Chapter 2 - Oakland High School

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Chapter 2 - Voltage, Current & Resistance
MECH 1100
Chapter 2 – Voltage, Current and Resistance
Topics
Atoms
Electrical Charge
Voltage
Current
Resistance
Basic Circuits
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What are ATOMS?
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The two simplest atoms: hydrogen and helium.
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Energy levels increase as the distance from the nucleus increases.
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The valence shell
The outer shell is called the valence
shell.
Electrons in this shell are involved in
chemical reactions and they account
for electrical and thermal conductivity
in metals.
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The valence shell
Metals have one, two or three electrons in the
valence shell. The atom illustrated here is a
sodium atom (Na), with only one electron in its
outer shell.
Sodium is highly reactive, and easily
gives up its single valence electron.
For this reason, it is not used in
electrical work.
Non-metals have either complete
or nearly compete outer shells, so
they make poor electrical
conductors (i.e. they are
insulators).
+
S he ll 1
S he ll 2
S he ll 3
Sodium atom
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A neutral Si atom is
shown. There are 4
electrons in the valence
shell.
+
Shell 1
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Shell 2
Shell 3
Is Si a conductor, insulator, or
semiconductor?
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The valence shell
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A neutral Si atom is
shown. There are 4
electrons in the valence
shell.
+
Shell 1
Shell 2
Shell 3
Is Si a conductor, insulator, or
semiconductor?
Semiconductor
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The copper atom
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Electrical charge
There is a force (F) between electrical charges. Like
charges repel; unlike charges attract.
• The force is directly proportional to charge.
• The force is inversely proportional to square of distance.
+
+
_
+
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Electrical charge
Coulomb – The unit of charge
One Coulomb is the total charge possessed by 6.25 × 1018
electrons.
Total Charge
š‘›š‘¢š‘šš‘š‘’š‘Ÿ š‘œš‘“ š‘’š‘™š‘’š‘š‘”š‘Ÿš‘œš‘›š‘ 
š‘„=
6.25 × 1018 š‘’š‘™š‘’š‘š‘”š‘Ÿš‘œš‘›š‘ /š¶
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Electrical charge
How many coulombs of charge do 88.7 × 1015 electrons
represent?
š‘›š‘¢š‘šš‘š‘’š‘Ÿ š‘œš‘“ š‘’š‘™š‘’š‘š‘”š‘Ÿš‘œš‘›š‘ 
š‘„=
6.25 × 1018 š‘’š‘™š‘’š‘š‘”š‘Ÿš‘œš‘›š‘ /š¶
88.7 × 1015 š‘’š‘™š‘’š‘š‘”š‘Ÿš‘œš‘›š‘ 
š‘„=
6.25 × 1018 š‘’š‘™š‘’š‘š‘”š‘Ÿš‘œš‘›š‘ /š¶
š‘„ = 14.91 × 10−3 š¶ = 0.01491š¶
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Voltage
Force is required to move a charge
against the electric field.
+
+
+
+
+
+
+
+
+
e-
-
When force is applied over a
distance, work is done. Work done
in moving a charge against the
electric field leads to the definition
of voltage:
Voltage is the work per charge
done against the electric field.
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The equation for voltage is
W
Vļ€½
Q
V - Volts
W – Energy in joules
Q – Charge in coulombs
One volt is the potential difference (voltage) between
two points when one joule of energy is used to move
one coulomb of charge from one point to the other.
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If 135 J of energy are required to move 15C
of Charge, what is the voltage?
W
Vļ€½
Q
135š½
š‘‰=
= 9š‘‰
15š¶
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Symbol for dc voltage sources (battery).
Voltage is responsible for establishing current.
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Batteries in Parallel
Batteries in Series
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Batteries
An automobile battery is an example of a multiple cell
battery. Like all batteries, the automotive battery does not
store charge – it stores chemical energy that can be
converted to current when an external path is provided to
allow the chemical reaction to proceed.
Rather than saying “charging” a
battery, it is more accurate to say
“reversing the chemical reaction” in a
battery.
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Fuel cells
A fuel cell is a device that converts chemical energy into dc voltage
directly by combining a fuel (usually hydrogen) with an oxidizing
agent (usually oxygen). The
hydrogen and oxygen react
to form water. The process
differs from batteries in that
the reactants constantly
flow into the cell where
they combine and produce
electricity.
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A basic power supply.
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Electrons flow from negative to positive when a
voltage is applied across a conductive or
semiconductive material.
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Current
Current (I in amperes or amps) is the amount of charge (Q in
coulombs) that flows past a point in a unit of time (t). The
defining equation is:
One coulomb equals the
Q
I ļ€½
t
W
Qļ€½
V
value of electrical charge in
6.24150965×1018 protons or
electrons
One ampere is a number of electrons having a total charge of 1
Coulomb moving through a given cross section in 1 s.
What is the current if 2 C passes a point in 5 s? 0.4 A
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Current sources are not as common as voltage sources
Symbol
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Conductance
Conductance is:
the reciprocal of resistance
the ease of with which voltage flows.
1
Gļ€½
R
G – Conductance (S siemens)
R – Resistance (ohms Ω)
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Resistance
Components designed to have a specific
amount of resistance are called resistors.
Color bands
Resistance material
(carbon composition)
Insulation coating
Leads
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Resistance/resistor symbol.
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Fixed resistors
(Value cannot be changed)
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Typical fixed resistors
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Resistance color-code
Color
Digit
Multiplier Tolerance
Black
0
10 0
Brown
1
10 1
1% (five band)
Resistance value, first three bands:
Red
2
10 2
2% (five band)
First band – 1st digit
Orange
3
10 3
Yellow
4
10 4
Second band – 2nd digit
Green
5
10 5
Blue
6
10 6
Violet
7
10 7
Gray
8
10 8
White
9
10 9
Gold
±5%
10 -1
5% (four band)
Silver
± 10%
10 -2
10% (four band)
No band
± 20%
*Third band – Multiplier (number of
zeros following second digit)
Fourth band - tolerance
* For resistance values less than 10 W, the third band is either gold or silver.
Gold is for a multiplier of 0.1 and silver is for a multiplier of 0.01.
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Color-code bands on a 4-band resistor.
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What is the resistance and tolerance of
each of the four-band resistors?
Color
Digit
Multiplier Tolerance
Green, brown, red, gold
Black
0
10 0
Brown
1
10 1
1% (five band)
Red
2
10 2
2% (five band)
Orange
3
10 3
Yellow
4
10
4
Green
5
10 5
Blue
6
10 6
Violet
7
10 7
Gray
8
10 8
White
9
10 9
Gold
±5%
10 -1
5% (four band)
Silver
± 10%
10 -2
10% (four band)
No band
± 20%
Gray, red, yellow, silver
5.1 kW ± 5%
820 kW ±10%
Yellow, violet, black, silver
47 W ± 10%
Brown, black, gold,gold
1.0 W ± 5%
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Three-digit labeling for a resistor.
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Alphanumeric Labeling
• Two or three digits, and one of the letters R, K, or M are
used to identify a resistance value.
• The letter is used to indicate the multiplier, and its position
is used to indicate decimal point position.
• R – multiplier of 1 and no zeros after the digits
• K - multiplier of 1000 and 3 zeros after the digits
• M - multiplier of 1,000,000 and 6 zeros after the digits
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Variable resistors
Variable resistors include the potentiometer and rheostat.
The center terminal of a variable resistor is connected to the
wiper.
R
Shaft
1
3
2
Wiper
Resistive
element
Variable resistor
(potentiometer –
divides voltage)
R
To make a potentiometer into a
rheostat, one of the outside
terminals is connected to the
wiper.
Variable resistor
(rheostat –
controls current)
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Symbols for resistance devices with sensitivities to
temperature, light, and force.
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A simple electric circuit
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Switches
Switches are commonly used to control circuits by
either mechanical or electronic means.
Different
from the
book
The throw refers to how many circuits are affected by
the movable arm of a switch.
The pole refers to the number of contacts that are
available to form a circuit.
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Normally open Pushbutton
Normally closed Pushbutton
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Closed and open circuits using a SPST switch for
control.
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SPDT switch controlling two lamps.
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Mechanical limit switches
• Various sizes
• Various shapes
• Various uses
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Mechanical limit switches
• Micro switches
• Small size
• Various ranges of
uses
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Mechanical limit switches
• Purpose
– To detect the
presence or absence
of an object
– Opens or closes a
contact
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Mechanical limit switches
4
1
2
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Mechanical limit switches
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GAUGING (MODULE #2)
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Mechanical limit switch in process sequence
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Mechanical limit switches
• What can go wrong •
• Broken arm
•
• Internal broken
•
contact
•
• Welded contacts
•
• Arc/pitted contacts
• Bad/loose
connection
• Incorrect adjustment
Possible symptoms?
No change in signal
No signal
Continual signal
Intermittent signal
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Mechanical limit switch
• How to check and
adjust
– Check continuity
– Check voltages
– Look for changes
when switching
– Check and readjust
as necessary
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Fuses, circuit breakers and their symbols.
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WIRE
Gauge – Size (AWG or mm)
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WIRE
Gauge – Size (AWG or mm)
In the AWG scale, the smaller the number the larger the wire
diameter and the greater the current carrying capacity.
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Wire resistance
Sometimes, the resistance of wires must be accounted for. The
equation for wire resistance is:
Rļ€½
ļ²l
A
where ļ² (rho) = resistivity in CM-W/ft
l = length in feet
A = cross sectional area in circular mils (CM)
What is the resistance of 400 feet of 22 gage copper wire where the
area is 642 CM and the resistivity of copper is 10.37 CM-W/ft and
the resistance/1000 feet is 16.14 W/1000 feet.
By proportion, the resistance of 400 feet is 0.4 x 16.14 W = 6.46 W
By the equation,
ļ² l ļ€Ø10.37 W-CM/ft ļ€©ļ€Ø 400 ft ļ€©
Rļ€½
ļ€½
ļ€½ 6.46 W
A
642 CM
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Ground.
(Reference Point for measuring Potential)
In U.S. this is typically a bare wire or green insulation or green
with yellow stripe insulation.
1 – Overvoltage protection – lightning
2 – Voltage Stabilization- Common reference
3 – Safety – What happens if neutral breaks?
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Simple circuit with ground connections.
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The electric circuit
Switch
A basic electric circuit consists of
1) a voltage source
2) a path
Battery (2
3) a load.
Lamp
cells)
An example of a basic circuit is a flashlight, which has
each of these plus a control element – the switch.
Switch
Metal reflector
Metal strip
Spring
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The DMM – Digital Multimeter
Volts - AC
Volts - DC
Ohms
Amperes
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Analog meters
An analog
multimeter is
also called a
VOM (voltohmmilliammeter).
No resistance will
show as infinite ohms
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Measuring current
Series Circuit
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Measuring voltage or voltage drop
Parallel Circuit
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Measuring resistance
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Schematic Symbols Review
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Key Terms
Ampere The unit of electrical current.
AWG (American Wire Gauge) A standardization based
on wire diameter.
Charge An electrical property of matter that exists
because of an excess or a deficiency of
electrons. Charge can be either + or -.
Circuit An interconnection of electronic components
designed to produce a desired result. A basic
circuit consists of a source, a load, and an
interconnecting path.
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Key Terms
Conductance The ability of a circuit to allow current. The unit
is the siemans (S).
Coulomb The unit of electrical charge.
Current The rate of flow of electrical charge.
Electron A basic particle of electrical charge in matter.
The electron possesses a negative charge.
Ground The common or reference point in a circuit.
Ohm (W) The unit of resistance.
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Key Terms
Potentiometer A three-terminal variable resistor.
Resistance The opposition to current. The unit is the ohm (W).
Rheostat A two-terminal variable resistor.
Siemens The unit of conductance.
Volt The unit of voltage or electromotive force.
Voltage The amount of energy per charge available to
move electrons from one point to another in an
electric circuit.
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Quiz
1. The atomic number is the number of
a. protons in the nucleus
b. neutrons in the nucleus
c. protons plus neutrons in the nucleus
d. electrons in the outer shell
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Quiz
2. Valence electrons are
a. in the outer shell
b. involved in chemical reactions
c. relatively loosely bound
d. all of the above
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Quiz
3. The atomic particle responsible for electrical current in
solid metallic conductors is the
a. proton
b. electron
c. neutron
d. all of the above
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Quiz
4. The symbol for charge is
a. C
b. W
c. Q
d. W
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Quiz
5. The definition for voltage is
Q
a. V ļ€½
t
b. V ļ€½
W
t
c. V ļ€½
W
Q
d. V ļ€½ It
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Quiz
6. A battery stores
a. electrons
b. protons
c. ions
d. chemical energy
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Quiz
7. The unit of conductance is the
a. ohm
b. coulomb
c. siemen
d. ampere
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Quiz
10. The circular mil is a unit of
a. length
b. area
c. volume
d. resistance
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