1. EENG 2610: Circuit Analysis Class 1: Basic Concepts, Ohm’s Law
Oluwayomi Adamo
Department of Electrical Engineering
College of Engineering, University of North Texas

2. Electro-technology is driving force in all engineering discipline Circuit analysis is fundamental to electro-technology
Power Grid
This course is your first course in Electrical Engineering. It’s very important to master the materials that you will learn in this course to become a good Electrical Engineer. This course is the foundation to almost all other EE courses that you will take in the future.
Even though you may not have noticed, electrical technology is one of the most fundamentally important technologies in this world and we have been relying on electrical technology for our daily life.
Motherboard of Computer
Integrated Circuits (IC chips)

3. Basic Strategy in Circuit Analysis
Typical Electric Circuit

4. The concept of node is extremely important.
ELECTRIC CIRCUIT IS AN INTERCONNECTION OF ELECTRICAL COMPONENTS
The concept of node is extremely important.
We must learn to identify a node in any shape or form

5. BASIC CONCEPTS LEARNING GOALS
System of Units: The SI standard system; prefixes
Basic Quantities: Charge, current, voltage, power and energy
Circuit Elements: Active and Passive

6. International System of Units – SI Standard System

7. Standard SI Prefixes
Give some example: For example 2x10^3 m = 2 km, 2x10^9 m = 2 Gm, ……
SI prefixes used to form decimal multiples and submultiples of SI units.
These standard prefixes are employed throughout our study of electric circuits.

8. Basic Quantities Electric Charge (unit: coulomb) Q, q, q(t)
The most elementary quantity in electric circuit analysis
Charged particle in matter: electron (-), proton (+), neutron (no charge)
Electric Circuit
A pipeline where electric charge can be transferred from one point to another
An interconnection of electrical components, each of which we will describe with a mathematical model
Electric Current (unit: ampere) i, i(t)
The time rate of change of charge:
1 A = 1 C/s (A: ampere, C: coulomb, s: second)
Conventional current flow represents the movement of positive charges, even though in metallic conductors current flow is resulted from the motion of electrons, negative charge.
I = 2 A means 2 C of charge pass from left to right each second
Must specify both magnitude and direction:
Show on white board: equivalent circuit for negative current.

9. Basic Quantities Two types of current we will study in this courseDC AC
Two types of current we will study in this course
Alternating current (AC)
Direct Current (DC)
Voltage (or potential) between two points in a circuit (unit: volt)
Defined as the difference in energy level of a unit charge located at each of the two points:
The energy required to move a unit positive charge is the defined voltage
1 V = 1 J/C = 1 N·m/C (V: volt, J: joule, C: coulomb, N: newton, m: meter)
The + and – signs define a reference direction for V
A unit charge moved between A and B will have energy change
Must specify both magnitude and direction

10. Basic Quantities Energy and Energy Transfer W, w(t)
When the element is absorbing energy, a positive current enters the positive terminal and leaves via the negative terminal.
When the element is supplying energy, a positive current enters the negative terminal and leaves via the positive terminal.
A negative current in one direction is equivalent to a positive current in the opposite direction, and vice versa. The same is the voltage.
Charges gain
energy as passing
Charges spend
Energy as passing
Vbulb
Vbattery

11. Basic Quantities Power (unit: watt) P, p(t)
Defined as the time rate of change of energy:
The change in energy in a period of time:
1 W = 1J/s = 1 V·A (W: watt, J: joules, V: volt, A: ampere)
Passive Sign Convention – Sign Convention for Power
Variables for the current and voltage should be arranged as shown in the figure:
Current enters an element via positive voltage reference point
If the sign of power is positive, power is being absorbed by the element; if the sign is negative, power is being supplied by the element. + -
Circuit Element
Power:

12. Example 1.2: Determine whether the elements are supplying or receiving power and how much

13. Circuit Elements
In general, all elements will be terminal devices that are completely characterized by the current through the element and the voltage across it.
Active or passive elements
Active element is capable of generating energy.
Passive element cannot generate energy.
Passive Elements
Resistor, Capacitor, Inductor
We will define these in coming classes
Independent Voltage Source
Maintain a specified voltage between its terminals regardless of the current through it.
Independent Current Source
Maintain a specified current between its terminals regardless of the voltage across its terminals.
Independent sources normally supply energy, but they can also absorb energy

14. Circuit Elements Dependent (or Controlled) Sources
Unlike independent sources, dependent (or controlled) sources generate a voltage or current that is determined by a voltage or current at a specified location in the circuit.
Four different types of dependent sources:
Voltage
Controlled
Sources
Current
controlled
Sources

15. The Principle of Conservation of Energy
Power supplied in a circuit network is exactly equal to the power absorbed. Electrical circuits satisfy this principle
Example 1.7: Use power balance to compute I0

16. Ohm’s Law Ohm’s Law Defines a passive element Resistor R (unit: ohm)
It only absorbs power; converts electrical energy to thermal energy
Ohm’s Law: The voltage across a resistor is directly proportional to the current flowing through it:
1 Ω = 1 V/A (Ω: ohm, V: volt, A: ampere)
Linear approximation
Linear range
Actual v-I relationship

17. Ohm’s Law Two specific values of resistance
Power absorbed by a resistor
Conductance G (unit: siemens S)
1 S = 1 A / V
Two specific values of resistance - = + v = i
Short Circuit
Open Circuit

18. Example 2.1: Determine voltage, current, and power absorbed by resistor