Electric Circuits 电路 FU Li-jun 付立军 Dalian Nationalities University 大连民族学院

Introduction Part 1 DC Circuits （直流电路） （交流电路） （高级电路分析） Part 2 AC Circuits Part 3 Advanced Circuit Analysis

1.1 Current 1.3 Power and Energy 1.2 Voltage 1.4 Circuit Elements Chapter 1 Basic Concepts

An electric circuit is an interconnection of electrical elements （元件）

1.1 Current Electric current is the time rate of change of charge, measured in amperes (A) 电流

Two main types Direct Current (DC) : Current remains constant with time, represented by the symbol I Alternating Current (AC) : Current varies sinusoidally with time, represented by the symbol i

The direction of current flow is conventionally taken as the direction of positive charge movement Direction of current must be specified by an arrow

Reference direction 参考方向

1.2 Voltage Voltage (or potential difference （电位差） ) is the energy required to move a unit charge through an element, measured in volts (V) Mathematically, 电压

The plus(+) and minus (-) signs are used to define reference direction or voltage polarity reference ground 参考地 v a =9V v b =- 9V

1.3 Power and Energy Power is the time rate of absorbing energy, measured in watts (W) The power p is called the instantaneous power 功率和能量 瞬时功率

Passive sign convention If p=vi>0, the element is absorbing power If p=vi<0, the element is supplying power The current enters through the positive terminal of an element. absorbed power supplied power 关联参考方向 吸收功率 发出功率

Law of conservation of power: Σp = 0 The algebraic sum of power in a circuit, at any instant of time, must be zero: 功率守恒定律

1.4 Circuit Elements 1. Active elements : 2. Passive elements: Two types of elements: Independent source Dependent source Capable of generating energy resistors, capacitors, inductors 电阻 电容 电感 电路元件 有源元件 独立源 受控源 无源元件

independent voltage source An ideal independent source is an active element that provides a specified voltage or current that is completely independent of other circuit elements. independent current source V I i + - v

Symbols for (a) controlled voltage source (b) controlled current source An ideal dependent (or controlled) source is an active element in which the source quantity is controlled by another voltage or current.

1. A voltage-controlled voltage source (VCVS) 2. A current-controlled voltage source (CCVS) 4. A voltage-controlled current source (VCCS) 3. A current-controlled current source (CCCS) Four possible types of controlled sources, namely: 电压控制电压源 电流控制电压源 电流控制电流源 电压控制电流源

Example 1.1 Calculate the current i in the circuit. Solution: Since so

Example 1.2 Calculate the power supplied or absorbed by each element. absorbed supplied absorbed Solution:

部分电路图和内容参考了： 电路基础（第 3 版），清华大学出版社 电路（第 5 版），高等教育出版社 特此感谢！