Presented by Byoung-Kuk Lee, Ph. D., Senior IEEE

Presented by Byoung-Kuk Lee, Ph. D., Senior IEEE

Energy Mechatronics Lab.

College of Information and Communication Eng.

Sungkyunkwan University Tel: +82-31-299-4581 Fax: +82-31-299-4612 http://seml.skku.ac.kr EML: bkleeskku@skku.edu

Circuit Variables

Circuit Theory

• Electric circuit: a mathematical model that approximates the behavior of an actual electrical system

• Circuit theory: A special case of electromagnetic (EM) field theory - the study of static and moving electric charges.

electric circuit model

Circuit Variables

Three Basic Assumptions in Circuit Theory

1. Electrical effects happen instantaneously throughout a system.

- If the dimension of the system is less than 1/10th of  = c/f, (c = 310⁸ m/s)

 modeled as a lumped parameter system

(parameters do not depend on the location within the system)

f = 60 Hz (power system),  = 510⁶ m [Circuit theory]

 Lumped parameter system

f = 10⁹ Hz (cell phone),  = 0.3 m [EM theory (microwave)]

f = 4.510¹⁴ Hz (red light),  = 65010^-9 m [EM theory (optics)]

 Distributed parameter system

2. The net charge on every component in the system is always zero.

- Charge conservation law

3. There is no magnetic coupling between the components in a system.

Circuit Variables

The Scope of Circuit Theory

• Circuit theory provides simple solutions with sufficient accuracy to problems that would become complicated if electromagnetic field theory is used.

• However, electromagnetic field theory is essential to obtain

solutions in nano-sized dimensions or visible wavelength ranges.

Circuit theory EM theory

Circuit Variables

The International System of Units (SI)

• The same units of measure used by all the major engineering societies

cd candela

Luminous intensity

mol mole

Amount of substance

K degree kelvin

Thermodynamic temperature

A ampere

Electric current

s second

Time

kg kilogram

Mass

m meter

Length

Symbol Basic Unit

Quantity

cd candela

Luminous intensity

mol mole

Amount of substance

K degree kelvin

Thermodynamic temperature

A ampere

Electric current

s second

Time

kg kilogram

Mass

m meter

Length

Symbol Basic Unit

Quantity

Circuit Variables

The International System of Units (SI)

Derived Units in SI

Prefixes to signify powers of 10

Circuit Variables

Circuit Analysis

 Circuit analysis is understanding the behavior of each ideal circuit element in terms of voltage and current.

A conceptual model for electrical engineering design

Circuit Variables

Voltage & Current

• Voltage is the energy per unit charge created by charge separation.

dq

v  dw v = the voltage in volts w = the energy in joules q = the charge in coulombs

• Electric current is the rate of charge flow.

dt

i  dq

Circuit Variables

The Ideal Basic Circuit Element

• Three attributes:

1. Only two terminals, which are points of connection to other circuit components.

2. Described mathematically in terms of current and/or voltage.

3. Cannot be subdivided into other elements.

[Polarity Reference]

Positive voltage: voltage drop from terminal 1 to terminal 2 Positive current : charge flow from terminal 1 to terminal 2

Circuit Variables

Passive Sign Convention

• Uses a positive sign in the expression that relates the voltage and current at the terminals of an element when the reference directi on for the current through the element is in the direction of the re ference voltage drop across the element.

Positive sign Negative sign

Circuit Variables

Power & Energy

• Power is energy per unit of time.

dt

p  dw

vi

dt dq dq

p dw



 

 



 



 



 

v = the voltage in volts i = the current in amperes

Circuit Variables

Algebraic Sign of Power

Polarity references and the expression

for power

• If p > 0, power is being delivered to the circuit inside the box.

 the circuit consumes power

• If p < 0, power is being extracted from the circuit inside the box.

 the circuit generates power

Circuit Variables

Sign Convention in Solar Cells

[Solar cell]

a device that directly

generates electricity by sunlight

Circuit model of the solar cell

( )

p vi

V I VI



   

1 2

Circuit Variables

I-V Characteristics of Solar Cells

Circuit Variables

Assessment problem 1.7