Welcome to Physics 361: Electronics I

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Welcome to Physics 361 Electronics I
Homework Barnaal Problems 8 (pg 47), 9, 11 (pg
48) and complete problem later in the notes Lab
1 (next Tuesday) Ohms Law and Kirchhoffs Laws,
Read Handouts, Barnaal 3-15, 37-42
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Welcome to Physics 361 Electronics I Analog
Electronics
Instructor Dr. Tom Böttger, Assistant
Professor office Harney 105, 415- 422-5716
(office) research lab Harney 110 email
tbottger_at_usfca.edu homepage http//physics.usfca.
edu/tbottger/bottger_site.html office hours
after class, T, Th 10.30 - 12 am (subject to
change), you are welcome to drop in outside my
office hours but I may not always be available
lectures Harney 143, R 1.30 PM, attendance is
strongly encouraged. If you miss a class, you are
responsible to obtain handouts and homework
assignments
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Text
Analog Electronics for Scientific Application,
1989 by Dennis Barnaal Please make an effort to
read the relevant text chapter before you come to
class. Book is a bit outdated but very cheap,
will be supplemented with handouts from other
texts Lab Manual Distributed in class.
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Labs

• Harney 139, Tuesday 1.30pm 3.15pm and on your
  own
• Magnetic card lock, check whether it works with
  your student ID
• Integral part of this course, work in groups of
  two or alone, lab report due at beginning of next
  lab

• lab manual will be distributed in class or lab
• see syllabus on lab rules and laboratory notebook
  style

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Physics 361 laboratory

• The lab course is a required co-requisite!
  attendance is mandatory and will be recorded
• make arrangements in advance if you are to miss a
  lab, you can usually make up labs outside regular
  lab hours (encouraged)
• If you miss more than two labs, you will
  automatically receive an F in the course
• Lab/tutorial manuals will be provided
• Labs dont start until next Tuesday

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Homework

• Problems will be assigned on Thursday, due at the
  beginning of lab on the following Tuesday
• Returned in class on Thursday
• No late homework accepted since solutions will be
  posted after class
• Cooperation with your classmates is strongly
  encouraged. Discussing and explaining physical
  concepts will give you a higher level of
  understanding. However, everything you turn in
  must be your own work.

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Homework grading
0 - minimal effort 1 - reasonable effort, but
serious mistake in physical reasoning 2 - physics
ok, but serious math error 3 - completely
correct, perhaps minor math error Cooperation
with your classmates is strongly encouraged.
Discussing and explaining physical concepts will
give you a higher level of understanding.
However, everything you turn in must be your own
work. Cheating in exams will result in an F in
the course.
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Cell phones and talking
Cell phones. Please turn off your cell phone
during lecture and laboratory sessions. Talking.
There will be plenty of opportunity to talk with
your neighbors in group activities. Feel free to
ask questions. Also remember that the lecture
hall transmits sound very effectively disturbing
others in the class (including the instructor).
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Exams

• 1 midterm exam 10
• Final exam (comprehensive) 20
• Cheating in exams will result in an F in the
  course.

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Explain your reasoning
Please show your work to receive full credit.
Empty pages earn you zero mercy points.
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Grading
Midterm exam 10 Final exam 20
Homework 20 Laboratory reports 50
A 90 100 B 80 - 89 C 70 - 79 D 60 -
69 F lt 60
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Class web page!

• http//physics.usfca.edu/tbottger/physics_361.html

• Username physics361
• password electronica
• There, you can find
• 1. Posted (pdf format) syllabus, homework
  solutions,
• 2. Posted (ppt format) lectures, (please dont
  print them out!)
• 3. Class announcements and other important items
  relating to this class.
• Get into the habit of checking this Web page
  often!

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Basic concepts
CHARGE
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Electric Charge

• A body is said to be charged when it has an
  excess or deficiency of electrons.
• The unit of electrical charge is the coulomb (C)
  6.24 1018 electrons.

How many electrons is it?
Typical 1108 !!

-
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Coulombs Law
Coulomb (1736 1806)

• not derived by observation
• A strong force! (will see in a moment)

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Similarities between Coulomb force and
gravitational force
Q is or - ? F is attractive or repulsive
m is only ? F is only attractive

• Both are inverse square laws and conservative
  forces

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Force between electron and proton in H atom
? 39 orders of magnitude smaller!
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Voltage

• When charges are transferred from one body to
  another, a potential difference or voltage
  results between them.
• The voltage between two points is 1 V if it
  requires 1 J of energy to move 1 C of charge from
  one point to the other i.e. ?V W / Q.
• Symbol for DC voltage sources

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Electromotive force (EMF)

• The battery acts like a (current) pump.
• EMF is the energy that one Coulomb of charge
  gains in passing through a battery.
• Measured in J/C or volts.
• Well just call this the battery voltage
• Also given the symbol ebatt.

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EMF and voltage
I
ebatt 12 volts
loses 12 J/C
R
12 J/C
-12 J/C
DVab V0
DVcd
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Potential in a resistor loop
What goes up must come down
V
V0
d
c
d
b
a
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Electric current
When charges of a like sign move a current is
established. Suppose the charges move
perpendicular to a surface of area A as shown.
If DQ passes through A in time Dt the average
current during that period is
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Current

• Measure of the rate of flow of charge.
• Average current

• Instantaneous current

• Units?
• Measured in Coulombs/second Ampere (A).

• 1 A is 1 C of charge passing a given point in 1
  s, i.e. I Q / t or Q I t.

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Current
Lamp

E
-
I
Direction of conventional current

• Electrons flow from negative terminal to positive
  terminal but conventional current flows in the
  opposite direction.

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Special note

• The current is actually carried by electrons (-)
  but we usually work as if ve charge is moving,
  which we call standard current convention.

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electron flow
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electron flow
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electron flow
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electron flow
30
electron flow
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electron flow
32
electron flow
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electron flow
34
electron flow
35
electron flow
36
-1
electron flow
37
electron flow
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Measuring V and I
Place a voltmeter across or in parallel with the
component where the voltage is to be measured.
To measure current through a component the
ammeter must be placed in series with the
component.
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Potential and current
Georg Simon Ohm (1787-1854) a German observed
that in metal wires the current in the wire was
proportional to the potential difference between
the two ends
This works just like gravity. For water flowing
down a hill the greater the elevation change
(greater change in gravitational potential) the
swifter the water flows. For electrical current,
the greater the electrical potential difference
(or voltage) the greater the current.
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Resistance

• The proportionality constant is called the
  resistance R
• Represents the ability of a circuit element to
  impede the flow of current.

• Units?

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Ohms Law

• R is different for different materials and for
  different shapes of wire.

• Not all materials follow Ohms Law!
• Those that do are called ohmic

Others are are called nonohmic
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Resistor color code

• Resistors are used to control the amount of
  voltage and/or current in a circuit

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Resistor color code
Big Brown Rabbits Often Yield Great Big Vocal
Groans When Gingerly Slapped.
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Resistivity

• ? different for different materials (copper,
  aluminum, iron, etc.)

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Types of Resistors

• Fixed Resistors e.g. molded carbon composition,
  carbon film, metal film, metal oxide, wire-wound,
  IC resistor network
• Variable resistors potentiometer rheostat

Fixed Resistor
Potentiometer
Rheostat
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Power
Power is defined as the rate of doing work
or, equivalently, as the rate of transfer of
energy.
(Watts, W)
where W is the work (or energy) in joules and t
is in seconds.
In terms of electrical quantities
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Kirchhoffs Laws
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Gustav Kirchhoff(1824-1887)
A German
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Kirchhoffs Laws
When a circuit cannot be reduced to a single loop
as we have done then we need something else. To
analyze these more complex circuits we will use
two laws known as Kirchhoffs Laws.

• Kirchhoffs current Law (KCL) The sum of the
  currents entering any junction must be equal to
  the sum of the currents leaving that junction.
  Conservation of charge.
• Kirchhoffs voltage Law (KVL) The algebraic sum
  of the changes in potential across all the
  elements in a closed circuit loop must be zero.
  Conservation of energy.

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Kirchhoffs Current Law (KCL)
R1
I1
I
I1
R2
I2
I2
I I1 I2
I
?V
What goes in must come out
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Kirchhoffs Voltage Law (KVL)
voltage rises and drops must be equal around any
closed loop
What goes up must come down
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Problem solving strategy Mesh (Loop) Analysis

• Draw the circuit diagram and label all known and
  unknown quantities. Assign a direction to the
  currents in each part of the circuit. If you
  guess the wrong direction your result will be
  negative, but will have the correct magnitude.
  Once you assign a current direction you must
  rigorously adhere to Kirchhoffs rules.
• Apply the current rule to any junction that
  provides a relationship between the various
  currents.
• Apply the voltage rule to as many loops as needed
  to solve for all the unknowns. Careful with the
  signs!
• Algebra solve equations for unknown quantities.

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Rules for using KVL
For all these we are traversing a loop in the
direction from point a to point b.
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Example
Find currents I1, I2, and I3 !
1. Apply the current rule at c.
2. Apply the voltage rule in the clockwise
direction for loop abcda.
3. Apply the voltage rule in the clockwise
direction for loop befcb.
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Example
?
?
?
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Example
?
?
?
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Voltage Divider
I have a 5 V supply but need 3 V for my
application
The voltage across the input source is
The voltage across the output terminals A and B
is
The output voltage is thus
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More general Voltage Divider Rule

• The voltage applied to a series circuit will be
  dropped across all the resistors in proportion to
  the magnitude of the individual resistors.
• Vx (Rx / RT) V

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Parallel Circuit KCL

• Elements/branches are said to be parallel when
  they have only 2 nodes in common. The voltage
  across all parallel elements in a circuit will be
  the same.

KCL
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Current Divider
The source current is divided between the two
resistors and is given by
The voltage at the output is
The output current from the divider is
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Parallel Circuits KCL

• Kirchhoffs Current Law ?I 0, or S Iin S
  Iout
• Overall conductance GTG1 G2 . . Gn 1/RT
• i.e. Total resistance, RT 1/(1/R1 1/R2 . .
  . 1/Rn)
• For 2 resistors in parallel RT R1R2 / (R1 R2)
• For n equal resistors in parallel RT R/n where
  R is the resistance of each resistor.
• Current divider rule Ix (RT/Rx)IT
• Total power dissipated PT P1 P2 . . . Pn
• where P1 V2/R1 or VI1 . . . . Pn V2/Rn
  or VIn

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Series - Parallel Networks
R1
R1
RT1
R2
R3
R2
(a)
R3
R4
RT3
R2
R5
R6
R1
RT2
R3
R4
R7
(c)
(b)
Complete this as a homework exercise!
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Circuit Ground

• Ground is an arbitrary electrical point of
  reference or common point in a circuit.
• The circuit ground is referred to as a chassis
  ground when it is connected to the metal chassis
  of an appliance.
• For safety purpose, the chassis ground is usually
  connected to the earth ground via the electrical
  outlet box.

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Ground Symbols