Revisiting What we Know…
After some more about speakers and amplifiers
Day 9:
Questions?
Speakers and amps
In-class- Review#1
Reminders/Updates:
Thurs class/review;Exam 1
Come to exam
With:
pencil, calculator (or slide rule),
3x5 card with equations etc
Look at signal driving speaker vs. microphone signal
How does voltage/current that comes out of bare microphone compare with
signal driving speaker?
a. bigger, b. smaller, c. about the same
do experiment
2
The big picture.
Audio amplifiers take little signals
(low power = low V and/or low I)
and boost them up to big powerful signals
(high power, big V and/or I).
Examples:
CD player or radio signals amplified to drive speakers.
Door signal boosted to open or close door.
Panel signals boosted to control microwave oven.
Built up from lots of little pieces hooked together by wires.
Power ultimately provided by
batteries or AC power from wall.
Function determined by the pieces
and how they are hooked up.
computers- millions of transistors
(show circuit board)
3
Basics of audio amp
voltage
+9V Power Source
0
50 Ohm
Resistor
current
time
Signal out drives 0
speaker
time
B
C
D
Small signal in from
CD Player
A
S
N
Gate
Permanent
Magnet
E
Ground
4
TRANSISTORS (STAR OF SHOW)
Power amplification comes from transistorother components needed for transistor to work.
Gate
Supporting cast (with symbols):
a. transformers to get different voltages.
b. few other little things not very important.
c.resistors- control currents, divide up voltages as desired.
d. diodes- make AC voltages from transformers into the DC
voltages transistors need.
e. capacitors- pass AC while blocking DC. Also store electric
charges.
Learning goals:
1) Basic physics of d. and e. General idea how they are used.
2) Basic function of transistors as adjustable current gate (voltage controlled
resistance) and how this allows amplification of signals.
3) NOT expected to learn detailed physics of transistor, will cover briefly.
5
How do you (I) figure out what will happen with
these new components :
Be the electron! Follow electron rules:
1. Opposite charges attract: Electrons are
attracted to excess positive charge
2. Like charges repel: Electrons will repel each
other.
If both of these, then force of attraction to positives will
balance force of repelling of each other, else electrons will
move (flow).
3. Rate of electron flow depends on resistance
and voltage difference between these two points
+9V
4. Higher voltage = more excess
-++
positive charge at that point.
-++
 Electrons flow from lower to
-++
higher voltage
6
appliance
Audio Amplifiers
Weak signal ->
small sound
V
time
Could we use a transformer to make the signal from microphone big
enough to drive a speaker?
a. yes, b. no, c. yes if big enough transformer but probably not practical.
7
transformer will not work. Does not add energy. Need more power (I x
V) to drive speaker. Transformer makes I or V bigger, but P=IV stays the
same!
•Audio amplifier adds energy.
same power, puny
signal
V
Amplifier
Larger current,
More power!
8
Voltage supply
voltage
0
current
time
0
time
So, is this the type of current we want to drive the speaker?
a. just what you want to drive speaker
b. speaker might work, but would waste a lot of power
c. speaker would not work at all
d. speaker would burn up
b. sound comes from motion back and forth,
this has big constant current on top of oscillation.
at best wastes energy, at worst burns up speaker d.
9
Capacitor can take voltage of ….
voltage
0
and make current like
current
0
time
turn this (bad) into
time
this (good)
What is a capacitor? Just two thin metal foil plates.
symbol
thin metal plates with
wires to each
add insulator between so can squish together
10
and roll up into little tube.
Start Exam Review
Exam mechanics
•
•
•
•
•
Designed for 1 hr (you can take 1.5)
In BESC 180, 7:30 pm prompt!
1 3x5 card (more on this)
Pencil, calculator
Do not cheat. It’s the surest way to piss me off (and
it probably won’t help)
• If you have an accommodation for extra time please
see me after class today if you haven’t already.
Exam:
some essay /long answer (25%), some m/c (75%) -- designed to be like HW
best preparation- go over homework and solutions,
also class notes (particularly questions in class).
Most importantly, be sure you make sense of the answers.
one 3 x 5 card. Important part of studying to make list of most important ideas and
formulas needed.
Resources:
- Help Room
- Labs Times will be review:
--- any person any lab
- Online.
Topics:
Electrostatic forces … will things attract, repel, and why.
Circuits with wires, batteries or regular outlets, bulbs, heaters…
-- think like an electron, how fast will you be able to flow and where will you lose your
energy. Power, current, resistance, voltage drop.
Conductors, insulators, and semiconductors …
--what are the differences on a macroscopic scale?
-- how is conductivity influenced by how electrons are found in bands
Photocopiers,
--how they work…, role of photoconductors and forces between electric charges.
Power Distribution …
--power loss in wires, why AC?, why HV and LV combo?
--transformers: how do they work, how would you design transformer for power
system to raise or lower voltages, currents creating magnetic fields and CHANGING
magnetic fields creating currents.
Speakers / sound
Hold off on details of Amplifiers for next time ; ) but should know basics.
Review in reverse order here.
Sound /amps
Power / Transformers
Energy Bands / Insulators, Conductors, Photoconductors.
Electric circuits
MidTerm
Cover
1020, Spring 2014,
EXAM 1.
Each m/c question is worth 2 points. The long answer 14 points. Total points = 60.
Beware of grabbing at a numerical answer simply because you happen to see that number as you are
calculating. We are sneaky and put in choices that are numbers you are likely to produce if you are not sure
how to do the problem correctly. For many problems, it is good to make a simple sketch to picture the
problem correctly.
Numbers / Equations you may need:
Speed of light in empty space (c)
Speed of sound in air
Speed of sound in water
3.0 x 108 m/s
331 m/s.
1540 m/s.
1 electron – Volt = 1.602 x 10-19 Joules
Household Voltage = 120V A/C
Some equations that we used in class:
DV = I R
P = I DV
Vsec = (Nsec/ Nprim) * Vprim
To ensure that you properly understand the question, we strongly recommend that you make a sketch of the
situation described by the problem before giving an answer.
Remember to write your name on your answer sheet.
Return both the answer sheet and the exam.
Bring your questions
• In addition I’ll review key ConcepTests from
the areas we’ve covered.
Everyday Life Experience at the Ballpark:
You are at the ball park sitting in the bleachers in the
outfield (~325 ft from the batter). You see the bat hit
the ball. About how long will it take before you hear
the bat hit the ball?
About 0.3 seconds
How fast is that sound traveling?
Speed = distance/time = 325 ft/0.3 s = 1083 ft/s or 330 m/s
Speed of Sound in Air =
331 m/s at 0 degree C
343 m/s at 20 degree C
(Speed of Light = 3 X 108 m/s… much, much faster)
About 0.3 second means ~ 325 ft away from batter.
In 0.03 seconds, travels ~ 32 ft and in 2 sec, ~2,160 ft
17
What produces the sound?
When bat hits ball, push the air causes a slight increase in the
pressure of the air followed by a slight decrease in pressure air.
What is it that your ear is detecting?
This pressure fluctuation travels out in all directions as a wave, as air
molecules push on the ones next to them and then they push on the ones
next to them. As the pressure wave reaches your ear, you hear sound.
AIR MOLECULES
Just after
clap
Slight decrease
More densely packed air molecule…
Slight increase in pressure
Later
Experiment with wave interference sim
18
Creating Musical Tones
To create a pure sustained tone (like concert A), the speaker
pushes on the air at regular intervals and this pushes on the air
creating a series of pressure waves.
In speaker we vibrate cone:
Higher P
Lower P
All instruments work with same principle... push on air at regular
intervals.
19
transformer will not work. Does not add energy. Need more power (I x
V) to drive speaker. Transformer makes I or V bigger, but P=IV stays the
same!
•Audio amplifier adds energy.
same power, puny
signal
V
Amplifier
Larger current,
More power!
20
Capacitor can take voltage of ….
voltage
and make current like
current
0
0
time
time
turn this (bad) into
this (good)
What is a capacitor? Just two thin metal foil plates.
symbol
thin metal plates with
wires to each
add insulator between so can squish together
21
and roll up into little tube.
Basics of audio amp
voltage
+9V Power Source
0
50 Ohm
Resistor
current
time
Signal out drives 0
speaker
time
B
C
D
Small signal in from
CD Player
A
S
N
Gate
Permanent
Magnet
E
Ground
22
current through coil gives Magnetic field, reverse current,
reverse magnetic field.
current in
current out
What happens if oscillating current in primary but there is no core?
a. The light bulb will not light because there is no conduction path for electrons to move
from one coil to another.
b. The light bulb will not light because there is no changing magnetic field present.
c. The light bulb will be dimmer than with a core.
d. The light bulb will be the same brightness as with core.
e. The light bulb will be brighter than with core.
NOTE: Not everything curly is a transformer; e.g. lightbulb filament is NOT.
Transformer construction detail. The core.
Magnetic field is always produced from current through primary coil.
Without core, magnetic field spreads out a lot.
Field going through secondary coil is weaker,
Less push (V) on elecs, produces less current,
less power transfer … power wasted.
Vsec = Vpri (Nsec/Npri)
current in
B
current out
What will happen to
light bulb?
iron core concentrates field (sucks it in), more through
second coil bigger current! (incredible graphics display…)
Does not carry current!
Transformers
Which would make the best core for a transformer if strength did not matter?
a.wood, b. copper, c. glass, d. iron wrapped in plastic insulator
If I took a transformer used to convert 100 V up to 1000 V and I hooked the
primary up to a 12 V car battery. If I then went to measure the voltage across
the secondary coil, what would I find?
a. 0 V, b. 12 V, c. 1200 V. d. 120 V, e. 1.2 V
What is ratio of turns on primary to secondary?
a. 10 pri. to 1 sec., b. 1 to 10, c. 100 to 1, d. 1000 to 1, e. 1 to 1000
power distribution system
500,000 V (on towers)
power plant
substation
5000V
5000 V
running around
town.
120 V
short wires
into houses
Vsec = Vpri (Nsec/Npri)
or
Vout / Nsecondary = Vin / Nprimary
I sec = I primary x (# turns primary/#turns secondary)
Know this from P=IV (power isn’t changing)
Generators
How did I generate power in class?
moving coil through magnetic field.
so if moved coil or magnet could generate
electric power.
Power plants: use steam or water
to spin magnets past coils (or vice-versa)
S
I, V out
magnets
N
N
S
S
N
N
S
iron core
spinning turbine
hydroelectric turbine
E = mgh, power = mass/sec x gh
~ 40% efficient
Pelectrical out = .4 (mass water/s x gh)
h
steam plants same idea,
boil water to make steam pressure
to spin turbine.
boiler
turbine
I
cooling pond
conduction of materials- energy levels and electron occupation.
3
2
1
How many energy levels are in band 1?
a. none, b. 1, c. between 1 and 10, d. an enormous number
Band 3
Band 2
Band 1
How many energy levels are in band 1?
a. none, b. 1, c. between 1 and 10, d. an enormous number
How many empty and filled levels are there in band 3?
a. 1 filled, none empty, b. 1 of each, c. very many of each,
d. many filled, 1 empty
3
many empty
levels close
together
2
1
How many empty and filled levels are
there in band 3?
Answer is c: very many of each
True / False
1. the upper electrons in band 3 can easily
move because there are very close energy
levels they can move into.
2. the upper electrons in band 2 can easily
move because there are very close levels to
move into.
a. 1T 2T, b. 1F 2F, c. 1T 2F, d. 1F 2T
many filled levels close
together.
Material A. What is it?
Band 2
Band 1
empty
full
Fill in the blank:
This material is a ______________.
When hooked to a battery, electrons in Band 1 will ______________.
When hooked to a battery, electrons in Band 2 will ______________.
a. conductor, move, move
b. semi-conductor in the dark, not move, move.
c. semi-conductor in the light, move, move.
d. semi-conductor in the light, not move, move.
e. insulator, not move, not move.
conductor- empty levels
very close
empty
insulator- big jump to empties.
empty
full
ENERGY gap- no ALLOWED levels
full
full
electron like ball rolling on
almost flat ground
move easily
electron like in pit.
Can’t move
without big boost.
semiconductor-- half way in between. Little gap to empty levels, shallow
pit.
empty
full
Making a Copy
Part I: a charge image
Photoconductor
Corona wire
metal
Light
velocity
+++++
+++++
Document
Light
+++++
Charge image
+++++
36
Part II: Transferring Charge Image to Toner and Paper
Just like Part I, in reverse.
Roller and
brush
Release toner
toner
Light
+
+++++
Positive
toner particle
Charge image
+ ++
---
+++++
+
+
+++++
Black image
Heat
Charge Paper
--+ ++
+ ++
+++++
37
All Pretty Simple Charge-Physics (opposites attract), except
for selectively removing the charges
Document
Charge image
+++++
Rumor has it Carlson tried trained squirrels
with little fingers for long time. Gave up and used physics.
(yes kidding)
38
Photoconductorsway to get charges to leak away when shine light on surface.
1. before light.
------------------ -- -----------(side
view)
e’s want to go to bottom, but R too high, stuck
V
conducting plate
2. where light hits, R low, electrons flow
away.
--------- -- ------------
-
-
V
-
-
-
-
39
Semi-conductor physics
where light hits, R low, electrons flow away,
then add ink, sticks only where charges.
--------- -- ------------
-
-
V
-
-
-
-
-
Very special material- low R (“conducts”) only when
light hits. “Photoconductor”
To understand, have to understand what determines
resistance of a material.
• insulators (wood, ceramic, plastic)- very high resistance.
• conductors (metals)- very low resistance
• “semiconductors”- in the middle. Resistance depends on temp., light,
cleanliness.
40
Where does the power go?
Wires
Hair Dryer, Lights, and Stereo plugged into same outlet
Hair Dryer
Lightbulb 1
Stereo
10 Amps
0.5 Amps
Lightbulb 2
2 Amps
What is the current through the wires?
a. 10 Amps
b. 12.5 Amps
c. 7.5 Amps
d. more than 12.5 Amps.
If the resistance of each light bulb is 100 Ohms, how much power is going into the
two bulbs combined?
a. 25 Watts,
b. 50 Watts
c. 100 Watts
d. 200 Watts
120 V
Why does light dim when heater on, how much less current through light?
(need to think like an electron!)
Job for electron man!
(on rollerblades)
e
boot camp for electrons.
bunch of them going through
obstacle course.
glide down easily, just a few bumps. Hardly any energy.
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
lots of energy
at start.
e
e
e
!?#%, bridge
out, stuck.
e
energy used up getting
through course. Vigor (V)
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
exhausted!
Rules:
a) no electron deaths
b) no passing of electrons
c) electrons have energy
(high at start , low at end, determined by V)
e
e
e
e
e
e
e
e
e
feet of
mud! takes lots
of energy to get
through. R (rottenness)
V=IR
P=IV
d) Material has resistance
(lets electrons pass easily or not)
!! Careful: know which
elements / system these
apply to !!
What happens when bridge gets
fixed so have another route?
e
e
e
e’s piled up down both routes, so
still divide up and go down both,
just end up faster on bridge route
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
lots of energy
at start.
e
e
e
e
(bulb)
deep
mud!
energy used up getting
through course. Vigor (V)
e
e
e
e
(heater)
pretty
easy
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
exhausted!
What happens now compared with bridge out?
a. Can go easy path across bridge. Takes less energy, can go much faster.
b. Have to go down entry and exit road much faster. (wires fro outlet)
Lose more energy hitting bumps at high speed than with no bridge.
Energy used going down road not so small as when small current
c. Ones that go through mud have a little less energy to get through it than they did
without bridge path, so get through it slower.
#/s in bulb = I_bulb = (Vtotal - Vroad)/ Rbulb,
but going faster so Vroad lot bigger than before,
go through mud slower than if bridge out. I smaller. (Vroad = I Rroad)
why different voltages, why different plugs, why 3 wires?
Are different voltages more dangerous?
230V more dangerous than 120V
- easier to fibrillate heart (but wastes less power in wires).
Different plugs partly historical, but modern European
plugs harder to touch “live” plugs than US. Probably
designed that way because voltage more dangerous.
3rd wire- ground. Protection
electric appliance
+120 to –120 V
“ground”, always 0V
protection if wire touches case
~0 V but goes up when
current flowing back
End of Review
Back to Amplifiers
Unless You Have More Questions
Capacitors – two metal plates that store charge;
insulator in between plates.
No current or electron flow
across here.
+9V “Above Ground”
GROUND
(0 V)
E
in electric circuits hook on part to “ground”, infinite source or sink of electrons.
Does not change anything compared to if wire straight from negative terminal to
47
capacitor.
Hook up to battery and close switch,
what is the immediate current through the meter?
a. no current flows
b. electrons flow down for a little while then stop
c. current continues to flow until battery dead.
d. electron current oscillates back and forth
I?
+9V “Above Ground”
GROUND
(0 V)
E
48
Capacitors – two metal plates that store charge; insulator in
What was current on the right
between plates.
side after hooked up battery?
a. no current flows.
b. same as on left, e’s flow down (away from
capacitor)
c. same as on left but opposite direction, e’s
flow up (towards capacitor)
d. e’s only go from battery to ground.
A B
I?
+9V “Above Ground”
GROUND
(0 V)
E
49
Capacitors – two metal plates that store charge;
insulator in between plates.
What is Voltage at B?
a. -9 V b. +9 V c. 0 V
A B
+9V “Above Ground”
Adjustable
Power Supply
GROUND
(0 V)
E
50
Capacitors – two metal plates that store charge;
insulator in between plates.
-  -  - -
-- -- -- -- -- -- -+++++++
+++++++
What if can control voltage instead of using 2) Attraction of electrons on Plate B to
battery? What happens if drop voltage to 4 positive charge on Plate A has decreased.
Volts?
Force of electrons repelling each other is
greater than attraction to Plate A, so
1) Plate A is at higher voltage than
some electrons leave … flow back to
Supply. Electrons flow towards
Ground.
Plate A until voltage difference is 0.
+0V
Excess positive charge on Plate A
will decrease.
-  -  - -
Adjustable
Power Supply
-  -  - -
E
-  -  - -
A B
GROUND
51
Capacitors – two metal plates that store charge;
insulator in between plates.
So if we can vary voltage at Plate A, we can create an alternating current
through the speaker and drive the speaker.
A B
-  -  - -  -  - -
-  -  - -
Adjustable
Power Supply
------+++++++
-  -  - -
+9V to
+4V “Above Ground”
E
GROUND
52
Simulator!
http://phet.colorado.edu/en/simulation/circuit-construction-kit-ac
53
capacitor
thin metal plates with wires to each
hook up to battery, what is the
current through the meter?
a. no current flows
b. electrons flow down for a little while
then stop
c. current continues to flow until battery
dead.
d. electron current oscillates back and forth
I?
9V
+
-
in electric circuits hook
on part to “ground”,
infinite source or sink of
electrons. Does not
change anything compared
to if wire straight from
negative terminal to capacitor.
54
+
+
+
+
+
+
+
+
---
I
I??
9V
--- -
0
+
time after voltage
hooked up.
-
in electric circuits hook
on part to “ground”,
infinite source or sink of
electrons. Does not
change anything compared
to if wire straight from
negative terminal to capacitor.
What was current on the right
side after hooked up battery?
a. no current flows.
b. same as on left,e’s flow down
c. same as on left but opposite
direction, e’s flow up.
55
d. e’s only go from battery to ground.
---
I
+
+
+
+
+
+
+
+
-
- - -
I
9V!
-
9V
--- - +
0
time after voltage
hooked up.
----
Charges keep flowing
until Vbattery =Vcapacitor
Amount of charge piled up = CV
C is “capacitance”
1farad x 1 V = 1 coulomb
56
If suddenly reduced voltage to 6 V, capacitor would push
harder than batteries, charges would flow onto + side of C
and off of negative side. So changing V gives changing
current on each side of Cap., but constant voltage does not.
so capacitor saves the day, gets rid of all that DC
current, keeps the AC needed to drive speaker.
voltage
and current
0
0
time
time
in
V, I
57
A word about Voltage, Current, Resistance
With V, concerned about about V
V = voltage difference between two points
V = I R
Voltage DROP across an element =
current (I) through an element of resistance (R)
NOTE: R depends upon physical property of material
V depends upon the energy source
(or for part of circuit with fixed I and R, then V is determined by others)
I is then usually determined by the other two…
58
resistors-
used to limit current and set voltage
to set voltage of part of
circuit to +5V relative to G
Still use V=IR
1000 ohms
R1
B
10 V
R2
G
What should R2 be selected to be in order to make
Voltage difference between B and G be 5 V?
a. 2000 , b. 1000, c. 500, d. 200, e. 100 ohms
59
resistors- limit current, set voltage
1000 ohms
R1
B
R2
10 V
G
If B-G is to be 6.7 V, R2 should be ?
60
Vary voltage (positive charges) on A  …. Use resistors to control voltage at A!
What is voltage at A?
Resistor
++++9 V
+++
50ohms
+++
50ohms
A
+++
If we can vary this resistance at will, we can change
voltage at A, make it mimic CD signal then good
music! … Job for TRANSISTOR!
GROUND
61
Up next … the transistor
capacitors- pass AC while blocking DC. Also store
electric charges. -- move the signal to the right range.
resistors- control currents, divide up voltages as
desired.
transistors acts like a valve by being a variable resistor…
can think of as changing from high resistance to low,
made up from . . .
diodes- let current only pass in one direction --- like a
turn-style.
62
THE STAR OF THE SHOW: The TransistorWe have a separate voltage signal (CD signal) that controls size of
effective resistance of the transistor. We can adjust it up and down,
and vary how much current flows through.
transistor
Like water faucet knob, valve for current!
Changing signal at Gate … like turning knob … changes
current!.
Gate
Transistor Amplifies!
1. little change amount of charge on (Valve control)
gate
2. Makes big change in current
(through transistor) … varies
resistance.
Gate, change amount of
+
positive charge at gate, control
valve.
e’s
63
THE STAR OF THE SHOW: The TransistorWe have a separate voltage signal (CD signal) that controls size of
effective resistance of the transistor. We can adjust it up and down,
and vary how much current flows through.
Note:
First Time we see Component with 3 connections
In / Out like other components
AND
a control connection
Note: NO direct connection..
NO current flow through here!
+
Gate, change amount of
positive charge at gate, control
valve.
e’s
64
Transistor like plunger valve on spring that blocks flow of water
as the block moves in and out.
Positive voltage applied is like pulling plunger back, big current,
Zero voltage lets plunger block flow, no current.
water flow
Little push of valve in or out changes
big volume of water (current) that
flows.
+++
+++
voltage
off, NO +excess
+
Lots of excess
charges
charges,
plunger closes,
on gate (voltage)
current
plungeroff
out, current on.
Transistor RESISTANCE IS
Transistor RESISTANCE IS
LARGE. (behaving like
SMALL.
insulator now)
What you are required to know about transistors.
Electrically transistor is like a resistance whose value depends on charge voltage
applied to the gate (i.e. plunger). Power amplification. Little work to move plunger
can change large amount of energy flow contained in large water flow or
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equivalently large current flow through transistor.
Transistors amplify signals
toward
speaker
+ 10 V
lots of power!!
(Power provided by
10 V power supply)
Resistor #1
Tiny
Signal
D
Gate
V
(Resistance of transistor
changes with Gate voltage)
time
+0V
Voltage at D depends on voltage split between
Resistor #1 and resistance of transistor.
But resistance of transistor depends on voltage on gate!
So voltage at D changes with voltage at Gate.
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