Transcript PHOTOELECTRIC EFFECT

PHOTOELECTRIC EFFECT
Photoelectric History
• In 1839, Alexandre Edmond
Becquerel discovered the
photovoltaic effect while studying
the effect of light on electrolytic
cells.
• Though not equivalent to the
photoelectric effect, his work on
photovoltaics was instrumental in
showing a strong relationship
between light and electronic
properties of materials.
Photoelectric History
• The actual
photoelectric effect
was first observed
by Heinrich Hertz
in 1887, the
phenomenon is also
known as the
"Hertz effect."
Photoelectric History
• Study of the photoelectric
effect led to important
steps in understanding the
quantum nature of light
and electrons and
influenced the formation
of the concept of wavepariticle duality.
Photoelectric History
• In 1905, Albert
Einstein formulated the
wave-particle duality
by describing light as
composed of discrete
quanta, now called
photons, rather than
continuous waves.
Photoelectric History
• Based upon Max Plank’s
theory of black-body
radiation, Einstein
theorized that the energy
in each quantum of light
was equal to the
frequency multiplied by
a constant, later called
Plank’s Constant.
Photoelectric History
• The photons of a
light beam have a
characteristic energy
determined by the
frequency of the
light.
Photoelectric History
• A photon above a
threshold frequency
has the required
energy to eject a
single electron,
creating the
observed effect.
Photoelectric History
• This discovery led
to the quantum
revolution in
physics and earned
Einstein the Nobel
Prize in Physics in
1921.
Photoelectric Basics
• IIn the photoelectric effect,
electrons are emitted from matter
(metals and non-metallic solids,
liquids or gases).
• TThe electrons are emitted because
they absorb energy from
electromagnetic waves of a very
short wavelength, such as visible
or ultraviolet light.
Photoelectric Basics
In the photoemission process, if
an electron with some material
absorbs the energy of one
photon and thus has more
energy than the work function
(the electron binding energy)
of the material, it (the electron)
is ejected.
Photoelectric Basics
• If the photon
energy is too low,
the electron is
unable to escape
the material.
Photoelectric Basics
One photon with a
high enough
frequency in, one
electron out.
• Increasing the intensity
of the light beam
increases the number
of electrons excited,
but does not increase
the energy that each
electron possesses.
Photoelectric Basics
Three photons with
a high enough
frequency in, three
electrons out.
• Increasing the intensity
of the light beam
increases the number
of electrons excited,
but does not increase
the energy that each
electron possesses.
Photoelectric Basics
Lots of photons with
a high enough
frequency in, lots of
electrons out.
• Increasing the intensity
of the light beam
increases the number
of electrons excited,
but does not increase
the energy that each
electron possesses.
Photoelectric Basics
• The energy of the
emitted electrons
does not depend on
the intensity of the
incoming light, but
only on the energy
or frequency of the
individual photons.
Photoelectric Basics
• Electrons can absorb energy from photons
when irradiated, but they usually follow an
“all or nothing” principle.
Photoelectric Basics
• All of the energy from one photon must be
absorbed and used to liberate one electron
from atomic binding, or else the energy is
re-emitted instead of the electron.
Photoelectric Uses and Effects
• Video camera tubes
in the early days of
television used the
photoelectric effect.
Photoelectric Effect and sound
production at the movies
•
Photoelectric Uses and Effects
• The photoelectric
effect will cause
spacecraft exposed
to sunlight to
develop a positive
charge.
Photoelectric Uses and Effects
• This can be a major
problem, as other
parts of the
spacecraft in
shadow develop a
negative charge
from nearby
plasma,
Photoelectric Uses and Effects
and the imbalance
can discharge
through delicate
electrical
components.
•
Photoelectric Uses and Effects
• Light from the sun
hitting lunar dust causes
it to become charged
through the
photoelectric effect.
Photoelectric Uses and Effects
• The charged dust then
repels itself and lifts
off the surface of the
Moon by electrostatic
levitation. This looks
almost like an
“atmosphere of dust.”
Photoelectric Uses and Effects
• Photons hitting a thin
film of alkali metal or
semiconductor material
such as gallium arsenide
can produce an image
even in low light level
conditions
Photoelectric Uses and Effects
• Still, the most common use
is panels that produce an
electrical current. From
solar calculators
Photoelectric Uses and Effects
• Still, the most common use
is panels that produce an
electrical current. From
solar calculators to solar
house panels
Photoelectric Uses and Effects
• Still, the most common use
is panels that produce an
electrical current. From
solar calculators to solar
house panels to electric cars
Photoelectric Uses and Effects
• Still, the most common use
is panels that produce an
electrical current. From
solar calculators to solar
house panels to electric cars
to satellites and spacecraft,
the uses for photoelectically
produced power keeps
expanding.
Atomic Fingerprints
Every atom has a unique signature due to a
combination of number of electrons and
energy levels for that atom.
What does quantized mean?
Terms to know…
• Spectroscopy- method of identifying
elements and chemicals.
• Emission- given off
• Absorption- absorbing
• Photon- packet of energy (light is one
example)
• Energy Level – Where electrons are found
spinning around the nucleus of atoms.
How Atoms give off light
• The emission spectrum of a chemical element
or chemical compound is the spectrum of
frequencies of electromagnetic radiation
emitted by the element's atoms or the
compound's molecules when they are returned
to a lower energy state.
• Each element's emission spectrum is unique.
Therefore, spectroscopy can be used to
identify the elements in matter of unknown
composition. Similarly, the emission
spectra of molecules can be used in
chemical analysis of substances.
When atoms receive energy, electrons can
move up into higher energy levels, they
don’t stay there long and when they fall to
lower energy levels, they give off energy in
the form of light. How far they “fall”
determines what energy (frequency) of light
they give off.
B
B
C
A
A
C
•
Quantum Processes
Quantum properties dominate the fields of
atomic and molecular physics. Radiation is
quantized such that for a given frequency of
radiation, there can be only one value of
quantum energy for the photons of that
radiation. The energy levels of atoms and
molecules can have only certain quantized
values. Transitions between these quantized
states occur by the photon processes
absorption and emission.
It is possible for excited electrons in atoms
and molecules to have some other kind of
interaction which lowers their energy before
they can make a downward transition. In
that case they would emit a photon of lower
energy and longer wavelength. This process
is called fluorescence if it happens
essentially instantaneously.
Atoms in a gaseous state will produce
Line Spectra. Gas atoms are far apart
and minimally interact with each other. If
all the gas atoms are the same they will
produce the same spectra.
Solids and liquids will produce a
Continuous Spectra because the
atoms are so closely packed that there
is lots of atomic interaction. Almost any
photon energy is possible
Emission
Spectrum
A spectrum that consists predominantly
or solely of emission lines. It indicates
the presence of hot gas and a nearby
source of energy, as found, for example,
in planetary nebulae and quasars.
A spectrum of absorption lines or bands, produced when light from a hot
source, itself producing a continuous spectrum, passes through a cooler
gas. A material's absorption spectrum shows the fraction of incident
•
electromagnetic
radiation absorbed by the material over a range of
frequencies. An absorption spectrum is, in a sense, the opposite of an
emission spectrum.
Every chemical element has absorption lines at several particular
wavelengths corresponding to the differences between the energy levels
of its atomic orbitals. For example, an object that absorbs blue, green
and yellow light will appear red when viewed under white light.
Absorption spectra can therefore be used to identify elements present in a
gas or liquid. This method is used in deducing the presence of elements
in stars and other gaseous objects which cannot be measured directly.
•
Notice the pattern between emission spectrum
and absorption spectrum.
•
Absorption Spectrum
A material's absorption spectrum is the
fraction of incident radiation absorbed by
the material over a range of frequencies.
The absorption spectrum is primarily
determined by the atomic and molecular
composition of the material. Radiation is
more likely to be absorbed at frequencies
that match the energy difference between
two quantum mechanical states of the
molecules. The absorption that occurs due
to a transition between two states is
referred to as an absorption line and a
spectrum is typically composed of many
lines.
Absorption Spectrum
The frequencies where absorption lines
occur, as well as their relative
intensities, primarily depend on the
electronic and molecular structure of
the molecule. The frequencies will also
depend on the interactions between
molecules in the sample, the crystal
structure in solids, and on several
environmental factors (temperature,
pressure, electromagnetic field). The
lines will also have a width and shape
that are primarily determined by the
spectral density or the density of
states of the system.
emission
spectrum
A spectrum that consists predominantly or
solely of emission lines. It indicates the
presence of hot gas and a nearby source of
energy, as found, for example, in planetary
nebulae and quasars.
A spectrum of absorption lines or bands, produced when light from a hot
source, itself producing a continuous spectrum, passes through a cooler
gas. A material's absorption spectrum shows the fraction of incident
•
electromagnetic
radiation absorbed by the material over a range of
frequencies. An absorption spectrum is, in a sense, the opposite of an
emission spectrum.
Every chemical element has absorption lines at several particular
wavelengths corresponding to the differences between the energy levels
of its atomic orbitals. For example, an object that absorbs blue, green
and yellow light will appear red when viewed under white light.
Absorption spectra can therefore be used to identify elements present in a
gas or liquid. This method is used in deducing the presence of elements
in stars and other gaseous objects which cannot be measured directly.
Day 2 – Nuclear
Radiation
Particle
Charge Mass #
Location
Electron
-1
0
Electron cloud
Proton
+1
1
Nucleus
Neutron
0
1
Nucleus
Nuclear Notation
Mass Number (A)
Nucleons (protons
+neutrons)
238
Atomic Number (Z)
Just protons
92
• Z = Atomic number or the number of protons
• A = Mass number or the number of protons plus
neutrons
Antoine-Henri Becquerel
Radioactivity
(1852 - 1908)
• Discovered accidentally in 1896, radioactivity
occurs when unstable nuclei emit a particle or
energy.
• In all nuclear reactions, charge and mass number
is conserved. Some mass is converted into energy.
• Three types of radiation
Alpha (a)
Beta (b)
Gamma (g)
Alpha Radiation
• Radiation is the same as a helium nucleus
4
He or a. Remember the helium nucleus
2
consists of two protons + two neutrons
• Alpha radiation is the least energetic type of
radiation and can be stopped or shielded by a
sheet of paper.
Beta b Radiation
•
234
90
Th 
234Pa
91
+ 0e
1
beta particle
• Beta (b) radiation also consists of a particle
which can be an electron or positron.
• Transforms either a neutron into a proton or a
proton into a neutron in the nucleus
• Shielded by heavy clothing or wood
Gamma g Radiation
• Pure energy photon and not a “particle”
• Very energetic form of light like an X-ray or
gamma ray but comes from the nucleus
• Requires thick concrete or lead to
shield or stop
• No change in atomic mass
or mass number
Radiation Summary
Symbol
Charge
a
2+
b
1-
g
None
Penetration
Power
Low, paper stops it
Medium, clothes
stop it
High, only thick
metal slows it
Geiger Counter
Half-Life
Half-Life (t1/2) is
the time required
for half of the
atoms of a
radioisotope to
emit radiation and
to decay to
products.
Examples of Half-Life
Isotope
C-15
Ra-224
Ra-223
I-125
C-14
U-235
Half life
2.4 sec
3.6 days
12 days
60 days
5700 years
710 000 000 years
Half-Life of a Radioisotope
• The half-life of cesium-137 is 30 years. If you start with
a 8mg sample, how much is left after 30 years? 4 mg
•
after 60 years? 2 mg
•
after 90 years? 1 mg
decay curve
initial
1
half-life
8 mg
4 mg
2
2 mg
3
1 mg
What is up with
2
E=mc ?
This famous equation from Einstein
represents the equivalency of mass and
energy.
E = resting energy
M = mass
C = the speed of light (3x108m/s in a vacuum)
2
E=mc
• Einstein saw that mass was a means of
energy storage.
• Mass is a super storage device for energy.
• Small mass differences have huge energies
because c is such a big number.
Do all protons have the same
mass?
Absolutley NOT.
The mass of a proton depends on which
atomic nuclei it’s in.
Hydrogen atoms have very massive protons.
Iron atoms have very low mass protons
Uranium atoms have fairly high proton
masses.
Binding Energy
The mass difference is related to the binding
energy of the nucleus.
Iron has low mass per nucleon but the highest
binding energy (hardest to pull apart)
Hydrogen has a high mass per nucleon but a
small binding energy.
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•
4 Fundamental Forces
Nucleons
This is the term that refers to particles of the
nucleus.
Protons and Neutrons
What holds the nucleus of an atom together? Why don’t
the like charges of the protons repel and break up the
nucleus?
The electrical repulsive forces are trying to separate each proton from
every other proton. BUT those forces are overpowered by the
strongest force in the Universe. The Strong Nuclear Force holds all
nucleons together. The Strong Nuclear Force is the strongest but it
acts over distances not much longer than protons themselves.
For large nuclei, Strong forces don’t act from one side of the nucleus
to the other. Repulsive forces DO act across the nucleus.
This causes instability in the nucleus, and sometimes the nucleus will
decay.
Nuclear Fission
When a larger nucleus breaks into smaller
nuclei. The nucleons lose mass because
they are in smaller atomic nuclei. The
difference in mass equals the energy
released.
Large amounts of energy are released and
fission is the idea behind nuclear power
generation and massive bombs.
Nuclear Fission
• Fission occurs when a large nucleus absorbs a
neutron, becomes highly unstable, and breaks up
into two smaller nuclei.
235
92
U 
92
36
Kr 
141
56
Ba  2 n +
1
0
Energy
•
•
Nuclear Power plants
The idea is to take fissionable materials
(Uranium 235 and Plutonium) and generate
heat to boil water. The steam produced
turns a turbine like most other power plants.
•
Problems
• Thermal pollution: Disposal of radioactive fission
fragments.Radioactive interaction with structural
components.Accidental release of radioactivity into
atmosphere.Leakage of radioactive waste. Life time of
30 yrs due to build up of radioactivity.Earthquakes.
Limited supply of fissionable materials
• Breeder Reactor: Some neutrons produced are
absorbed by 238U.239Pu is produced, and is
fissionable.So the supply of fuel can increase 100
times.However, Plutonium is highly toxic and can
readily be used in bombs, and it involves a graphite
moderator, as was used in Chernobyl.
Radiation Dangers
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Nuclear Fusion
This nuclear reaction occurs when smaller
atoms smash into one another and fuse
together. This process makes the nucleons
lose mass. The difference in mass is equal
to energy released by E=mc2. Much more
energy is released in this process than by
Fission.
Why would 2 positive protons (Hydrogens) stick together?
Wouldn’t they repel because of being like charges?
• They do “want” to repel, thats why Fusion
doesn’t work at normal temperatures.
• Extremely high temperatures are required, these
great speeds (15million Kelvin) enable them to
get close enough for the stronger Strong Nuclear
Force to overpower the weaker electrical
replusion and cause them to fuse.
• When two hydrogen’s fuse into a helium atom a
lot of mass is lost and converted to energy.
Nuclear Fusion
• Fusion happens when two or more small nuclei
collide to form a larger nuclei
2H
1
+
3H
1
4He
2
+ 1n +
0
• Occurs in the sun and other stars
• A clean and powerful source of energy
Energy
Fusion
• Fusion is much cleaner with very little
harmful by-products.
• The fuel is easy to come by.
• The energy released is greater than fission
per unit mass.
• Many technical hurdles will have to be
overcome before we use this practically.
One main issue is containment. Containers
melt when subjected to Fusion
temperatures.
Manhattan Project
Purpose: Develop Nuclear weapons
First bomb – Trinity- exploded over American
soil (near Alamogordo, NM) in a test on
July 16, 1945. It was a Plutonium bomb
(very complicated and needed a test, a
similar bomb was dropped over Nagasaki
Japan)
Little Boy
First nuclear weapon used in War.
It was a Uranium Bomb. Much simpler to build, but
Uranium 235 was very hard to come by.
It was dropped over Hiroshima, Japan on August 6,
1945.
It was untested when dropped.
It exploded with the energy equivalence of 18,000
tons of dynamite.
Over 100,000 Japanese were killed by this bomb.
Little Boy
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Fat Man
Dropped on Nagasaki, Japan August 9, 1945
It had the energy release of 21,000 tons of dynamite.
The death toll was less than Hiroshima due to bad weather and
the bombing run flying slightly off course.
It was a Plutonium Bomb and much more complicated than the
Uranium bomb.
Thermonuclear Device
•
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•
•
Fusion Devices
Stars are under tremendous gravity
Creates tremendous pressure
High pressure means high temperature
High temperature means particles collide violently
On earth high temperatures and densities not easily
achieved
• Fission Bomb can ignite Fusion Bomb:
Thermonuclear Device or H bomb
•
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