Download ground

Updates
This week and next:
 Homework #4 (uses online applet)
 Reading Quiz on Tuesday will cover today’s reading and
Tues reading: Review of 7.1 (Clocks) and 7.2 (Violins) for
Today and 10.1 (Amplifiers) and 11.1(Radios) for
Thursday
Upcoming Dates:
 Feb. 20: Deadline for students who have three or more
final exams on the same day to make arrangements to
change their final exam schedule
 Feb. 17,18,19: LAB # 2, EKGs
Today and next class:
•
Sound waves and speakers – Short review of
1010 … See Phys1010 October 7 Lecture notes.
•
Audio Amplifiers – electronics that makes this
work.
Looks complicated … new circuit part (capacitors, transistors,
resistors.) Our goal is for you to be able to understand how
components work and reason through where electrons are
+9V Power Source
flowing and why.
50 Ohm
Resistor
B
Signal in from
CD Player A
Gate
Signal out drives
speaker
C D
S
N
Permanent
Magnet
E
Ground
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
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 ideal gas simulation
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
Look at sound:
Microphone detects changes in pressure.
Sound waves traveling out
Hit microphone,
It flexes, Creates
electrical signal
Higher P
Lower P
pressure
time
Microphone
Higher P
Lower P
pressure
time
Question: If I increase the volume, what will happen to the signal from the
microphone?
a. The peaks will go up and the valleys will go down.
b. The peaks will get closer together.
c. The whole signal will go up.
d. Both a and b.
e. Nothing will happen
DO EXPERIMENT….
Microphone
Higher P
Lower P
pressure
time
Question: If I increase the volume, what will happen to the signal from the
microphone?
Answer is a. The peaks will go up and the valleys will go down. When I
turn up the volume, the speaker cone moves further and piles up
more air molecules. High pressure is higher. Low pressure is lower.
But on average pressure is the same.
So, a louder volume means a larger pressure difference between peak
and valley.
Microphone
Higher P
Lower P
Amplify Voltage
Ear is detects very, very small pressure changes:
Normal pressure of air (at sea) = 1 atmosphere
Minimum pressure change detectible by ear = 2 X 10-10 atmospheres
minimum change is 1 part in 5 billion,
Maximum pressure change detectible by ear = 3 X 10-4 atmospheres
maximum is 1 part in 3600.
More than max === OUCH!!!!!
Thinking about waves:
Frequency (f)
(Pitch)
Wavelength (l)
# of oscillations/sec
Distance of one complete cycle
(Hz = 1/s)
(m)
(e.g. distance between pressure maximums)
Period (T)
Time for one complete oscillation
(s)
Speed (v)
Distance traveled per second
(m/s)
Relationships among these variables:
Speed of sound = l X f
 distance of one oscillation * # oscillations per second
= total distance per second.
Speed of sound = l / T
f = 1/T  # oscillations/second = (1 second)/(time for one oscillation)
End of sound review.
On to new stuff:
How can we make speaker cone move back and forth?
Two parts:
1) Currents through coils produce magnetic
fields
NEW 2) Controlling the current through the coils.
Speaker need to oscillate back and forth work?
• Electricity causes speaker to move
Currents through coils produce magnetic fields!
Coil of wire
Cone
moves
Permanent
Magnet
S
N
I
N
S
N
N
S
S
Drive voltage
V
time
Cone
moves
I
Drive voltage
Speakers and Microphones
Speaker:
Electrical signal Sound
Microphone:
Sound electrical signal
Microphone
Coil of wire
Cone
moves
S
N
I
N
Permanent
Magnet
S
Drive voltage
V
time
V
Oscilloscope
time
How does voltage driving speaker need to change in order to increase
volume from speaker? (reasoning?)
a. Voltage oscillations need be more rapid (more oscillations per second).
b. Voltage oscillations need to be less rapid (fewer oscillations per second).
c. Voltage oscillations need to be larger (higher highs and lower lows).
d. Voltage oscillations need to be both larger and more rapid.
e. Voltage oscillations need to be smaller (less high and low).
c. Larger peak voltages, gives larger current flow through speaker.
More current  means bigger magnetic field in coil
Bigger magnetic field in coil means speaker moves back and forth more.
Controls volume.
How does voltage driving speaker need to change in order to change pitch of
tone to make it lower? (reasoning?)
a. Voltage oscillations need be more rapid (more oscillations per second).
b. Voltage oscillations need to be less rapid (fewer oscillations per second).
c. Voltage oscillations need to be larger (higher highs and lower lows).
d. Voltage oscillations need to be smaller (less high and low).
e. Voltage oscillations need to be both smaller and more rapid.
b. lower pitch means lower frequency,
So speaker goes back and forth fewer times in one second,
Voltage oscillations need to be less rapid. (farther apart in time).
Current in speaker changes directions fewer times per second.
Look at signal driving speaker vs. microphone signal
How does signal driving speaker compare with voltage/current that
comes out of bare microphone?
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.
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.
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!
Voltage supply
voltage
current
0
0
time
time
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.
Capacitor can take voltage of …. and make current like
current
voltage
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
and roll up into little tube.
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 capacitor.
Hook up to battery and close switch, 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 “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
capacitor.
b. Electrons flow down for a little while
then stop. Are attracted to + terminal of
battery. Stop flowing when shortage of
electrons at top (extra + charges) gives
strong enough force so no more electrons
can flow.
Current
Capacitors – two metal plates that store charge; insulator in
between plates.
0
time after voltage
hooked up.
-  -  - -
+++++++
+++++++
A B
+9V “Above Ground”
GROUND
(0 V)
E
Capacitors – two metal plates that store charge; insulator in
What was current on the right
between plates.
-  -  - -
A B
+++++++
+++++++
b. Electrons flow down for a little
while then stop. Are attracted to
+ terminal of battery. Stop
flowing when shortage of
electrons at top (extra + charges)
gives strong enough force so no
more electrons can flow.
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.
I?
+9V “Above Ground”
GROUND
(0 V)
E
Capacitors – two metal plates that store charge; insulator in
between plates.
c. same as on left but opposite direction, electrons flow up towards capacitor.
Extra positive charge on left side of cap. attracts negatives.
So end up with exactly equal and opposite charge on each side
of capacitor.
Flows until 9 V difference across Capacitor balances force from battery.
e’s flow out of ground. Equal number of e’s flow from battery into ground as
flow into + side of battery.
-- -- -- -- -- -- -+++++++
+++++++
A B
+9V
-  -  - -
GROUND
(0 V)
-  -  - -
+9V “Above Ground”
-  -  - -
Capacitor
Demo
-  -  - -
E
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
Answer is c: 0 Volts. If not 0 Volts, would still have flow of electrons to or
from ground. Voltage difference across capacitor plates EQUAL to voltage
difference across Power Supply
-- -- -- -- -- -- -+++++++
+++++++
A B
+9V
0V
+9V “Above Ground”
Adjustable
Power Supply
GROUND
(0 V)
E
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
1) Plate A is at higher voltage than Supply. greater than attraction to Plate A, so some
Electrons flow towards Plate A until
electrons leave … flow back to Ground.
voltage difference is 0. Excess positive
A B
charge on Plate A will decrease.
+4V
+9V
+0V
-  -  - -
+4V
9V “Above Ground”
-  -  - -
E
GROUND
-  -  - -
Adjustable
Power Supply
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
+4V “Above Ground”
E
GROUND