Chapter 01 Electricity

ElectricityElectricity

Topics Covered in Chapter 11-1: Negative and Positive Polarities

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

1-4: The Coulomb Unit of Electric Charge1-5: The Volt Unit of Potential Difference

Chapter 1

© 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

1-1: Negative and Positive 1-1: Negative and Positive PolaritiesPolarities

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 a substance 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.

1-2: Electrons and Protons in the 1-2: Electrons and Protons in the AtomAtom

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


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


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

1-3: Structure of the Atom1-3: 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.

1-3: Structure of the Atom1-3: Structure of the Atom 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 electronsP ring = 8 or 18 electronsQ ring = 8 electrons

1-3: Structure of the Atom1-3: Structure of the Atom The maximum number of electrons in the outermost ring

is always 8.

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

An atom’s nucleus contains neutrons as well as protons.

Neutrons have no net electric charge.

One Atom of Copper

K

K = 2(complete)

L

L = 8(complete)

MM = 18

(complete)

N = 1(incomplete)N

Atomic number = 29

1-3: Structure of the Atom1-3: Structure of the Atom

1-3: Structure of the Atom1-3: Structure of the AtomThe valence electron is weakly bound to the nucleus. This makes copper an excellent conductor.

29 protonsatomic number = 29

29 electrons(net charge = 0)

1 valence electron

29 protonsatomic number = 29

29 electrons(net charge = 0)

1 valence electron

1-4: The Coulomb Unit of Electric 1-4: The Coulomb Unit of Electric ChargeCharge

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


Charge of an Electron The charge of a single electron, or Qe, is 0.16 × 10−18 C. It is expressed

−Qe = 0.16 × 10−18 C (−Qe indicates the charge is negative.)

The charge of a single proton, QP, is also equal to 0.16 × 10−18 C .

However, its polarity is positive instead of negative.

1-5: The Volt Unit of Potential 1-5: The Volt Unit of Potential DifferenceDifference

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 difference 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 is the 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 in moving 1 C of charge between those two points. 1 V = 1 J / 1 C

9 joulescoulomb9 joulescoulomb9 joulescoulomb



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).


1-6: Charge in Motion Is 1-6: Charge in Motion Is CurrentCurrent

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 as electrons.


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 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 I or i, for intensity.

1-7: Resistance Is 1-7: Resistance Is Opposition 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 low

resistance. Insulators, like glass and rubber, have very high

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

1-7: Resistance Is 1-7: Resistance Is Opposition 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.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1-8: The Closed Circuit1-8: 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.

1-8: The Closed Circuit1-8: The Closed Circuit 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 theresistor is to limitcurrent (flow) or

togenerate heat.

1-8: The Closed Circuit1-8: The Closed Circuit


An open circuit(no current is flowing)

1-8: The Closed Circuit1-8: The Closed Circuit


1-9: Direction of the Current1-9: 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).

1-9: Direction of the Current1-9: Direction of the CurrentFig. 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.


1-10: Direct Current 1-10: Direct Current and 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.

1-10: Direct Current1-10: 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.


1-11: Sources of Electricity1-11: 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.

1-11: Sources of Electricity1-11: 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

1-12: The Digital Multimeter1-12: 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.


Chapter 01 Electricity

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Transcript of Chapter 01 Electricity