Download Chapter 02 Voltage and Current

Atomic Theory Review
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Chapter 02
Voltage and Current
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Source:
Circuit Analysis: Theory and Practice Delmar Cengage Learning
Atom
 Contains a nucleus of
protons and neutrons
 Nucleus is surrounded by a
group of orbiting electrons
Electrons are negative,
protons are positive
Electrically neutral atom
 Equal number of electrons
and protons
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Atomic Theory Review
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Atomic Theory Review
Bohr model
 Electrons orbit the nucleus in discrete orbits
called shells
 Designated by letters K, L, M, N, etc.
Only certain numbers of electrons can exist
within any given shell
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Valence shell
 Outermost shell of an atom
 Electrons in this shell are called valence
electrons
No element can have more than eight
valence electrons
Number of valence electrons affects its electrical
properties
Copper atom
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Electrical Charge
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Example: Charge
Objects become charged when they have an
excess or deficiency of electrons
 Occurs when electrons are stripped from
parent atoms, free electrons
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An initially neutral body has 1.7μC of negative
charge removed. Later, 15×1011 electrons
are added. What is the body’s final charge?
For example: Spinning clothes in dryer
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Rubbing ebonite rod with fur
Unit of charge is the coulomb (C)
1 C = 6.24 × 1018 electrons (or protons)
The charge on one electron (or proton)
=1 / 6.24×1018 C or 1.6 × 10-19 C
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Free Electrons
Coulomb’s Law
French scientist Charles Coulomb:
Studied force between charges
 Coulomb’s law
F = k Q1Q2 /r 2 [newtons, N]
where k = 9x109 (based on MKS)
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Amount of energy required to escape:
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Few valence electrons
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Q1 and Q2 are charges in coulombs
r is center to center spacing in meters
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Depends on number of electrons in valence shell
Weak attraction
Small amount of energy needed
Example: copper material
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Heat can provide enough energy to escape
Valence electrons wander from valence shell of one
atom to valence shell of another
Material remains electrically neutral
Presence of large number of free electrons
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Indicates material is a good conductor
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Ions
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Electrically neutral atom
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Ion
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Conductors
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Equal number of electrons and protons
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Atom with an excess or deficit of electrons
Positive ion has lost an electron
Negative ion has gained an electron
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Conductors: Materials through which charges
move easily
Metals are good conductors because they
have few loosely bound valence electrons
Excellent conductors
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Insulators
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Semiconductors
Materials that do not conduct because
their valence shells are full or almost full
Glass, porcelain (or china), plastic, and
rubber are good insulators
High voltage will cause an insulator to
break down and conduct
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Silver
Copper
Gold
Aluminum
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Half-filled valence shells (4 electrons)
 Neither good conductors nor good
insulators
Silicon (Si) and Germanium (Ge)
 Primary materials in semiconductor
devices
 Used to make transistors, diodes, and
integrated circuits (ICs)
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Voltage
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Voltage
When two objects have a difference in charges
 They have a potential difference or voltage
between them. Unit of voltage is the volt
Thunderclouds: Millions of volts between them
Difference in potential energy
Voltage between two points
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DC Voltage Sources
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If it takes 35 J of energy to move a charge of
5 C from one point to another, what is the
voltage between two points?
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1 coulomb
Voltage is always measured between two points
Potential energy (Work), W = QV
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Example: Voltage
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One volt if it requires one joule of energy to
move one coulomb of charge from one point to
another
1 joule
V = Work/Charge = W /Q 1 volt 
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Batteries
 Alkaline
 Carbon-Zinc
 Lithium
 NickelCadmium
 Lead-Acid
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Symbols of Voltage Sources
Battery Capacity
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Series / Parallel Batteries
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Other Voltage Sources
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Specified in amp-hours
Life: Capacity/current drain
Affected by
 Discharge rates, operating schedules,
temperatures, and other factors
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Electronic Power Supplies
Solar Cells
DC Generators
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Current
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Unit of Current
Movement of charge is electric current
More electrons per second passing through a
circuit, the greater the current
Current is rate of flow of charge
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Example: Current
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Current direction
Direct Current (DC)
If 840 coulombs of charge pass through a
conducted wire during a time interval of 2
minutes, what is the current?
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Unit of current is ampere (A)
One ampere (1 A)
 Current in a circuit when one coulomb of
charge passes a given point in one second
Current = Charge / time
I = Q /t
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Electron current flow
 From the negative terminal of
a battery to the positive
terminal
Conventional current flow
(used in this course)
 Flow from positive to
negative
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Alternating Current (AC)
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Alternating current (AC) changes direction
cyclically. Most common ac source
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Commercial ac power system supplies power to
your home
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Electronic systems require dc for operation
Portable units use batteries
Commercial ac power lines used with built-in power
supply
AC adapter converts ac to dc for powering
circuits and is used in laptop computers,
modems, etc.
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Terminal Designations
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Digital multimeter (DMM)
Analog multimeter (VOM)
Electronic Power Supplies
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Measure Voltage and current
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Voltage-Meter Symbol
Multimeter terminals
 Marked VΩ, A, and COM
Terminal VΩ
 Used to measure voltage and resistance
Terminal A
 Used to measure current
COM: common terminal for measurements
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Measure the voltage:
 Place voltmeter leads across components
 Color coded test leads are industry standard
 Red lead is connected to the VΩ socket
 Black lead is connected to COM
 If leads are reversed, you will read the opposite
polarity
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Amper-Meter Symbol
How to Measure Voltage
Measure the current:
 Measurable current must pass through meter
 Open circuit and insert meter
 Positive current if it enters the A terminal
 Negative current if it enters the COM terminal
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How to Measure Current
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Switches
Measurable
current must pass
through meter
Open circuit and
insert meter
Connect with
correct polarity
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Place voltmeter
leads across
components
Red lead is positive,
Black lead is
negative
If leads are
reversed, you will
read the opposite
polarity
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Single-pole, single-throw (SPST)
Single-pole, double-throw (SPDT)
Double-pole, single-throw (DPST)
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Switches
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Fuses and Circuit Breakers
Push-button - normally open (NO) or
normally closed (NC)
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Protect equipment or wiring against
excessive current
Fuses use a metallic element that
melts slow-blow and fast-blow fuses
If current exceeds rated value of a
circuit breaker, magnetic field
produced by the excessive current
operates trips open a switch
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Kernel abilities
Exercise 1
1. What is the charge of an electron? Please give an
example.
1. What unit is for the charge?
2. What are the differences between conductor, semiconductor, and insulator. Please give their
representative materials?
3. Please show the priority of excellent
conductors, gold, copper, aluminum, and
silver.
2. What the charge is on one electron?
3. What are differences between DC and AC voltages
and currents? Please give their examples.
4. What two semiconductors are used for
electronic devices?
4. How to measure DC and AC voltages and currents?
Please give their examples.
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5. Please show the definition of voltage.
6. Please show the definition of current
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Problem 45: What is wrong here?
Problem 42: Show their reading
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Exercise 2
7. Please show the schematic with Multisim to
use DMM to measure AC 110V, DC 12V, AC
current, and DC current.
8. Please show the schematic with Multisim to
use Oscilloscope to measure AC 110V, DC
12V, AC current, and DC current.
9. Please extend the two-way switch control of
a light to 3 down-up-stair floors.
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