Cours d'éléctronique pour les débutants 1

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ElectricityElectricity

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 Chapter 11

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



Topics Covered in Chapter 1Topics 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



11--1: Negative and Positive Polarities1: Negative and Positive Polarities

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.




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.




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.




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.



11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom

Electrons and protons in an atom assemble in specific

combinations for a stable arrangement.

Each stable combination makes one particular kind of atom.




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.





Electrons are distributed in orbital rings around the

nucleus.

The distribution of electrons determines the atom¶s

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 f ree electrons. Free electrons can move from one atom to the next and

are the basis of electric current.




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.





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.




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.




Semiconductors are materials that are neither good

conductors nor good insulators.

Examples of semiconductors include:

carbon silicon.

germanium



11--3: Structure of the Atom3: Structure of the Atom

Atomic Number

The atomic number of 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.




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




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

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

An atom¶s nucleus contains neutrons as well asprotons.

Neutrons have no net electric charge.



One Atom of Copper

K

K = 2(complete)

L

L = 8

(complete)

M

M = 18(complete)

N = 1

(incomplete)N

Atomic number = 29





The valence electron is weakly bound to the nucleus. This

makes copper an excellent conductor.

29 protons

atomic number = 29

29 electrons(net charge = 0)

1 valence electron

29 protons

atomic number = 29

29 electrons(net charge = 0)

1 valence electron



11--4: The Coulomb Unit of Electric4: The Coulomb Unit of ElectricChargeCharge

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 × 1018

electrons or protons.

The symbol for electric charge is Q or q, for quantity .




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 because

electrons 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)




Fig. 1-5: Physical force between electric charges. (a) Opposite charges attract. (b) Two

negative charges repel each other. (c ) Two positive charges repel.





Charge of an Electron

The charge of a single electron, or Qe, is 0.16 × 10í18 C.

It is expressed

íQe = 0.16 × 10í18 C

(íQe indicates the charge is negative.)

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

0.16 × 10í18 C .

However, its polarity is positive instead of negative.



11--5: The Volt Unit of Potential5: The Volt Unit of PotentialDifferenceDifference

Potential refers to the possibility of doing work.

Any charge has the potential to do the work of moving

another charge, either by attraction or repulsion.

Two unlike charges have a diff erence of potential. Potential difference is often abbreviated PD.

The volt is the unit of potential difference.

Potential difference is also called voltage.




The volt is a measure of the amount

of work or energy needed to move an

electric charge.

The metric unit of work or energy isthe joule (J). One joule = 0.7376

ft·lbs.

The potential difference (or voltage)

between two points equals 1 volt

when 1 J of energy is expended inmoving 1 C of charge between those

two points.

1 V = 1 J / 1 C

9 jou

ou o b

9 jou

ou o b

9 jou

ou o b





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 ).




11--6: Charge in Motion Is Current6: Charge in Motion Is Current

When the potential difference between two charges

causes a third charge to move, the charge in motion is

an electric current.

Current is a continuous flow of electric charges such aselectrons.




Fig. 1-9: Potential difference across two ends of wire conductor causes drift of free electrons

throughout the wire to produce electric 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 theampere (A).

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

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

The letter symbol for current isI or i , for int ensity .



11--7: Resistance Is7: Resistance IsOpposition to CurrentOpposition 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 .



11--7: Resistance Is7: Resistance IsOpposition to CurrentOpposition 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.




11--8: The Closed Circuit8: The Closed Circuit

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.




Open and Short Circuits

When a current path is broken (incomplete) the circuit is

said to be open. The resistance of an open circuit is

infinitely high. There is no current in an open circuit. When the current path is closed but has little or no

resistance, the result is a short circuit. Short circuits

can result in too much current.



A closed circuit

(current is flowing)

The purpose of the

resistor is to limit

current (flow) or to

generate heat.





An open circuit

(no current is flowing)





11--9: Direction of the Current9: 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 f low.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).



11--9: Direction of the Current9: Direction of the Current

Fig. 1-13: Direction of I in a closed

circuit, shown for electron flow

and conventional current. The

circuit works the same way no

matter which direction you

consider. (a) Electron flow

indicated with dashed arrow in

diagram. (b) Conventional

current indicated with solid

arrow. (c ) Electron flow as in

(a) but with reversed polarity of

voltage source. (d ) Conventional

I as in (b) but reversed polarity

for V .




11--10: Direct Current10: Direct Currentand Alternating Currentand Alternating Current

Direct current (dc) flows in only one direction.

Alternating current (ac) periodically reverses direction.

The unit for 1 cycle per second is the hertz (Hz). This

unit describes the frequency of reversal of voltage

polarity and current direction.



11--10: Direct Current10: Direct Currentand Alternating 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.



11--11: Sources of Electricity11: Sources of Electricity

All materials have electrons and protons.

To do work, the electric charges must be separated to

produce a potential difference.

Potential difference is necessary to produce current

flow.



11--11: Sources of Electricity11: 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



11--12: The Digital Multimeter 12: The Digital Multimeter

A digital multimeter

(DMM) is a device used to

measure the voltage,

current, or resistance in a

circuit.

Fig. 1-16: A handheld digital multimeter and a

benchtop unit.


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Cours d'éléctronique pour les débutants 1