Transcript Circuit Elements

ECE 101
Exploring Electrical Engineering
Chapter 5
Circuit Elements
Herbert G. Mayer, PSU
Status 1/30/2016
Derived with permission from PSU Prof. Phillip Wong
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Syllabus
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

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Electric Charge
Electric Field
What is Conductance
References
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Intro
 Many SI units you saw earlier
 Now we discuss detail of a few key SI units
 With their: name, symbol, equation, unit etc.
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Electric Charge
 Electric charge (q or Q) is an intrinsic property of
certain subatomic particles
 A particle’s charge affects its motion in the presence
of electric and also in magnetic fields
 The SI unit for electric charge is the coulomb [C]
1 [C] = 1 [A] * 1 [s]
 The charge of 1 electron is ≈ 1.602×10-19 C
 Can be positive or negative
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Electric Field
 Lorentz Force Law → The force F, acting on a charged
particle in the presence of an electric field and a
magnetic field is:
F, E, B, & v are vectors
q = electric charge of the particle
v = velocity of the particle
E = electric field, B = magnetic field
F  qE  qv  B
 If the magnetic field is zero, then the electric field that
pervades the space around the charge is:
F
E= q
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What Is?
 An electron? Subatomic particle with electric
charge; we call that charge negative
 Electron is part of the lepton family
 Called an elementary particle, since it seems
to have no sub-particles
 Has mass of approx. 1/1836 of a proton
 Yet electrons have some properties of
particles AND waves
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What Is?
 Coulomb is the fundamental unit of electrical charge
 And identifies the SI derived unit of electric charge
 Symbol for Coulomb is C; the symbol for charge
flowing, creating a current, is: Q or q
 A coulomb is equal to a charge of 6.24150934 × 1018
electrons (or protons)
 What exactly a charge really is, we don’t understand,
but we do know some key properties, and we can
measure such properties quite accurately
 Similar to gravity: we can measure and use it, even
fly to the moon with rockets overcoming gravity, but
don’t fundamentally understand how gravity works;
theories exist
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7 SI Base Units: No Coulomb, No Volt
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7 SI Base Units
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Units Derived from 7 SI
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What was Ampere Again?
 Unit of current. One of the base units of the SI
 Named after André Marie Ampère, French physicist
1775 – 1836
 When about 6.24150934 × 1018 electrons stream
though a conductor in 1 second, the charge moved
is 1 C and the current is 1 A; ACA “amp”.
i = dq / dt
1A=1C/s
assumes even flow of current during one second s
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AC DC Current
 Carriers of charge may be:
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
Electrons Ions +
Holes +
André Ampère
 In direct current (DC), charge carriers move
in one direction, viewed macroscopically
 In alternating current (AC), charge carriers
alternate direction periodically; frequency in
Hertz [Hz]
 SI unit for current is ampere is A = C / s
 Picture of physicist André-Marie Ampère
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What is?
 We discussed definition of Volt, Power, Resistance
 Discuss inverse of resistance: Conductance
 In later presentation we discuss Ohm’s Law in detail
 Here highlight:
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Ohm’s Law
 At constant temperature, the current I
through some material is directly
proportional to the potential difference ΔV
between its ends; Δ is generally omitted!
I ≈ ΔV
 The resistance R is defined as:
V
R
I
 The general form of Ohm’s Law is:
V  IR
V
R
I
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V
I
R
Georg Ohm
Application of Ohm’s Law
Given:
Material of known resistance R
Voltage V is applied across the material
Result: Current I = V / R will flow through it.
Given:
Material of known resistance R
Known current I flowing through it
Result: Voltage V = I · R exists across the
material (known as a “voltage drop”).
Given:
Known voltage V across the material
Known current I through the material
Result: Resistance of the material is R = V / I.
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What is?
 Conductance G is a measure of the ease with which
a current flows through a material; i.e. it is the
reciprocal of resistance:
1
G
R
 Ohm’s Law in terms of conductance (I below, not 1):
I
G
V
I
 I  VG
 V
G
 SI unit for conductance is Siemens [S], the inverse
of resistance
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References
1. Ohm’s law:
https://en.wikipedia.org/wiki/Ohm%27s_law
2. https://en.wikipedia.org/wiki/André-Marie_Ampère
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