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Atoms and Energies
But first, a synopsis of storage
Optical Storage
Light shining through an opening spreads
and produces diffraction patterns
 Light traveling through a circular opening
produces a circular pattern, such as the one
shown below
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d sin q = 1.22 l
y
d
q
D
tan q = y/D
y
More Optical Storage
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Laser light is focused through a (circular) lens
onto the surface of a CD
The central max of the diffraction pattern must be
no larger than one bit if data is to be resolved
The diameter d of the lens, the distance D between
the CD and the surface (which should be the focal
length of the lens, and the wavelength of light l
all affect the size of the central maximum
The ratio D/d for a lens is constrained to be >1 by
practical manufacturing considerations
Final Optical Storage Remarks
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Bits are written as lands or pits on a disk
Light striking a (smooth) land undergoes total
reflection toward the detector
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This is a 1
Light striking a (rough) pit undergoes diffuse
reflection so the detector receives a much smaller
amount of light and doesn’t trigger
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This is a 0
Electrical Storage
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The electric force FE on a charge q0 can be considered due
to an electric field which is produced by other charges qi in
the area
FE = q0 E
If the charge is positive, the force points in the same
direction as the field
If the charge is negative, the force points in the opposite
direction as the field
Electric fields, then, point from positive charges toward
negative charges, or from high “potential” to low potential
More Electrical Storage
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We can draw field lines using two guidelines:
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Electric field lines point in the direction of the field
The spacing of the field lines represents the
strength of the field (Closer spacing means
stronger field)
If moving a charge between two points requires
work (or does work), the charge gains (or loses)
potential energy:
DU = –W = –  F  dx = – q0  E  dx
DV = –  E  dx
Still More Electrical Storage
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An electric field can be “stored” using two oppositelycharged plates, an arrangement called a capacitor
Capacitors storing charge Q will maintain a potential
difference, or voltage V, between the plates that is
proportional to Q:
Q = CV
Capacitance C is independent of V and Q and depends only
on geometry and materials of device:
C = e0 A/d,
Where A is the area of each parallel plate, and d is the
spacing between the two plates
Final Electrical Storage Remarks
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Capacitors are used in RAM:
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A charged capacitor is a 1
An uncharged capacitor is a 0
Reading a capacitor discharges it, so you must
continually re-write when reading
Capacitors lose charge over time (charge will
slowly leak into air or other insulator), so
capacitors are not good for long-term storage
Magnetic Storage
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The smallest region with uniform magnetism is
called a “domain”
Each bit requires two domains to allow for error
identification
If two domains are magnetized in same direction,
the bit is a 0
If two domains are magnetized in opposite
directions, the bit is a 1
Direction of magnetization must change at the
start of each new bit.
Magnetic Storage: Writing
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Magnetic fields have two sources:
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Currents (electromagnetism)
Alignment of intrinsic “spin” of particles
(ferromagnetism)
Magnetic data is written by running a current
through a loop of wire near the disk
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resulting magnetic field aligns spins in region of
disk and produces magnetic domain
switching current produces magnetic domain with
magnetism in opposite direction
Magnetic Storage: Faraday’s
Law
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A changing magnetic field induces a current in a
coil of wire proportional to the derivative (rate of
change) of the field
The field, emf, and current also depend upon the
area A of the loop, and the number of turns in a
coil.
This is summarized in Faraday’s Law:
e
d B
dB
 iR  
 A
dt
dt
 B   B  dA  BA
Magnetic Storage: Reading by
Induced Currents
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As magnetic data passes by coil of wire,
changing field induces currents
increase in field (more positive or less
negative) induces current in opposite
direction of that induced by a decrease in
field (more negative or less positive)
 Number of changes in a bit indicates
whether bit is 0 or 1
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Magnetic Storage: Reading by
Magnetoresistance
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Charges traveling through magnetic field experience
magnetic force (provided velocity and field are not
aligned):
FB = qv x B
Force is perpendicular to velocity (and to field), so charges
are pushed “off track”, resulting in more frequent
collisions and thus an increased resistance
Current through a loop of wire near magnetic data will
vary as magnetic field does, giving a very sensitive
indication of magnetic data
Magnetic Storage: Reading by
Giant Magnetoresistance
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Giant Magnetoresistance (GMR) is a completely
different effect from Magnetoresistance (MR)
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Both utilize magnetic data’s effect on resistance,
but that’s the only similarity
MR is the regular “Lorenz” force on charges
moving in a magnetic field
GMR exploits spin-dependent scattering and
requires very carefully-crafted devices such as
spin valves
Giant
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magnetoresistance
When magnetic field is present in magnetic
superlattice, scattering of electrons is cut
dramatically, greatly decreasing resistance
Superlattices are hard to mass-produce, but the
effect has been seen in three-layer devices called
“spin valves”
The origin of giant magnetoresistance is very
different from that of regular magnetoresistance!
On To Atoms
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Around the turn of the century, Bohr proposed that
electrons in atoms can only occupy certain, quantized
energy “states”
When an electron moves from one allowed state to another,
it needs to absorb or emit a particular amount of energy
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Often that energy takes the form of light
Only specific energies (and therefore wavelengths) of light
will be emitted by a particular element
The collection of energies emitted or absorbed by an
element is called the atomic spectrum of that element
Do Today’s Activity
Our Model of the Atom
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If the atom is in the “ground state” of lowest energy, electrons fill the
states in the lowest available energy levels. The first shell has two
possible states, and the second shell has eight possible states. Higher
shells have more states, but we’ll represent them with the eight states
in the first two sub-shells.
Electrons in the outermost shell are called “valence” electrons. We’ll
make them green to distinguish from e- in filled shells
E=0 (unbound)
n=4
n=3
n=2
n=1
Really eight closely spaced
energies, since no two electrons
can occupy same state
The Hydrogen Atom
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Has one electron, normally in the ground state n=1
This electron can absorb energy and go to a higher state, like n=3
The atom will eventually return to its ground state, and the electron
will emit the extra energy in the form of light.
This light will have energy E = (13.6 ev)(1/1 – 1/32) = 12.1 eV
The corresponding wavelength is l = hc/E = 1020 Å
E=0 (unbound)
n=4
n=3
n=2
n=1
Other Atoms
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Electrons can absorb energy and move to a higher level
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White light (all colors combined) passing through a gas will come
out missing certain wavelengths (absorption spectrum)
Electrons can emit light and move to a lower level
Calculating the allowed energies extremely complicated for anything
with more than one electron
But can deduce allowed energies from light that is emitted
n=4
n=3
n=2
n=1
E=0 (unbound)
Really eight closely spaced
energies, since no two electrons
can occupy same state
Stuff to remember about
Spectra
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ENERGY IS QUANTIZED
Different elements have different allowed energies (since
different numbers of protons and electrons provide
different structure of attraction
Light emitted when electrons move from a high energy
level to a lower energy level in an atom will have only
certain, QUANTIZED, allowed energies and wavelengths.
Those wavelengths depend solely on the element emitting
the light and compose the characteristic emission spectrum
for that element