Download magnetized - eLisa UGM

Electromagnetism
&
Its Applications
1
Electricity and Magnetism
• An electricity can be converted to be a
magnetism or vice versa.
– With electricity, it turned out to be useful to define
an electric field rather than always working in terms
of electric forces.
Electricity
Magnetism
2
Magnetic Field
• Magnetic fields are produced by
electric currents, which can be
– macroscopic currents in wires, or
– microscopic currents associated with
electrons in atomic orbits.
• The magnetic field B is defined in
terms of force on moving charge in
the Lorentz force law.



F  qv xB
• Magnetic field sources are essentially
dipole in nature, having a north and
south magnetic pole.
3
Biot-Savart Formula
Magnetic field at P, produced by dl, is given by:
i dl sin 
dB  k
2
r
Current (i)
dl α
r
dB
P
R
4
Magnetic Field Sources
5
Magnetic Field of Current
• The magnetic field lines around a long wire which carries an electric current
form concentric circles around the wire.
• The direction of the magnetic field is perpendicular to the wire and is in the
direction the fingers of your right hand would curl if you wrapped them
around the wire with your thumb in the direction of the current.
The constant 0 is the permeability of free space.
6
Magnetic Field of Current Loop
• Electric current in a circular
loop creates a magnetic field
which is more concentrated
in the center of the loop than
outside the loop.
• Examining the direction of
the magnetic field produced
by a current-carrying segment
of wire shows that all parts of
the loop contribute magnetic
field in the same direction
inside the loop.
Stacking multiple loops concentrates the field even more into what is called a solenoid.
7
Field at Center of Current Loop
• The form of the magnetic field
from a current element in the
Biot-Savart law becomes
• which in this case simplifies
greatly because the angle =90 °
for all points along the path and
the distance to the field point is
constant. The integral becomes
8
Field on Axis of Current Loop
• The application of the Biot-Savart law on the
centerline of a current loop involves integrating
the z-component.
• The symmetry is such that all the terms in this
element are constant except the dL , which when
integrated just gives the circumference of the
circle. The magnetic field is then
9
Field on Axis of Current Loop …
10
Solenoid
• A long straight coil of wire can be used to generate a nearly uniform magnetic
field similar to that of a bar magnet.
• Such coils, called solenoids, have an enormous number of practical applications.
• The field can be greatly strengthened by the addition of an iron core. Such
cores are typical in electromagnets.
11
Solenoid Field from Ampere's Law
• Taking a rectangular path about which to
evaluate Ampere's Law such that the length
of the side parallel to the solenoid field is L
gives a contribution BL inside the coil.
– The field is essentially perpendicular to the
sides of the path, giving negligible
contribution.
– If the end is taken so far from the coil that
the field is negligible, then the length inside
the coil is the dominant contribution.
• This admittedly idealized case for Ampere's
Law gives
• This turns out to be a good approximation
for the solenoid field, particularly in the case
of an iron core solenoid.
12
Solenoid Magnetic Field Calculation
• At the center of a long solenoid
– Active formula: click on the quantity you wish to calculate.
Magnetic field = permeability x turn density x current
13
Bar Magnet
• The lines of magnetic field
from a bar magnet form closed
lines.
• By convention, the field
direction is taken to be
outward from the North pole
and in to the South pole of the
magnet.
• Permanent magnets can be
made from ferromagnetic
materials.
14
Electric and Magnetic Sources
• The electric field of a point
charge is radially outward from a
positive charge.
– Electric sources are inherently
"monopole" or point charge
sources.
• The magnetic field of a bar
magnet.
– Magnetic sources are inherently
dipole sources - you can't isolate
North or South "monopoles".
15
Bar Magnet and Solenoid
• The magnetic field produced by electric current in a
solenoid coil is similar to that of a bar magnet.
16
Iron Core Solenoid
• Electromagnets are usually in the
form of iron core solenoids. The
ferromagnetic property of the iron
core causes the internal magnetic
domains of the iron to line up with
the smaller driving magnetic field
produced by the current in the
solenoid.
– The effect is the multiplication of
the magnetic field by factors of
tens to even thousands. The
solenoid field relationship is
– and k is the relative permeability of
the iron, shows the magnifying
effect of the iron core.
17
Magnetic Field of the Earth
• The earth's magnetic field is similar to that of a
bar magnet tilted 11 degrees from the spin axis of
the earth.
• The problem with that picture is that the Curie
temperature of iron is about 770 C .
• The earth's core is hotter than that and therefore
not magnetic. So how did the earth get its
magnetic field?
• The earth's magnetic field is attributed to a
dynamo effect of circulating electric current, but it
is not constant in direction.
– Rock specimens of different age in similar
locations have different directions of permanent
magnetization. Evidence for 171 magnetic field
reversals during the past 71 million years has been
reported.
18
Electric Motors
• Electric motors involve rotating coils of wire which are
driven by the magnetic force exerted by a magnetic field
on an electric current.
• They transform electrical energy into mechanical energy.
19
How Does an Electric Motor Work?
20
DC Motor Operation
21
DC Motor Operation: electric current
22
DC Motor Operation: magnetic field
23
DC Motor Operation: magnetic force
24
Tape Recording Process
25
Tape Head Action
•
An electric current in a coil of wire produces a magnetic field similar to that
of a bar magnet, and that field is much stronger if the coil has a ferromagnetic
(iron-like) core.
• Tape heads are made from rings of ferromagnetic material with a gap where
the tape contacts it so the magnetic field can fringe out to magnetize the tape.
– A coil of wire around the ring carries the current to produce a magnetic
field proportional to the signal to be recorded.
– If an already magnetized tape is passed beneath the head, it can induce a
voltage in the coil.
– Thus the same head can be used for recording and playback.
26
Cassette Tape Head Arrangement
• The basic tape head action
involves an oscillating
current in a coil.
– The magnetic field produced
in a ring of ferromagnetic
material fringes out to the tape
material at the gap.
– For stereo cassette tape heads,
there are two such
mechanisms to record and
playback from parallel tracks
on the tape.
27
Magnetic Emulsions
• The recording medium for the tape recording process is typically made by
embedding tiny magnetic oxide particles in a plastic binder on a polyester
film tape.
– Iron oxide has been the most widely used oxide, but chromium oxide and metal
particles provide a better signal-to-noise ratio and a wider dynamic range.
– The oxide particles are on the order of 0.5 micrometers in size and the polyester
tape backing may be as thin as 0.5 mil (.01 mm).
– The oxide particles themselves do not move during recording.
– Rather their magnetic domains are reoriented by the magnetic field from the tape
head.
28
Erase Head
• Before passing over the record head, a tape
in a recorder passes over the erase head
which applies a high amplitude, high
frequency AC magnetic field to the tape to
erase any previously recorded signal and to
thoroughly randomize the magnetization of
the magnetic emulsion.
– Typically, the tape passes over the erase head
immediately before passing over the record head.
• The gap in the erase head is wider than those
in the record head; the tape stays in the field
of the head longer to thoroughly erase any
previously recorded signal.
29
Biasing
• High fidelity tape recording requires a high
frequency biasing signal to be applied to the tape
head along with the signal to "stir" the
magnetization of the tape and make sure each part
of the signal has the same magnetic starting
conditions for recording.
– This is because magnetic tapes are very sensitive to
their previous magnetic history, a property called
hysteresis.
• A magnetic "image" of a sound signal can be stored
on tape in the form of magnetized iron oxide or
chromium dioxide granules in a magnetic emulsion.
– The tiny granules are fixed on a polyester film base,
but the direction and extent of their magnetization
can be changed to record an input signal from a tape
head
30
Tape Playback
• When a magnetized tape passes under the playback
head of a tape recorder, the ferromagnetic material in
the tape head is magnetized and that magnetic field
penetrates a coil of wire which is wrapped around it.
– Any change in magnetic field induces a voltage in the
coil according to Faraday's law.
– This induced voltage forms an electrical image of the
signal which is recorded on the tape.
• Problem: The magnetization of the magnetic
emulsion is proportional to the recorded signal while
the induced voltage in the coil is proportional to the
rate at which the magnetization in the coil changes.
– This means that for a signal with twice the frequency,
the output signal is twice as great for the same degree
of magnetization of the tape.
– It is therefore necessary to compensate for this increase
in signal to keep high frequencies from being boosted
by a factor of two for each octave increase in pitch.
– This compensation process is called equalization.
31
Biasing in Tape Recording
• A music signal alone cannot be used to produce a faithful tape recording of a
sound because the magnetization of the tape is so sensitive to its previous
magnetic history, even the effects of the signal recorded just ahead of it.
• A high frequency bias signal is typically applied to the tape through the tape
head along with the music signal to remove the effects of this magnetic
history.
• This large bias signal (typically 40 to 150 kHz in frequency) keeps "stirring"
the magnetization so that each signal to be recorded encounters the same
magnetic starting conditions.
• The necessity for biasing has its origin in the magnetic property called
hysteresis - the magnetic material tends to hold onto any magnetization it
receives and must be actively driven back to zero to start over.
• Magnetic emulsions made with chromium dioxide require a larger biasing
signal to make use of their wider dynamic range, so modern recorders have
different bias settings for iron oxide, chromium dioxide, and metal tapes.
• With optimum biasing, the recorded magnetic image is proportional to the
signal current applied to the record head.
32
Bias During Recording
• To record a sine wave on tape, you mix it with a high
frequency bias signal.
– The bias keeps the magnetic domains "stirred", with an average
magnetization in the direction of the signal voltage you wish to
record.
– As the head passes, a net magnetization proportional to the sine
wave signal remains.
33
Optimum Biasing
34
Hysteresis in Magnetic Recording
• Because of hysteresis, an input signal at the level indicated by the
dashed line could give a magnetization anywhere between C and D,
depending upon the immediate previous history of the tape (i.e., the
signal which preceded it).
– This clearly unacceptable situation is remedied by the bias current which cycles
the oxide grains around their hysteresis loops so quickly that the magetization
averages to zero when no signal is applied.
The result of the bias signal is like a
magnetic eddy which settles down to
zero if there is no signal
superimposed upon it. If there is a
signal, it offsets the bias signal so that
it leaves a remnant magnetization
proportional to the signal offset.
35
Hysteresis in Magnetic Recording
36
Dynamic Loudspeaker Principle
• A current-carrying wire
in a magnetic field
experiences a magnetic
force perpendicular to
the wire.
37
Loudspeaker Details
• A light voice coil is mounted so that it can
move freely inside the magnetic field of a strong
permanent magnet.
• The speaker cone is attached to the voice coil
and attached with a flexible mounting to the
outer ring of the speaker support.
– Because there is a definite "home" or equilibrium
position for the speaker cone and there is elasticity of
the mounting structure, there is inevitably a free cone
resonant frequency like that of a mass on a spring.
– The frequency can be determined by adjusting the
mass and stiffness of the cone and voice coil, and it
can be damped and broadened by the nature of the
construction, but that natural mechanical frequency
of vibration is always there and enhances the
frequencies in the frequency range near resonance.
– Part of the role of a good enclosure is to minimize
the impact of this resonant frequency.
38
Types of Enclosures
• The production of a good high-fidelity
loudspeaker requires that the speakers be
enclosed because of a number of basic
properties of loudspeakers.
• Just putting a single dynamic loudspeaker in a
closed box will improve its sound quality
dramatically.
• Modern loudspeaker enclosures typically
involve multiple loudspeakers with a crossover
network to provide a more nearly uniform
frequency response across the audio frequency
range.
• Other techniques such as those used in bass
reflex enclosures may be used to extend the
useful bass range of the loudspeakers.
39
Use of Multiple Drivers in
Loudspeakers
• Even with a good enclosure, a single loudspeaker cannot be
expected to deliver optimally balanced sound over the full
audible sound spectrum.
– For the production of high frequencies, the driving element should
be small and light to be able to respond rapidly to the applied signal.
Such high frequency speakers are called "tweeters".
– On the other hand, a bass speaker should be large to efficiently
impedance match to the air. Such speakers, called "woofers", must
also be supplied with more power since the signal must drive a
larger mass.
40
Hard disk
• A hard disk drive (HDD,
or also hard drive or the
now-obsolete usage hard
file) is a non-volatile data
storage device that stores
data on a magnetic surface
layered onto hard disk
platters.
41
Hard disk
• A hard disk uses rotating platters (disks).
– Each platter has a smooth magnetic surface on which digital
data is stored. Information is written to the disk by applying a
magnetic field from a read-write head that flies very close
over the magnetic surface.
– The magnetic medium (film) on the disk surface changes its
magnetization in microscopic spots (bits) due to the head's
write field.
– The information can be read back by a magnetoresistive
(MR) read sensor which is part of the same head structure on
the trailing end of the flying slider.
– The read sensor detects the magnetic flux emanating from
the bit transitions passing underneath it through a small
change of the MR sensor's electric resistance.
42
Hard disk
• Due to the extremely close spacing between the heads and the
disk surface, any contamination of the read-write heads or disk
platters can lead to a head crash — a failure of the disk in which
the head scrapes across the platter surface, often grinding away
the thin magnetic film.
• For Giant Magnetoresistive (GMR) heads in particular, a minor
head crash from contamination (that does not remove the
magnetic surface of the disk) will still result in the head
temporarily overheating, due to friction with the disk surface,
and can render the data unreadable for a short period until the
head temperature stabilizes (so called "thermal asperity," a
problem which can partially be dealt with proper electronic
filtering of the read signal).
43
Hard disk
• Using rigid platters and sealing the unit allows much
tighter tolerances than in a floppy disk.
• Consequently, hard disks can store much more data
than floppy disk and access and transmit it faster.
• In 2005, a typical workstation hard disk might store
between 80 GB and 500 GB of data, rotate at 7,200
to 10,000 rpm, and have a sequential media transfer
rate of over 50 MB/s.
• The fastest workstation and server hard drives spin at
15,000 rpm, and can achieve sequential media
transfer speeds up to and beyond 80 MB/s.
• Notebook hard drives, which are physically smaller
than their desktop counterparts, tend to be slower
and have less capacity.
• Most spin at only 4,200 rpm or 5,400 rpm, whereas
the newest top models spin at 7,200 rpm.
44
Hard disk
• The platters are made from a non-magnetic material, usually glass or
aluminum, and coated on both sides with a thin layer of magnetic material.
– Older drives used iron(III) oxide, but current drives use a thin film of a cobaltbased alloy, applied by sputtering.
• The magnetic surface in the hard drive is divided into small sub-micrometresized magnetic regions, each of which is used to represent a single binary unit
of information.
– Each of these magnetic regions is further subdivided into a few hundred magnetic
grains. Each grain is considered to be a single magnetic domain.
– Each grain will thus be a magnetic dipole which points in a certain direction,
creating a magnetic field around it.
– All of the grains in a magnetic region are expected to point in the same direction,
so that the magnetic region as a whole also has a magnetic dipole moment and an
associated magnetic field.
45
Hard disk
• The data is encoded through the change in magnetization at a
region boundary, rather than the direction of magnetization of a
region.
– If the magnetization reverses between two magnetic domains, this
signifies one state, while no change in magnetization signifies the other
state.
– For various reasons, the actual binary data is encoded using consecutive
sequences of these two possible states, rather than the states themselves.
– Most hard drives use a form of Run Length Limited coding, for example.
– At a boundary where the magnetization reverses, magnetic field lines will
be dense and perpendicular to the medium.
– The read head is designed to detect these changes.
46
Hard disk
• In older hard drives, the read head was usually a small
inductor, often filled with a paramagnetic material in
order to enhance the signal.
– As it passes over a boundary with a magnetization reversal,
the read head experiences magnetic flux, which is converted
by the inductor into an electric current.
• Modern hard drives usually have a read head that makes
use of the Giant Magnetoresistive effect, which causes
the resistance of certain materials to change in response
to a strong magnetic field.
– As this type of read head passes over a boundary with a
magnetization reversal, the strong magnetic field will cause its
resistance to change in a detectable way.
47
Hard disk
• The magnetic surface and how it operates. In this case
the binary data encoded using frequency modulation
48
Hard disk
• Comparison of the transition width caused by Neel
Spikes in continuous media and granular media, at a
boundary between two magnetic regions of opposite
magnetization
49
Credit Cards
• The phone companies, gas
companies and department stores
have their own numbering
systems, ANSI Standard X4.131983 is the system used by most
national credit-card systems
50
Credit Cards
• The stripe on the back of a credit card is a magnetic stripe, often called a
magstripe.
– The magstripe is made up of tiny iron-based magnetic particles in a plasticlike film. Each particle is really a tiny bar magnet about 20-millionths of
an inch long.
• The magstripe can be "written" because the tiny bar magnets can be
magnetized in either a north or south pole direction.
– The magstripe on the back of the card is very similar to a piece of cassette
tape
• A magstripe reader can understand the information on the three-track stripe.
• If the ATM isn't accepting your card, your problem is probably either:
– A dirty or scratched magstripe
– An erased magstripe (The most common causes for erased magstripes are
exposure to magnets, like the small ones used to hold notes and pictures
on the refrigerator, and exposure to a store's electronic article surveillance
(EAS) tag demagnetizer.)
51