Download Lecture 2 Principles of Electricity File

Principles of Electricity
Electronic Concepts
Hussam Al-Hertani
The Oldest Analogy in Electronics
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Electronic Concepts 2010 - Hussam Al-Hertani
Elements, Atoms and Charge
 Matter
 Matter – anything that has weight and occupies space
 Element – substance that cannot be broken down into a
combination simpler substances
 Atomic Structure
 Atom – smallest particle of matter that retains the physical
characteristics of an element
 Bohr Model
 Simplest model of an atom
 Central core (nucleus) – contains protons and neutrons
 Electrons revolve around nucleus
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Electronic Concepts 2010 - Hussam Al-Hertani
Elements, Atoms and Charge
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Electronic Concepts 2010 - Hussam Al-Hertani
The Starting Point: Elements, Atoms
and Charge
 Atomic Structure (Continued)
 Atomic Number of an Atom – number of protons
 Atoms contain an equal number of protons and electrons
 Electrons travel in orbital paths (shells)
 Valence Shell
 Outermost shell
 Cannot hold more than eight electrons
 Complete shell contains eight electrons
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Electronic Concepts 2010 - Hussam Al-Hertani
The Starting Point: Elements, Atoms
and Charge
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Electronic Concepts 2010 - Hussam Al-Hertani
The Starting Point: Elements, Atoms
and Charge
 Charge
 Force that causes two particles to be attracted to, or repelled
from, each other
 Two types – positive and negative
 Atom – proton (positive), electron (negative), neutron
(electrically neutral)
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Electronic Concepts 2010 - Hussam Al-Hertani
Elements, Atoms and Charge
 Attraction and Repulsion - Like charges repel each other
and opposite charges attract each other
 Ions
 Outside force can cause an electron to leave its orbit -atom is
referred to as a positive ion
 Outside force can cause an atom to gain an electron -atom is
referred to as a negative ion
 Free Electrons
 An electron that is not bound to any particular atom
 Can neutralize a positive ion
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Electronic Concepts 2010 - Hussam Al-Hertani
Elements, Atoms and Charge
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Electronic Concepts 2010 - Hussam Al-Hertani
Current
• Current – the directed flow of charge through a conductor
– Thermal energy (heat) is sufficient to free electrons in copper
– Free electron motion is random unless outside force is applied
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Electronic Concepts 2010 - Hussam Al-Hertani
Current
• Represented by the letter I (for intensity)
• Measured in charge per unit time
Q
I
t
where
I = the intensity of the current
Q = the amount of charge
t = the time (in seconds) required for the
charge (Q) to pass
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Electronic Concepts 2010 - Hussam Al-Hertani
Current
• Coulomb (C) – represents the total charge of
approximately 6.25 x 1018 electrons
• Unit of Current – 1 Ampere (A) = 1 coulomb/second
(C/s)
• Example:
3 coulombs of charge pass a point in a wire every two
seconds. Circuit current is found as
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Q 3C
I 
 1.5 C/s  1.5 A
t 2s
Electronic Concepts 2010 - Hussam Al-Hertani
Current
• Electron Flow Versus Conventional Current
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Electronic Concepts 2010 - Hussam Al-Hertani
Current
 Direct Current Versus Alternating Current
 Direct Current (dc) – unidirectional, always flows in one
direction
 Alternating Current (ac) – bidirectional, periodically changes
direction
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Electronic Concepts 2010 - Hussam Al-Hertani
Current
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Electronic Concepts 2010 - Hussam Al-Hertani
Voltage
• Voltage – a “difference of potential” that generates a directed
flow of charge (current) through a circuit
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Electronic Concepts 2010 - Hussam Al-Hertani
Voltage
• Often referred to as electromotive force (EMF)
• Unit of Voltage – volt (V) = 1 joule/coulomb
E
V
Q
• Volt – the difference of potential that uses one joule
of energy to move one coulomb of charge.
• 1 V = 1 J/C
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Electronic Concepts 2010 - Hussam Al-Hertani
Resistance and Conductance
• Resistance – opposition to current
• Unit of Resistance – ohm ( - Greek letter omega)
• Ohm – the amount of resistance that limits current to one
ampere when one volt is applied
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Electronic Concepts 2010 - Hussam Al-Hertani
Resistance and Conductance
 Conductance – a measure of the ease which current will pass
through a component
where
1
G
R
G = conductance
R = resistance
 Unit of Conductance – siemens (S)
 Old Unit of Conductance – mhos (upside down – omega
symbol)
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Electronic Concepts 2010 - Hussam Al-Hertani
Conductors, Insulators and
Semiconductors
• Examples
– Calculate the conductance of a 10 K resistor.
1
1
G 
 100 μS
R 10 kΩ
– Calculate the resistance of a circuit that has a conductance
of 25 mS.
1
1
R 
 40 Ω
G 25 mS
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Electronic Concepts 2010 - Hussam Al-Hertani
Conductors, Insulators and
Semiconductors
 Conductors – materials that provide little opposition to the
flow of charge (current)
 Example: copper
 Few valence shell electrons, one valence shell electron per atom
makes the best conductor
 Insulators – materials that normally block current
 Example: rubber
 Complete valence shell (8 electrons)
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Electronic Concepts 2010 - Hussam Al-Hertani
Conductors, Insulators and
Semiconductors
 Semiconductors – materials that are neither good conductors
nor good insulators
 Examples: - graphite (used to make resistors)
- silicon
- germanium
 Half-complete valence shells (four valence electrons)
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Electronic Concepts 2010 - Hussam Al-Hertani
Conductors, Insulators and
Semiconductors
 Other Factors that Affect Resistance
 Resistivity – the resistance of a specified volume of an element
or compound
 CM-/ft – Circular-mil ohms per foot
Mil is one thousandth of an inch (0.001 in)
 The circuilar mil (CM) is the area of a 1mil diameter circle
 -cm – Ohm-centimeters

 Length
 Cross-Sectional Area
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Electronic Concepts 2010 - Hussam Al-Hertani
Conductors, Insulators and
Semiconductors
• Calculating the Resistance of a Conductor

R
A
• Resistivity table provided on page 39 of text
where
 = resistivity (greek letter, rho)
ℓ = length
A = cross-sectional area
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Electronic Concepts 2010 - Hussam Al-Hertani
Conductors, Insulators and
Semiconductors
• Example
Calculate the resistance of a 25 cm length of copper that
has a cross-sectional area of 0.04 cm2.

l
R    1.723 X 10 6 Ω  cm
A
 1.08 X 10 3 Ω  1.08mΩ
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Electronic Concepts 2010 - Hussam Al-Hertani

 25cm 

2 
 0.04cm 
Conductors, Insulators and
Semiconductors
• The Effects of Temperature on Resistance
– Positive Temperature Coefficient
• Resistance increases as temperature increases and vice versa
• Example: most conductors
– Negative Temperature Coefficient
• Resistance decreases as temperature increases and vice versa
• Example: most semiconductors and insulators
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Electronic Concepts 2010 - Hussam Al-Hertani
In Class Problem Solving
 P41 Q. 3, 7, 13, 15, 17
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Electronic Concepts 2010 - Hussam Al-Hertani