Chapter 01 asas elektrik

7/29/2019 Chapter 01 asas elektrik

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Electricity

Topics Covered in Chapter 1

1-1: Negative and Positive Polarities

1-2: Electrons and Protons in the Atom1-3: Structure of the Atom

1-4: The Coulomb Unit of Electric Charge

1-5: The Volt Unit of Potential Difference

Chapter

1

2007 The McGraw-Hill Companies, Inc. All rights reserved.


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Topics Covered in Chapter 1

1-6: Charge in Motion Is Current

1-7: Resistance Is Opposition to Current

1-8: The Closed Circuit

1-9 The Direction of Current

1-10: Direct Current (DC) and Alternating Current (AC)

1-11: Sources of Electricity

1-12: The Digital Multimeter


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Electrons and Protons:

All the materials we know, including solids, liquids and

gases, contain two basic particles of electric charge: the

electron and the proton. The electron is the smallest particle of electric charge

having the characteristic called negative polarity.

The proton is the smallest particle of electric charge

having the characteristic called positive polarity.


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The arrangement of electrons and protons in a

substance determines its electrical characteristics.

When the number of protons and electrons in asubstance are equal, they cancel each other out,

making the substance electrically neutral.


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To use the electrical forces associated with the negative

and positive charges in matter, the electrons and

protons must be separated.

Changing the balance of forces produces evidence of

electricity.


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Fig. 1-1: Positive and negative

polarities for the voltage output of a

typical battery.

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


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1-2: Electrons and Protons in theAtom

A hydrogen atom contains one proton in its nucleus. This

is balanced by one orbiting electron. A hydrogen atom

contains no neutrons in its nucleus.

Fig. 1-2: Electron and proton in the hydrogen (H) atom.



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Electrons are distributed in orbital rings around the

nucleus.

The distribution of electrons determines the atoms

electrical stability. The electrons in the orbital ring farthest from the

nucleus are especially important.

If electrons in the outermost ring escape from the atom

they become free electrons. Free electrons can move from one atom to the next and

are the basis of electric current.


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Fig. 1-3: Atomic structure showing the nucleus and its orbital rings of electrons. (a) Carbon (C)

atom has 6 orbital electrons to balance 6 protons in the nucleus. (b) Copper (Cu) atom has 29

protons in the nucleus and 29 orbital electrons.



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When electrons in the outermost ring of an atom can

move easily from one atom to the next in a material, the

material is called a conductor.

Examples of conductors include: silver

copper

aluminum.


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When electrons in the outermost ring of an atom do not

move about easily, but instead stay in their orbits, the

material is called an insulator.

Examples of insulators include: glass

plastic

rubber.


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Semiconductors are materials that are neither good

conductors nor good insulators.

Examples of semiconductors include:

carbon silicon.

germanium


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1-3: Structure of the Atom

Atomic Number

The atomic numberof an element is the number of

protons in the nucleus of the atom balanced by an equal

number of orbiting electrons. The number of electrons in orbit around the nucleus of a

neutral atom is equal to the number of protons in the

nucleus.


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Orbital Rings

Electrons are contained in successive rings beyond the

nucleus. The rings are called K, L, M, N, O, P, and Q,

respectively. Each ring has a maximum number of electrons for

stability. They are:

K ring = 2 electrons.

L ring = 8 electrons.M ring = 8 or 18 electrons.

N ring = 8,18, or 32

electrons.

O ring = 8 or 18 electrons

P ring = 8 or 18 electrons

Q ring = 8 electrons


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The maximum number of electrons in the outermost ringis always 8.

The electron valence of an atom is the number ofelectrons in an incomplete outermost shell. The valenceindicates how easily the atom can gain or loseelectrons.

An atoms nucleus contains neutrons as well asprotons.

Neutrons have no net electric charge.


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One Atom of Copper

K

K = 2

(complete)

L

L = 8

(complete)

M

M = 18(complete)

N = 1

(incomplete)N

Atomic number = 29



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1-4: The Coulomb Unit of ElectricCharge

Most common applications of electricity require the

charge of billions of electrons or protons.

1 coulomb (C) is equal to the quantity (Q) of 6.25 10

18

electrons or protons.

The symbol for electric charge is Q orq, forquantity.


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Negative and Positive Polarities

Charges of the same polarity tend to repel each other.

Charges of opposite polarity tend to attract each other.

Electrons tend to move toward protons becauseelectrons have a much smaller mass than protons.

An electric charge can have either negative or positive

polarity. An object with more electrons than protons has

a net negative charge (-Q) whereas an object with moreprotons than electrons has a net positive charge (+Q).

An object with an equal number of electrons and

protons is considered electrically neutral (Q = 0C)


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Charge of an Electron

The charge of a single electron, orQe, is 0.16 1018 C.

It is expressed

Qe= 0.16 1018 C

(Qe indicates the charge is negative.)

The charge of a single proton, QP, is also equal to

0.16 1018 C .

However, its polarity is positive instead of negative.


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1-5: The Volt Unit of PotentialDifference

Fig. 1-7: The amount of work required to move electrons between two charges depends on their

difference of potential. This potential difference (PD) is equivalent for the examples in (a), (b),

and (c).



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1-6: Charge in Motion Is Current

The amount of current is dependent on the amount of

voltage applied.

The greater the amount of applied voltage, the greater

the number of free electrons that can be made to move,producing more charge in motion, and therefore a larger

value of current.

Current can be defined as the rate of flow of electric

charge. The unit of measure for electric current is the

ampere (A).

1 A = 6.25 1018 electrons (1C) flowing past a given

point each second, or 1A= 1C/s.

The letter symbol for current is Iori, forintensity.


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1-7: Resistance IsOpposition to Current

Resistance is the opposition to the flow of current.

A component manufactured to have a specific value of

resistance is called a resistor.

Conductors, like copper or silver, have very lowresistance.

Insulators, like glass and rubber, have very high

resistance.

The unit of resistance is the ohm (). The symbol for resistance is R.


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1-7: Resistance IsOpposition to Current

Fig. 1-10: (a) Wire-wound type of resistor with cement coating for insulation. (b) Schematic

symbol for any type of fixed resistor.



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A circuit can be defined as a path for current flow. Any

circuit has three key characteristics:

1. There must be a source of potential difference

(voltage). Without voltage current cannot flow.

2. There must be a complete path for current flow.

3. The current path normally has resistance, either to

generate heat or limit the amount of current.


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A closed circuit

(current is flowing)

The purpose of the

resistor is to limit

current (flow) or to

generate heat.




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An open circuit

(no current is flowing)




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1-9: Direction of the Current

With respect to the positive and negative terminals of

the voltage source, current has direction.

When free electrons are considered as the moving

charges we call the direction of current electron flow.

Electron flow is from the negative terminal of the voltage

source through the external circuit back to the positive

terminal.

Conventional current is considered as the motion of

positive charges. Conventional current flows in the

opposite direction from electron flow (positive to

negative).


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0 i t C t


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1-10: Direct Currentand Alternating Current

Fig. 1-14: Steady dc voltage of fixed polarity,

such as the output of a battery. Note the

schematic symbol at left.

Fig. 1-15: Sine-wave ac voltage with

alternating polarity, such as from an ac

generator. Note the schematic symbol at

left. The ac line voltage in your home has

this waveform.Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


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1-11: Sources of Electricity

Common sources of electricity include:

Static electricity by friction

Example: walking across a carpeted room

Conversion of chemical energy wet or dry cells; batteries

Electromagnetism

motors, generators

Photoelectricity materials that emit electrons when light strikes their surfaces;

photoelectric cells; TV camera tubes


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2-1: Types of Resistors

The two main characteristics of a resistor are its

resistance, R, in ohms and its power rating, P, in Watts.

The resistance, R, provides the required reduction incurrent or the desired drop in voltage.

The wattage rating indicates the amount of power the

resistor can safely dissipate as heat. The wattage ratingis always more than the actual amount of power

dissipated by the resistor, as a safety factor.


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2-2: Resistor Color Coding

Resistors under 10 :

The multiplier band is either gold or silver.

For gold, multiply by 0.1.

For silver, multiply by 0.01.

Fig. 2-9: Examples of color-coded Rvalues, with percent tolerance.



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2-2: Resistor Color Coding

What is the nominal value and permissible ohmic

range for each resistor shown?

1 k (950 to 1050 )

390 (370.5 to 409.5 )

22 k (20.9 to 23.1 k)

1 M (950 k to 1.05 M)


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2-4: Rheostats and Potentiometers

Rheostats and potentiometers are variable resistances used

to vary the amount of current or voltage in a circuit.

Rheostats:

Two terminals.

Connected in series with the load and the voltage source.

Varies the current.



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Potentiometers:

Three terminals.

Ends connected across the voltage source.

Third variable arm taps off part of the voltage.



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Potentiometers

Potentiometers are three-

terminal devices.

The applied Vis input to

the two end terminals ofthe potentiometer.

The variable Vis output

between the variable arm

and an end terminal.

Fig. 2-18: Potentiometer connected across voltage

source to function as a voltage divider. (a) Wiring

diagram. (b) Schematic diagram.



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2-5: Power Rating of Resistors

In addition to having the required ohms value, a resistor

should have a wattage rating high enough to dissipate

the power produced by the current without becoming

too hot.

Power rating depends on the resistors construction.

A larger physical size indicates a higher power rating.

Higher-wattage resistors can operate at higher

temperatures.

Wire-wound resistors are physically larger and have

higher power ratings than carbon resistors.


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2 6 R i t T bl


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2-6: Resistor Troubles

All experienced technicians have seen a burnt resistor.

This is usually caused by a short somewhere else in the

circuit which causes a high current to flow in the

resistor.

When a resistors power rating is exceeded, it can burn

open or drift way out of tolerance.

T t i l


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