Alternating Current
On the other hand, a current that varies with time, reversing direction periodically, is called alternating current, abbreviated as AC.
3Ω
4Ω 2Ω 3Ω a 6Ω
10V
Congsidering current I， N=4,B=6,L=7
N=3,B=5,L=6
1.4 Kirchhoff’s current LAW (KCL)
The net(净，总的) current a node is zero.
Ｉ= 0


entering
• Electrical Engineering Subdivisions
• • • • • • • • Communication systems Computer systems Control systems Electromagnetics Electronics Photonics Power systems Signal processing
P1 P2 P3 P4 P5 90 20 120 90 80 0
功率平衡！电路中所有元件的功率之和为 0 ！ 常用作对分析结果的检验准则。
•About Kirchhoff
古斯塔夫· 罗伯特· 基尔霍夫(Gustav Robert Kirchhoff )
——/wiki/Gustav_Kirchhoff
Box3
P3 U 3 I 3 60V ( 2)A 120W<0
U1 1
U2 2 U5
source Box4 P4 U 4 I 4 30V 3A 90W>0 Box5 load
4
U4
3
U3
5
P5 U 5 I 5 80V ( 1)A 80W<0 source 注 意：
R2 +
U2 -
I1+ I2 = I3
（入） （出）
•推广到广义节点上使用，任一闭合面
IA+IB+IC=0 【Prove】 IA A
IA= IAB- ICA
IB= IBC- IAB IC= ICA- IBC
IAB
IB B IBC
ICA
C
IC Add all three equations： IA+IB+IC=0 or ∑I=0
• The units are amperes (A), which are equivalent to coulombs per second (C/s).
The charge on an electron is -1.602E-19 C
•Electrical Current
•Actual direction： 电流的实际方向为正电荷的运动方向。
a
＋
I U
b
－
P UI U IR
The current reference enters the positive polarity of the voltage.
•Function of Element
Passive reference direction:
a i p + u -
p ui
•Example
N、B、L=？ I、Uab=？
a
+
2Ω
I
b
+
6V
12V
-
5Ω
-
1Ω
1Ω
5Ω
N=2,B=3,L=2 I=0,Uab=0
•Example
图示电路中，已知I1=11mA，I4=12mA，I5=6mA。 求I2，I3和I6。 solution： I3=I1－I5=11－6=5（mA)
I6
1.2 Circuits, Currents and Voltages
•Electrical Circuit
An electrical circuit consists of various types of circuit elements connected in closed paths by conductors.
if
if
P UI 0
P UI 0
then
then
positive
negative
power
power
absorb
sup ply b
a i p
Non passive reference direction:
p ui
then then
if if
P UI 0 P UI 0
Electrical Engineering and Electronics II
Chapter 1 Introduction
4 Course Hours
1. Introduction
1.1 Overview of Electrical Engineering 1.2 Circuits, Currents, and Voltages 1.3 Power and Energy 1.4 Kirchhoff's Current Law 1.5 Kirchhoff's Voltage Law 1.6 Introduction to Circuit Elements 1.7 Introduction to Circuits
• Why Study Electrical Engineering?
•To pass the Fundamentals of Engineering (FE) Examination •To have a broad knowledge and you can lead design projects in your own field •To be able to operate and maintain electrical systems •To communicate with electrical engineering consultants
ia=ib=ic
The current is identical.
Ex.1 Use KCL to determine the values of the unknown currents shown in Fig. 1.21
Answer: ia=4A ib=-2A ic=-8A
Identify the groups of the circuit elements that are connected in series shown in Fig. 1.21
U1 1 U2 2
Solution：box 1
P1 U1 I1 30V 3A 90W>0
Box 2 Load: Absorb
4
P2 U 2 I 2 20V 1A 20W>0
Load: Absorb
U4
3
U3
5
U5
U 1 30V, U 2 20V, U 3 60V, U 4 30V, U 5 80V I1 3A, I 2 1A, I 3 2A, I 4 3A, I 5 1A
R2
I2
I4
I2=I3－I4=5－12=－7（mA)
I1 I6=I1－I2=11－(－7)=18（mA) I6=I4+I5=12+6=18（mA)
R1
R3
I3
I5
•Series Circuits
Series: Elements are connected end to end, no other element can be connected to their common node.
i (t ) 200e 100t
t 0
200
t0 0
t (ms)
Direct Current
When a current’s direction is constant with time, we say that we have direct current, abbreviated as DC.
dq(t ) i (t ) dt q(t ) i (t )dt q(t0 )
t0 t
Example 1.1 Determining Current Given Charge
q(t )( C )
2
0
q( t ) 0
t0
100t
q( t ) 2 2e
t
400
t0
dq(t ) Solution : i (t ) dt 0 t0 200e 100t
Alternatively, the sum of the currents entering a node equals the sum of the currents leaving a node.
Ｉ入 = Ｉ出
I1 c
a
I2 d
+ R1 I1+ I2-I3=0
Or -I1- I2+I3=0 也可写成： U1 R3 I3 b
•Concepts
支路——branch:流过同一电流的电路分支 节点——node: 三条及以上支路的交点
回路——loop：由支路组成的的闭合路径（可
回到出发点）
I1
c + R1 E1 -
a R2 + R3 I3 b
I2
d
E2 -
b=3 n=2 l=3
2Ω 4Ω
a