1. ECE 4991 Electrical and Electronic Circuits Chapter 2

2. Hitchhikers Guide to the Galaxy
Chapter 2 - The basic concepts and practice at analyzing simple electric circuits with sources and resistors
Chapter 3 – More harder networks to analyze and the notion of equivalent circuits
Chapter 4 – Capacitors and inductors added to the mix
Chapter 5 – Analyzing transient situations in complex passive networks
Chapter 8 – New subject – the wonders of operational amplifiers as system elements
Chapter 9 – Introduction to semiconductors – the basics and diodes – more network analysis
Chapter 10 – Bipolar junction transistors and how they work – now you can build your own op amp

3. What’s Important in Chapter 2
Definitions
KCL & applications
KVL & applications
Electric power / power ratings
Ohm’s Law
Equivalent resistance computation
Voltage Dividers / Current Dividers
Ideal v real voltage sources

4. 1. Definitions Voltage Current Charge Power Energy Network Branch Node
Loop
Mesh

5.  in = 0 2. KCL & Applications
First, note that for current to flow, there must be a closed circuit
Kirchhoff’s Current Law (KCL)
 in = 0

6. 2. Applying KCL
Find nodes
Label currents
Write node equations

7. 2 ½ . Current Speed
Metals are seas of electrons – each atom gives up an electron which can move and be part of the current
i = dq/dt = charge density/length of wire * carrier velocity
Therefore carrier velocity = v = i / charge density
Charge density = e * ~3x1023/m3 = ~5x104 Coul/m3
For a 1 amp current, v = ~ 2x10-5 m/s = ~ .02 mm/s !

8.  vn = 0 3. KVL & Applications
First, note that for KVL to work, there must be a closed circuit
Kirchhoff’s Voltage Law (KVL)
 vn = 0

9. 3. Applying KVL
Label voltages
Pick loops
Write loop equations

10. 4. Electric Power / Power Ratings
Power is work done per unit time
For an electronic element, Power is the product of the voltage across the element and the current flowing through it
P = VI
“Positive Power” is dissipated by a load
“Negative Power” is provided by an energy source

11. 4 ½. I-V Characteristics i v

12. 5. Ohm’s Law
A resistor is a passive electronic circuit element with the following property – the voltage developed across it is directly proportional to the electric current through it
V = IR
R is measured in volts per amp
1 volt/amp = 1 ohm (Ω)
What are some ways to calculate the power dissipated in resistor R?

13. 5. Working with Ohm’s Law

14. 6. Equivalent Resistance Computation

15. 6. Equivalent R For series resistors Req =  Ri For parallel resistors

16. 6. Equivalent R Practice

17. 7. Voltage & Current Dividers
A voltage divider accesses a fraction of a voltage
A current divider accesses a fraction of a current

18. 8. Ideal v Real Sources Ideal sources are independent of loading
Real sources have equivalent series or parallel internal resistances I I