical needs. Electrical systems can be divided into five major classifications.

5 Linked Classifications of Electrical Systems Communications Systems Computer Systems Control Systems Power Systems Signal Processing Systems

Communications Systems Telephony –Analog and Digital –Switched Channels Radio –Broadcast AM, FM, & SW –Two-Way Television

Computer Systems Programmable Microcircuits High-speed switching of logic circuits Used for –Computation –Control

Control Systems Automated Adaptable Faster Operation than Manual More Reliable than Manual Modern high-performance aircraft rely on automated control systems

Power Systems Large networks connected by low frequency a-c transmission lines Small networks in aircraft and spacecraft Electromechanical Energy Conversion Power Electronics –High frequency switching converters –High efficiency, high power density

Signal Processing Systems Transform and manipulate signals and the information they contain Image processing –Data from weather satellites –MRI scans of the human body Noise reduction Encryption

Circuit Theory Mathematical modeling of physical phenomena Circuit theory is a special case of electromagnetic field theory –General EM theory is more complex than circuit theory –General EM theory requires more complex mathematics

Three Basic Assumptions of Circuit Theory Electric effects happen instantaneously throughout a system Net Charge on every component in the system is zero No magnetic coupling between components

Problem Solving Identify what is given and what is to be found. Sketch a circuit diagram or other visual model Think of several solution methods and a way of choosing between them Calculate a solution

Problem Solving (continued) Use your creativity –If your efforts are not converging to a solution, you may want to rethink your assumptions. Test your solution –Is your answer reasonable? –Does your answer validate your assumptions?

International System of Units –Frequencyhertz (Hz)s -1 –Forcenewton (N)kg·m/s 2 –Energy or workjoule (J)N·m –Powerwatt (W)J/s –Electric chargecoulombA·s –Electric potentialvolt (V)W/A –Electric resistanceohm (  )V/A –Electric conductancesiemens (S)A/V

International System of Units (continued) –Electric capacitancefarad (F)C/V –Magnetic fluxweber (Wb)V·s –Inductancehenry (H)Wb/A

Standardized Prefixes attoa femtof picop nanon10 -9 micro  millim10 -3 centic10 -2 decid10 -1 dekada10 hectoh10 2 kilok10 3 megaM10 6 gigaG10 9 teraT10 12

Circuit Analysis: An Overview A circuit model is used to connect our visualization to our analysis of a physical system The elements of our circuit model are ideal circuit components. The behavior of output parameters is governed by physical/mathematical laws for the elements of the circuit model.

Voltage and Current Voltage is the energy per unit of charge. Current is the rate of flow of charge.

Voltage and Current (continued) The relationship between voltage and current in a circuit element defines that circuit element. Both voltage and current have associated polarities. These polarities determine the direction of power flow.

Ideal Basic Circuit Element Three attributes of an ideal circuit element: –There are only two terminals –Described mathematically in terms of current and/or voltage –Cannot be subdivided into smaller components

Voltage Polarity Definitions Positive v –voltage drop from 1 to 2 or –voltage rise from 2 to 1 Negative v –voltage drop from 2 to 1 or –voltage rise from 1 to 2

Current Polarity Definitions Positive i –Positive charge flowing from 1 to 2 –Negative charge flowing from 2 to 1 Negative i –Positive charge flowing from 2 to 1 –negative charge flowing from 1 to 2

Passive Sign Convention Whenever the reference direction for the current in an element is in the direction of the reference voltage drop, use a positive sign in any expression that relates voltage to current. Otherwise, use a negative sign.

Power and Energy Power associated with a circuit element is consumed by that circuit element when the value of power is positive. Conversely, power is generated, or produced by the element if the value consumed is negative.

Expression of Power