updates this week and next: homework #4 (uses online applet) reading quiz on tuesday will cover...

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.

Ground

Gate

+9V Power Source

B

A

C D

E

S N

PermanentMagnet

50 OhmResistor

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 flowing and why.

Signal in from CD Player

Signal out drivesspeaker

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

Speed of Sound in Air = 331 m/s at 0 degree C343 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

How fast is that sound traveling? Speed = distance/time = 325 ft/0.3 s = 1083 ft/s or 330 m/s

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.

Just after clap

Later

AIR MOLECULES

Experiment with ideal gas simulation

More densely packed air molecule… Slight increase in pressureSlight decrease

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.

Higher P

Lower P

In speaker we vibrate cone:

SHOW SPEAKER IN ACTION

All instruments work with same principle... push on air at regular intervals.

Look at sound: Microphone detects changes in pressure.

Higher P

Lower P

Sound waves traveling out

Hit microphone,It flexes, Creates electrical signal

time

pressure

Higher P

Lower P

Microphone

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….

time

pressure

Higher P

Lower P

Microphone

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.

time

pressure

Higher P

Lower P

Amplify Voltage

Microphone

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 atmospheresminimum change is 1 part in 5 billion,

Maximum pressure change detectible by ear = 3 X 10-4 atmospheresmaximum is 1 part in 3600.

More than max === OUCH!!!!!

Thinking about waves:Frequency (f) # of oscillations/sec (Hz = 1/s)

(Pitch) Wavelength () Distance of one complete cycle (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 = X f distance of one oscillation * # oscillations per second

= total distance per second.

Speed of sound = / Tf = 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

2) Controlling the current through the coils. NEW

Speaker need to oscillate back and forth work?• Electricity causes speaker to move

Drive voltage

Coil of wire

V

time

N

S

I

Permanent Magnet

SN

Conemoves

Drive voltage

N

S

INS

Conemoves

Currents through coils produce magnetic fields!

Speakers and MicrophonesSpeaker: Microphone:

Electrical signal Sound Sound electrical signal

Permanent Magnet

Drive voltage

Coil of wire

V

time

N

S

ISN

Conemoves

Microphone

Oscilloscope

V

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 coilBigger 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), radioare similar to microphone. Need to get much more power to drivea speaker.

Audio Amplifiers

V

time

Weak signal ->small sound

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.

Amplifier

same power, punysignal

V

Larger current,More power!

time

current

0

Is this the type of current we want to drive the speaker?a. just what you want to drive speakerb. speaker might work, but would waste a lot of powerc. speaker would not work at alld. 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.

time

voltage

0

Voltage supply

Capacitor can take voltage of …. and make current like

time0

time0

turn this (bad) into this (good)

thin metal plates with wires to each

What is a capacitor? Just two thin metal foil plates.

add insulator between so can squish togetherand roll up into little tube.

voltage

symbol

current

GROUND(0 V)

Capacitors – two metal plates that store charge; insulator in between plates.

No current or electron flow across here.

+9V “Above Ground”

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.

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.

I?

Hook up to battery and close switch, what is the current through the meter?a. no current flowsb. electrons flow down for a little while then stopc. current continues to flow until battery dead.d. electron current oscillates back and forth

GROUND

(0 V)


++

++

++

++

++

++

++


- - - -

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.

A B

time after voltagehooked up.

0

Cur

rent

E

GROUND

(0 V)


++

++

++

++

++

++

++


- - - -

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. A B

What was current on the rightside 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 oppositedirection, e’s flow up (towards capacitor)d. e’s only go from battery to ground.

E

I?

GROUND

(0 V)


++

++

++

++

++

++

++

- - - - - - -- - - - - - -

+9V


- - - -- - - -

A B

E

- -

- -

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 sideof 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.

-

-

-

-

Capacitor Demo

GROUND(0 V)


++

++

++

++

++

++

++

- - - - - - -- - - - - - -

+9V

Adjustable Power Supply


A B

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

0 V

E

- - - -

GROUND


++

++

++

++

++

++

++

- - - - - - -- - - - - - -

+9V +0V


+9V “Above Ground”- - - -

-

-

-

-- -

- -

What if can control voltage instead of using battery? What happens if drop voltage to 4 Volts?1) Plate A is at higher voltage than Supply. Electrons flow towards Plate A until voltage difference is 0. Excess positive charge on Plate A will decrease.

2) Attraction of electrons on Plate B to positive charge on Plate A has decreased. Force of electrons repelling each other is greater than attraction to Plate A, so some electrons leave … flow back to Ground.

A B


+4V

E

GROUND


++

++

++

+- - - - - - -


A B


E

So if we can vary voltage at Plate A, we can create an alternating current through the speaker and drive the speaker.

updates this week and next: homework #4 (uses online applet) reading quiz on tuesday will cover...

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