electricity, magnetism and some applications to music

Electricity, Magnetism and some applications to music

Spring 2009 Final Exam ScheduleTuesday, April 28 - Monday, May 4No Exams on Sunday

EXAMTIMES

Class Meeting Times

EXAM DAY 1TUES 4/28

EXAM DAY 2WED 4/29

EXAM DAY 3THURS 4/30

EXAM DAY 4FRI 5/1

EXAM DAY 5SAT 5/2

EXAM DAY 6MON 5/4

7:00 a.m. -9:50 a.m.

7:30-10:20 T 9:00-10:15 TR (all a.m.)

7:30-10:20 W 8:30-9:20 MWF 9:00-10:15 MW (all a.m.)

7:30-8:45 TR 7:30-10:20 R (all a.m.)

7:30-10:20 F 9:00-10:15 M/7:30-8:45 F 9:30-10:20 MWF (all a.m.)

Finals For Saturday Classes Are Held During Regular Class Meeting Times

7:30-8:20 MWF 7:30-8:45 MW 7:30-10:20 M (all a.m.)

10:00 a.m. -12:50 p.m.

10:30-11:45 TR 10:30-1:20 T

10:30-1:20 W 11:30-12:20 MWF 12:00-1:15 MW 12:00-1:15 WF

10:30-1:20 R 12:00-1:15 TR

10:30-1:20 F 12:30-1:20 MWF 12:00-1:15 M/10:30-11:45 F

Finals For Saturday Classes Are Held During Regular Class Meeting Times

10:30-11:20 MWF 10:30-11:45 MW 10:30-1:20 M

1:00 p.m. -3:50 p.m.

1:30-2:45 TR 1:30-4:20 T

1:30-4:20 W 2:30-3:20 MWF 3:00-4:15 WF 3:00-4:15 MW

1:30-4:20 R 3:00-4:15 TR

1:30-4:20 F 3:30-4:20 MWF 3:00-4:15 M/1:30-2:45 F

FREE PERIOD

1:30-2:20 MWF 1:30-2:45 MW 1:30-4:20 M

Exam in this room. BringPencils and SCANTRONs

April 2009Sunday Monday Tuesday Wednesday Thursday Friday Saturday

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19WEBASSI

GNDUE

20Charge

Magnetism

21 22Charge

Magnetism

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onsRecordin

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26 27LAST DAY

OF CLASSESReviewReview

28 29FINALEXAM10:20

AM

30 1 2

3 4 5 6 7 8 9

ScheduleThis week we take a quick look at electricity

and magnetism and applications to musicRead in Textbook: (Mostly Qualitative)

Chapter 20: 402-407, 411-412, 414-420Chapter 21: 427-432, 433-436Chapter 22: 446-451,

Read in Measured Tones268-2748, 281-284,173,178, (mostly

qualitative/historical)Last WebAssign has been postedWe are almost there!

IntroductionThe early Greeks knew that amber—a fossilized

tree sap currently used in jewelry—had the interesting ability to attract bits of fiber and hair after it was rubbed with fur. This was one way of recognizing an object that was electrified.

In modern terminology we say the object is charged.

This doesn’t explain what charge is, but is a handy way of referring to this condition.

Probable First Observation ElectricityElectricity

Idiot!

If lightening had actually traveled down the kite string, old Ben Franklin

would have been toast!

Probably never happened, but good story!

A Quick Experiment that may not work

Demonstration Procedure

Pivot

The sequence of Experiments1. Identify two rods2. Treat each rod3. Bring one rod near to the other4. Observe what happens5. See what we can learn

If you rubbed the rods longer and/or harder, do you think the

effect that you see would be

A. StrongerB. WeakerC. The same

A. StrongerB. WeakerC. The same

We DEFINE the “stuff” that we put on the rods by the rubbing process as CHARGE.

We will try to understand what charge is and how it behaves.

We add to the properties of materials:

Mass

ChargeCharge

What’s Going On?All of these effects involve rubbing two surfaces

together.Or pulling two surfaces apart.Something has “happened “to each of these

objects.These objects have a new PROPERTY

Other properties are mass, color

We call this NEW PROPERTY .………. ………

CHARGE.There seems to be two types of charge.

We call these two types of charge

PositiveNegative

An object without either a (+) or (-) charge is referred

to as being NEUTRAL.NEUTRAL.

Example - Tape

Separation

An Example

Effect of Charge

We have also observed that there must be TWO kinds of charge.Call these two types

positive (+)negative(-)

We “define” the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE.

The charge on the glass rod or the dead cat is consequently defined as POSITIVE.

Old Ben screwed up more than once!!

++++++++++-------------+++---++---+-++-??

From whence this charge???

-+

Easily Removed

The nucleus of a certain type of neon atom contains 10 protons and 10 neutrons. What is the total charge of the nucleus?

Coulombs 101.6 )(

isproton aon Charge

Coulombs 10(-)1.6

iselectron an on Charge

19-

19-

MaterialsConductors

Charge easily moves in conducting materialsUsually metals … Cu, Ag, Al, Au, etc.

InsulatorsCharge does NOT move

OthersSemiconductors – Transistors, etc.Semimetals – Don’t ask!

Electrical PropertiesWhy doesn’t the charge flow to ground

through our bodies? It stays on the rod because the rod is an

insulator; charge generated at one end remains there.

The charge can be removed by moving our hands along the charged end.

As we touch the regions that are charged, the charges flow through our bodies to ground.

Electrical PropertiesA metal rod cannot be charged by holding it

in our hands and rubbing it with a cloth because metal conducts the charge to our hands. A metal rod can be charged if it is mounted on

an insulating stand or if we hold it with an insulating glove; that is, the rod must be insulated from its surroundings.

Conservation of ChargeLike Gilbert, Benjamin Franklin believed that electricity was

a single fluid and that an excess of this fluid caused one kind of charged state, whereas a deficiency caused the other. Because he could not tell which was which, he arbitrarily

named one kind of charge positive and the other kind negative.

By convention the charge on a glass rod rubbed with silk or plastic film is positive, whereas that on an amber or rubber rod rubbed with wool or fur is negative.

Conservation of ChargeIn our modern physics world view, all objects are

composed of negatively charged electrons, positively charged protons, and uncharged neutrons. The electron’s charge and the proton’s charge

have the same size.An object is uncharged (or neutral) because it

has equal amounts of positive and negative charges, not because it contains no charges.

For example, atoms are electrically neutral because they have equal numbers of electrons and protons.

Conservation of ChargePositively charged objects may have an excess

of positive charges or a deficiency of negative charges; that is, an excess of protons or a deficiency of electrons. We simply call them positively charged because the electrical effects are the same in both situations.

The modern view easily accounts for the conservation of charge when charging objects. The rubbing simply results in the transfer of

electrons from one object to the other; whatever one object loses, the other gains.

The Electric ForceSimple observations of the attraction or repulsion

of two charged objects indicate that the size of the electric force depends on distance. For instance, a charged object has more effect on

an electroscope as it gets nearer. But we need to be more precise.

How does the size of this force vary as the separation between two charged objects changes? And how does it vary as the amount of charge on

the objects varies?

The Electric ForceIn 1785 French physicist Charles Coulomb measured

the changes in the electric force as he varied the distance between two objects and the charges on them. He verified that if the distance between two charged objects is

doubled (without changing the charges), the electric force on each object is reduced to one-fourth the initial value.

If the distance is tripled, the force is reduced to one-ninth, and so on.

This type of behavior is known as an inverse-square relationship; inverse because the force gets smaller as the distance gets larger, square because the force changes by the square of the factor by which the distance changes.

The Electric ForceCoulomb also showed that reducing the charge on

one of the objects by one-half reduced the electric force to one-half its original value. Reducing the charge on each by one-half reduced the

force to one-fourth the original value. This means that the force is proportional to the product of

the two charges.

These two effects are combined into a single relationship known as Coulomb’s law:

In this equation, q1 and q2 represent the amount of charge on objects 1 and 2, r is the distance between their centers, and k is a constant (known as Coulomb’s constant) whose value depends on the units chosen for force, charge, and distance.

The Electric ForceEach object feels the force due to the other. The forces

are vectors and act along the line between the centers of the two objects. The force on each object is directed toward the other if the charges have opposite signs and away from each other if the charges have the same sign.

Because the two forces are due to the interaction between the two objects, the forces are an action– reaction pair. According to Newton’s third law, the forces are equal in magnitude, point in opposite directions, and act on different objects.

The Electric ForceBecause the existence of an elementary,

fundamental charge was not known until the 20th century, the unit of electric charge, the coulomb (C), was chosen for convenience in use with electric circuits. (We will formally define the coulomb later.)

Using the coulomb as the unit of charge, the value of Coulomb’s constant is determined by experiment to be:

The Electric ForceThe coulomb is a tremendously large unit for the

situations we have been discussing. For instance, the force between two spheres, each having 1 coulomb of charge and separated by 1 meter, is:

This is a force of 1 million tons!

The Electric FieldImplicitly, we have assumed the force between two

charges to be the result of some kind of direct interaction—sort of an action-at-a-distance interaction. This type of interaction is a little unsettling because there is no

direct pushing or pulling mechanism in the intervening space. Electrical effects are evident even in situations in

which there is a vacuum between the charges. If this were the only purpose of the field idea, it would

play a minor role in our physics world view. In fact, it probably seems like we are trading one unsettling

idea for another. However, as we continue our studies, we will find that the

electric field takes on an identity of its own. As we will learn in Chapter 22, electric and magnetic fields can travel through space as waves.

The Electric FieldWe define the electric field E at every point in space

as the force exerted on a unit positive charge placed at the point. This is equivalent to the way that the gravitational field was

defined, with the unit mass replaced by a unit positive charge. Because force is a vector quantity, the

electric field is a vector field; it has a size and a direction at each point in space.

You could imagine the space around a positive charge as a “porcupine” of little arrows pointing outward, as shown in figure to the left.

The arrows farther from the charge would be shorter to indicate that the force is weaker there.

The Electric FieldThe values for an actual electric field can be measured

with a test charge. The unit of charge that we have been using is 1 coulomb. This is a very large amount of charge, and if we actually used 1

coulomb as our test charge, it would most likely move the charges that generated the field, thus disturbing the field.

Therefore, we use a much smaller charge, such as 1 microcoulomb, and obtain the size of the field by dividing the measured force F by the size q of the test charge:

Notice that the units of electric field are newtons per coulomb (N/C).

The Electric FieldIf we know the sizes and locations of the

charges creating the electric field, we can also calculate the value of the field at any point of interest by assuming that we place a 1 coulomb charge at the location and calculating the force on this charge using Coulomb’s law.

In doing this, we can take advantage of the fact that each charge acts independently; the effects simply add. This means that we calculate the

contribution of each charge to the field and then add these contributions vectorially.

The Electric FieldOnce we know the value of the electric field at any

point, we can calculate the force that any charge q would experience if placed at that point:

This is read as, “The force on an object is equal to the net charge q on the object times the electric field E at the location of the object.”

The Electric FieldAs an example, let’s assume that we have

generated a uniform electric field that points downward and has a size of 1000 newtons per coulomb. If we place an object in this field that has a positive charge of 1 microcoulomb, the object will experience a downward force of:

If we change the charge on the object, it is very easy to calculate the new force; we do not have to deal with the charges that produced the electric field.

What is the electric field at a distance of 1 m from a 1 C charge?Imagine a UNIT charge (1 coulomb) placed at

the point where we want to know the electric field.

Calculate the FORCE on the unit charge

The Electric Field is then

Nkr

qqkF 9

2221 109

1

11

CNC

N

Q

FE /109

1

109 99

Electric PotentialBecause objects with different charges have different

electric potential energies at a given point, it is often more convenient to talk about the energy available due to the electric field without reference to a specific charged object.

The electric potential V at each point in an electric field is defined as the electric potential energy EPE divided by the object’s charge q:

Electric PotentialNotice that it doesn’t matter which charged object

we use to define the electric potential. This quantity is numerically equal to the work

required to bring a positive test charge of 1 coulomb from the zero reference point to the specified point.

The units for electric potential are joules per coulomb (J/C), a combination known as a volt (V). Because of this, we often speak of the electric

potential as a voltage.If you have a potential difference across a

conducting material, you will have a motion of electrons. This motion of charge is called a CURRENT and is measured in Coulombs per second.

1 C/sec is defined as a current of 1 AMPERE

NoLight

V

CurrentI

I~V

The ConstantI~VV=IR (R is proportionality constant)R is a property of the materialSome Materials are more “resistive to” the flow of current.

R is called the resistance.Units: Volts/Ampere = OHMs

Resistors

What happens here??

04/19/23 The Science of Music 57

S NS N +Q

No Impact!


S NS N +Q

There is nowA force!

Move

Force ~ q x v x strength of the magnet(B is magnetic field of magnet)


A Changing Magnetic Field Induces a CurrentMagnet Induces a Current in a Closed Circuit


MagnetsMagnets Do NOT attract chages.Magnetism is a very different

phenomenon. Magnets have N and S polesLike poles repelUnlike poles attractWhere have we seem this before??


Other ObservationsA magnet moving into a coil produces an

electric current (and voltage!).A wire moving near a magnet will have a

current generated in it.There is a “magnetic field” around a wire.A loop of wire acts like a small magnet.A Magnet produces a FORCE on a current

carrying conductor. (IMPORTANT)

Back to ….


What Reached the Ear?

This is an ANALOG signal. The ear doesn’t respond to digital signals.

A “Record”


The ProcessAnalog Source

Digital Storage Analog Storage

Convert to Analog Retain Analog

Speaker Speaker


Storage MethodsAnalog Storage

Mechanical Electrical (Record, cylinder)Magnetic (Tape, Wire)

Digital StorageMagnetic (Tape)Optical (CD)Electrical (MP3 file on your “Flash Memory”)


IssuesWe want the process to be fast.We want to be able to widely distribute the

recorded product.We want the product to reproduce, as well as

possible, the original sound.We want to ENJOY the final reproduction.


OLDEN DAYS – (Screech of Chalk)Bell's ear Phonautograph was a very unusual variation on the basic technology. The recording mechanism was the human ear. By removing a chunk of skull including the inner ear from a human cadaver, and attaching a stylus to the moving parts of the ear, he was able to use this bio-mechanical device to make a recording of the sounds that entered a recording horn. It recorded on a moving glass strip, coated with a film of carbon, so there are probably no original recordings from it.

Sound

Hum

anEar


Gramophone The graphophone in its original form was an improved form of the phonograph. One main difference, which Edison would soon adopt, was the use of a cardboard-coated wax cylinder instead of a sheet of tinfoil. The exact construction of the cylinders and the materials used changed considerably in later years, though the basic concept of recording into a soft, plastic material was retained. (image from NMAH)


Development - Platter


“HIS MASTERS VOICE”


Western ElectricWestern Electric's recorder used electronic amplifiers to drive an electromagnetic cutting head, rather than relying on the acoustic horn. The result was a louder, clearer record.


The Need for the Microphone


An Old Carbon Microphone


The MicrophoneThe microphone is a device that received the

sound vibrationsconverts it to an electrical “signal”Which is then sent to the next stage in the

process (later).The signal tends to be small and gets weaker

as it travels down a long wire.


The Microphone Process

MicrophoneMicrophoneSignal onSignal on

a wirea wire

MECHANCAL ---> ---------------MECHANCAL ---> --------------- ELECTRICAL ELECTRICAL


Particles of Metal are pressed closer together.

Resistance is reduced


How does it work?


The “Crystal” Microphone


The Record


Dynamic Microphone


Movies??

StretchedStretchedHorizontallyHorizontally


1930s Magnetic Tape


Playback


Today

Analog Record CD Digital


Back to your head


Exploded View


FULL CIRCLE!

electricity, magnetism and some applications to music

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