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
MUCH smaller!!
Sound going to microphone has little power, makes smaller electrical signal.
Need big current to drive speakers.
Signals from CD player, phonograph (is just needle with microphone), radio
are similar to microphone. Need to get much more power to drive
a speaker.
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
Put pieces together - basics of audio
amp
+9V Power Source
50 Ohm
Resistor
B
Signal in from
A
CD Player
Gate
Signal out drives
speaker
C D
S N
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.
NO:
No extra power!
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.
SHOW SPEAKER IN ACTION
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.
ans. c. some field will miss secondary coil
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
d. iron- concentrates magnetic field. Does not conduct electrons so does
not matter if insulated. In fact cores are painted to insulate and avoid rust. Also
note, can not use permanent magnets as cores, because cores have to be able
to reverse back and forth!
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
a. 0 V. Battery would make a constant magnetic field through the secondary.
Need a changing magnetic field to give electrons push to get voltage and
current out of secondary
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
ans. b) 1 on primary for 10 sec.
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)