Download Electricity, Magnetism and Applications

Electricity, Magnetism and some
applications to music
Spring 2009 Final Exam Schedule
Tuesday, April 28 - Monday, May 4
No Exams on Sunday
EXAM
TIMES
7:00 a.m. 9:50 a.m.
Class Meeting Times
EXAM DAY 1
TUES 4/28
EXAM DAY 2
WED 4/29
7:30-10:20 W
7:30-10:20 T
8:30-9:20 MWF
9:00-10:15 TR (all
9:00-10:15 MW
a.m.)
(all a.m.)
EXAM DAY 3
THURS 4/30
EXAM DAY 4
FRI 5/1
EXAM DAY 5
SAT 5/2
EXAM DAY 6
MON 5/4
7:30-8:45 TR
7:30-10:20 R (all
a.m.)
7:30-10:20 F 9:0010:15 M/
7:30-8:45 F 9:3010:20 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.)
10:30-1:20 W
11:30-12:20 MWF 10:30-1:20 R
12:00-1:15 MW
12:00-1:15 TR
12:00-1:15 WF
10:00 a.m. 12:50 p.m.
10:30-11:45 TR
10:30-1:20 T
1:00 p.m. 3:50 p.m.
1:30-4:20 W 2:301:30-2:45 TR 1:30- 3:20 MWF 3:00- 1:30-4:20 R 3:004:20 T
4:15 WF 3:00-4:15 4:15 TR
MW
Finals For
10:30-1:20 F 12:30Saturday Classes 10:30-11:20 MWF
1:20 MWF 12:00Are Held During 10:30-11:45 MW
1:15 M/
Regular Class
10:30-1:20 M
10:30-11:45 F
Meeting Times
1:30-4:20 F 3:304:20 MWF 3:004:15 M/
1:30-2:45 F
FREE PERIOD
Exam in this room. Bring
Pencils and SCANTRONs
1:30-2:20 MWF
1:30-2:45 MW
1:30-4:20 M
Calendar
April 2009
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
12
19
WEBASSIGN
DUE
26
18
20
21
Charge
Magnetism
27
22
4
24
25
Applications
Recording
Speakers
Charge
Magnetism
28
29
LAST DAY
OF CLASSES
Review
3
Saturday
30
1
2
7
8
9
FINAL
EXAM
10:20 AM
5
6
Schedule
 This week we take a quick look at electricity and
magnetism and applications to music
 Read in Textbook: (Mostly Qualitative)
 Chapter 20: 402-407, 411-412, 414-420
 Chapter 21: 427-432, 433-436
 Chapter 22: 446-451,
 Read in Measured Tones
 268-2748, 281-284,173,178, (mostly qualitative/historical)
 Last WebAssign has been posted
 We are almost there!
Introduction
 The 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 Electricity
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 Experiments
1. Identify two rods
2. Treat each rod
3. Bring one rod near to the other
4. Observe what happens
5. 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. Stronger
B. Weaker
C. The same
If the two rods are brought closer together, the force
acting between them will get …
Stronger
B. Weaker
C. The same
A.
Definition of sorts
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
Charge
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
Positive
Negative
An object without either a (+) or (-) charge is referred
to as being
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?
Charge on an electron is
(-)1.6 10-19 Coulombs
Charge on a proton is
() 1.6 10-19 Coulombs
Materials
 Conductors
 Charge easily moves in conducting materials
 Usually metals … Cu, Ag, Al, Au, etc.
 Insulators
 Charge does NOT move
 Others
 Semiconductors – Transistors, etc.
 Semimetals – Don’t ask!
Electrical Properties
 Why 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 Properties
 A 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 Charge
 Like 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 Charge
 In 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 Charge
 Positively 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 Force
 Simple 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 Force
 In 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 Force
 Coulomb 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 Force
 Each 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 Force
 Because 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 Force
 The 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 Field
 Implicitly, 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 Field
 We 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 Field
 The 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 Field
 If 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 Field
 Once 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 Field
 As 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
q1q2
1 1
F  k 2  k 2  9 109 N
r
1
 The Electric Field is then
F 9 109 N
E 
 9 109 N / C
Q
1C
Electric Potential
 Because 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 Potential
 Notice 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
CREATING A VOLTAGE
Current has to have a PLACE to go!
No
Light
Look at the bulb
A “Complete” Circuit
“Let there be Light”
Current
I
V
I~V
The Constant
 I~V
 V=IR (R is proportionality constant)
 R is a property of the material
 Some Materials are more “resistive to”
the flow of current.
 R is called the resistance.
 Units: Volts/Ampere = OHMs
Resistors
More resistance (2x), less current (1/2)
What happens here??
OR HERE??
A Magnet
S
N
+Q
No Impact!
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A Magnet
S
Move
N
+Q
There is now
A force!
Force ~ q x v x strength of the magnet
(B is magnetic field of magnet)
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A Changing
Magnetic
FieldinInduces
a Current
Magnet
Induces
a Current
a Closed
Circuit
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Magnets
 Magnets Do NOT attract chages.
 Magnetism is a very different phenomenon.
 Magnets have N and S poles
 Like poles repel
 Unlike poles attract
 Where have we seem this before??
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Other Observations
 A 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)
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Back to ….
What Reached the Ear?
This is an ANALOG signal. The ear doesn’t
respond to digital signals.
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A “Record”
The Process
Analog Source
Digital Storage
Convert to Analog
Speaker
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Analog Storage
Retain Analog
Speaker
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Storage Methods
 Analog Storage
 Mechanical Electrical (Record, cylinder)
 Magnetic (Tape, Wire)
 Digital Storage
 Magnetic (Tape)
 Optical (CD)
 Electrical (MP3 file on your “Flash Memory”)
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Issues
 We 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.
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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.
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Gramophone
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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)
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Development - Platter
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“HIS MASTERS VOICE”
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Western Electric
Western 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.
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The Need for the Microphone
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An Old Carbon Microphone
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The Microphone
 The microphone is a device that received the sound
vibrations
 converts 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.
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The Microphone Process
MECHANCAL
--->
---------------
ELECTRICAL
Microphone
Signal on
a wire
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Consider a powder of
metal
Particles of Metal are pressed
closer together.
Resistance is reduced
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How does it work?
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The “Crystal” Microphone
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The Record
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Dynamic Microphone
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Movies??
Stretched
Horizontally
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1930s Magnetic Tape
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Playback
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Today
Analog Record
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CD Digital
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Back to your head
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Exploded View
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FULL CIRCLE!
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