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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.
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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
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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.
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11--1: Negative and Positive Polarities1: Negative and Positive Polarities
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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11--1: Negative and Positive Polarities1: Negative and Positive Polarities
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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11--1: Negative and Positive Polarities1: Negative and Positive Polarities
Fig. 1-1: Positive and negative
polarities for the voltage output of a
typical battery.
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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.
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11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom
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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11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom
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.
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11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom
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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11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom
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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11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom
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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11--2: Electr ons and Pr otons in the2: Electr ons and Pr otons in theAtomAtom
Semiconductors are materials that are neither good
conductors nor good insulators.
Examples of semiconductors include:
carbon silicon.
germanium
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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.
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11--3: Structure of the Atom3: 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 electrons
P ring = 8 or 18 electrons
Q ring = 8 electrons
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11--3: Structure of the Atom3: Structure of the Atom
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.
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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
11--3: Structure of the Atom3: Structure of the Atom
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11--3: Structure of the Atom3: Structure of the Atom
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
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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 .
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11--4: The Coulomb Unit of Electric4: The Coulomb Unit of ElectricChargeCharge
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)
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11--4: The Coulomb Unit of Electric4: The Coulomb Unit of ElectricChargeCharge
Fig. 1-5: Physical force between electric charges. (a) Opposite charges attract. (b) Two
negative charges repel each other. (c ) Two positive charges repel.
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11--4: The Coulomb Unit of Electric4: The Coulomb Unit of ElectricChargeCharge
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.
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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.
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11--5: The Volt Unit of Potential5: The Volt Unit of PotentialDifferenceDifference
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
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11--5: The Volt Unit of Potential5: The Volt Unit of PotentialDifferenceDifference
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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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.
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11--6: Charge in Motion Is Current6: Charge in Motion Is Current
Fig. 1-9: Potential difference across two ends of wire conductor causes drift of free electrons
throughout the wire to produce electric current.
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11--6: Charge in Motion Is Current6: 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 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 .
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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 .
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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.
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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.
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11--8: The Closed Circuit8: 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.
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A closed circuit
(current is flowing)
The purpose of the
resistor is to limit
current (flow) or to
generate heat.
11--8: The Closed Circuit8: The Closed Circuit
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An open circuit
(no current is flowing)
11--8: The Closed Circuit8: The Closed Circuit
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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).
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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 .
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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.
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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.
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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.
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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
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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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