1. Chapter 12: Electrical current and related parameters
서강대학교
전자공학과

2. Electric current as a fundamental dimension
Ampere
Base unit for electric current
Propose at 1946
Constant current which, if maintained in tow straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in a vacuum, woul produce 2X10-7 newton per meter
Electric current (or simply current)
The flow of electric charge
1 A defined as the flow of 1 unit of charge per second 2

3. Electric current as a fundamental dimension
Subatomic particles
Electrons: negative charge
Nucleus
Protons (양자): positive charge
위 쿼크 2개, 아래 쿼크 1개
Neutrons: (중성자) no charge
위쿼크 1개, 아래 쿼크 2개
응용: 원자력 발전 (핵분열에 의한 에너지 발생을 이용)
Cf. down quark: -(1/3) e 전하량, up quark: +(2/3) e 전하량
핵자 질량의 대부분은 쿼크를 합쳐주는 글루온의 에너지에서 유래
Basic law of electric charge
Unlike charge attract each other
Like charge repel each other 3

4. Electric Charge 1 coulomb (C) Electric force
Unit of charge
Amount of charge that passes a point in a wire in 1 second when a current of 1 ampere is flowing through the wire
Electric force
Attractive electric force between two opposite charge particles
Coulomb’s law
k = 8.99 X 109 Nm2/C2
q = point charge (C)
r = distance (m)
Electric charge is conserved
It is not created nor destroyed
It can only transfer from one object to another 4

5. Voltage
Voltage represents the amount of work required to move charge between two points.
Electromotive force (emf) represents the electric potential difference between an area with an excess of free electrons and an area with an electron deficit.
The most common sources of electricity are chemical reaction, light, and magnetism. 5
Electric current and related parameters

6. Voltage Batteries Electricity is produced by the chemical reaction.
Size: AA, AAA, C, and D
Primary cell vs. secondary cell 6

7. Voltage Photoemission Another principle used to generate electricity.
When light strikes a surface that has certain properties, electrons can be freed – electric power is generated.
Sun’s radiation is converted to electricity
Photothermal
Sun’s radiation is used to make steam by heating water.
Photovoltaic
The light is converted directly to electricity.
Gallium arsenide: conversion efficiencies of 20% 7

8. Voltage
Power Plants
Water is used in all steam power-generating plants to produce electricity.
Figure 12.3 8

9. Direct current and alternating current
Direct current (dc)
The flow of electric charge that occurs in one direction.
dc is typically produced by batteries and dc generators.
Alternating current (ac)
The flow of electric charge that periodically reverses.
Domestic and commercial power: 60Hz 9

10. Direct current and alternating current
Kirchhoff’s current law
At any given time, the sum of the currents entering a node must be equal to the sun of the current leaving the node.
The law represents the physical fact that charge is always conserved in an electrical circuit. 10

11. Direct current and alternating current
Residential power distribution 11

12. Direct current and alternating current
Residential power distribution 12

13. Electrical circuits and components
The combination of various electrical components that are connected together.
Electrical components: wires, switches, outlets, resistors, and capacitors.
Resistivity
A measure of resistance of a piece of material to electric current.
superconductivity 13

14. Electrical circuits and components
Resistivity 14

15. Electrical circuits and components
Ohm’s Law
The law describes the relationship among voltage, resistance, and current.
When there exists an electrical potential of 1 volt across a conductor with a resistance of 1 ohm, then 1 ampere of electric current will flow through the conductor.
The American Wire Gage (AWG)
The actual size of wires is commonly expressed in terms of gage number.
AWG is based on successive gage numbers having a constant ratio of approximately 1.12 between their diameters. 15

16. Electrical circuits and components
Electric power
The electric power consumption of various electrical components can be determined using the following power formula:
The kWh units are used in measuring the rate of consumption of electricity by homes and business. 16

17. Electrical circuits and components
Series circuit
Resistor: an electrical component that resists the flow of either direct or alternating current.
Commonly used to protect sensitive components or to control the flow of current in a circuit.
Fixed-value and variable
The electric current flowing through a series of elements in an electric circuit is the same.
The voltage drop across each element can be determined using Ohm’s law.
The sum of the drop across each element is equal to the total voltage supplied to the circuit.
The total resistance is the sum of resistance in the circuit. 17

18. Electrical circuits and components
Series circuit 18

19. Electrical circuits and components
Parallel circuit 19

20. Electrical circuits and components
Capacitor
An electrical component that stores energy.
A capacitor has two oppositely charged electrodes with a dielectric material inserted between the electrodes.
In tuned circuits for radio receivers, audio filtering applications, timing elements for strobe lights, and intermittent wipers in automobiles.
Farad(F) is basic unit used to designate the size of a capacitor. 20

21. Electric motors
Factors that engineers consider when selecting a motor for an application
Motor type, motor speed, motor performance in terms of torque output, efficiency, duty cycles, cost, life expectancy, noise level, maintenance and service requirements.
Motor type
Table 12.3 21

22. Electric motors Duty cycle
Motors can be classified according to the amount of time the motor needs to be operated.
Continuous duty
Over a period of an hour or longer
Intermittent duty 22