PHOTOELECTRIC EFFECT

1
PHOTOELECTRIC EFFECT
2
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.

3
Photoelectric History

• The actual photoelectric effect was first
  observed by Heinrich Hertz in 1887, the
  phenomenon is also known as the "Hertz effect."

4
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 wave-pariticle
  duality.

5
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.

6
Photoelectric History

• Based upon Max Planks 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 Planks
  Constant.

7
Photoelectric History

• The photons of a light beam have a characteristic
  energy determined by the frequency of the light.

8
Photoelectric History

• A photon above a threshold frequency has the
  required energy to eject a single electron,
  creating the observed effect.

9
Photoelectric History

• This discovery led to the quantum revolution in
  physics and earned Einstein the Nobel Prize in
  Physics in 1921.

10
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.
  

11
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.

12
Photoelectric Basics

• If the photon energy is too low, the electron is
  unable to escape the material.

13
Photoelectric Basics

• Increasing the intensity of the light beam
  increases the number of electrons excited, but
  does not increase the energy that each electron
  possesses.

One photon with a high enough frequency in, one
electron out.
14
Photoelectric Basics

• Increasing the intensity of the light beam
  increases the number of electrons excited, but
  does not increase the energy that each electron
  possesses.

Three photons with a high enough frequency in,
three electrons out.
15
Photoelectric Basics

• Increasing the intensity of the light beam
  increases the number of electrons excited, but
  does not increase the energy that each electron
  possesses.

Lots of photons with a high enough frequency in,
lots of electrons out.
16
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.

17
Photoelectric Basics

• Electrons can absorb energy from photons when
  irradiated, but they usually follow an all or
  nothing principle.

18
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.

19
Photoelectric Uses and Effects

• Video camera tubes in the early days of
  television used the photoelectric effect.

20
Photoelectric Effect and sound production at the
movies

21
Photoelectric Uses and Effects

• The photoelectric effect will cause spacecraft
  exposed to sunlight to develop a positive charge.

22
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,

23
Photoelectric Uses and Effects

• and the imbalance can discharge through
  delicate electrical components.

24

25
Photoelectric Uses and Effects

• Light from the sun hitting lunar dust causes it
  to become charged through the photoelectric
  effect.

26
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.

27
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

28
Photoelectric Uses and Effects

• Still, the most common use is panels that produce
  an electrical current. From solar calculators

29
Photoelectric Uses and Effects

• Still, the most common use is panels that produce
  an electrical current. From solar calculators to
  solar house panels

30
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

31
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.

32
Atomic Fingerprints

• Every atom has a unique signature due to a
  combination of number of electrons and energy
  levels for that atom.

33

34
What does quantized mean?

35
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.

36
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.

37

• When atoms receive energy, electrons can move up
  into higher energy levels, they dont 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.

38
(No Transcript)
39
B
C
A
40

41
(No Transcript)
42

• 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.

43

44

• 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.

45

46

• 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

47
(No Transcript)
48

49

50
(No Transcript)
51
Notice the pattern between emission spectrum and
absorption spectrum.

52

53

54
(No Transcript)
55

56
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.

57
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.

58
(No Transcript)
59
(No Transcript)
60

61
Radiation
Day 2 Nuclear Radiation
62
Particle Charge Mass Location
Electron -1 0 Electron cloud
Proton 1 1 Nucleus
Neutron 0 1 Nucleus
63
Nuclear Notation
Mass Number (A) Nucleons (protons neutrons)
238
U
Atomic Number (Z) Just protons
92

• Z Atomic number or the number of protons
• A Mass number or the number of protons plus
  neutrons

64
Radioactivity
Antoine-Henri Becquerel (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)

65
Alpha Radiation

• Radiation is the same as a helium nucleus
• or ?. Remember the helium nucleus 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.

66
Beta ? Radiation

• 234 Th 234Pa 0e
• 90 91 ?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

67
Gamma ? 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 massor mass number

68
Radiation Summary
Symbol Charge Penetration Power
a 2 Low, paper stops it
b 1- Medium, clothes stop it
g None High, only thick metal slows it
69
(No Transcript)
70
(No Transcript)
71
(No Transcript)
72
Geiger Counter
73
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.
74
Examples of Half-Life

• Isotope Half life
• C-15 2.4 sec
• Ra-224 3.6 days
• Ra-223 12 days
• I-125 60 days
• C-14 5700 years
• U-235 710 000 000 years

75
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?
• 
  after 60 years?
• 
  after 90 years?
• decay curve
• 8 mg 4 mg 2 mg 1 mg

4 mg
2 mg
1 mg
initial
1 half-life
2
3
76
(No Transcript)
77
What is up with Emc2?

• 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)

78
Emc2

• 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.

79
Do all protons have the same mass?

• Absolutley NOT.
• The mass of a proton depends on which atomic
  nuclei its in.
• Hydrogen atoms have very massive protons.
• Iron atoms have very low mass protons
• Uranium atoms have fairly high proton masses.

80
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.

81

82

83
4 Fundamental Forces
84
Nucleons

• This is the term that refers to particles of the
  nucleus.
• Protons and Neutrons

85
What holds the nucleus of an atom together? Why
dont 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 dont 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.
86
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.

87
Nuclear Fission

• Fission occurs when a large nucleus absorbs a
  neutron, becomes highly unstable, and breaks up
  into two smaller nuclei.

Energy

88

89

90
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.

91
(No Transcript)
92
(No Transcript)
93

94
(No Transcript)
95
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.

96
Radiation Dangers
One of the many, many problems that came up from
the explosion was the release of massive amounts
of radioactive Cesium-137.
If a forest fire were to occur, the ash cloud
from the fire could prove to be very dangerous to
any life the cloud passes over.

• In April, 1986, one of four nuclear reactors near
  Chernobyl in the Ukraine exploded.

Cesium-137 has a half-life of 30 years and is
easily absorbed by trees and other plants as a
salt.
97
Radiation Dangers
More recently, the Fukushima nuclear power plant
in Japan has caused a wide-spread evacuation
because of the release of numerous radioactive
isotopes.
Among the isotopes released are Cesium-137,
Iodine-131, and Plutonium. All of these elements
are dangerous in high enough quantities.
Because of these radioactive isotopes being
released, up to a 30 km evacuation has been
called for, fishing is affected, and tap water
usage is restricted.
98
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
  Emc2. Much more energy is released in this
  process than by Fission.

99
Why would 2 positive protons (Hydrogens) stick
together? Wouldnt they repel because of being
like charges?

• They do want to repel, thats why Fusion doesnt
  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 hydrogens fuse into a helium atom a lot
  of mass is lost and converted to energy.

100
Nuclear Fusion

• Fusion happens when two or more small nuclei
  collide to form a larger nuclei
• 2H 3H 4He 1n
• 1 1
  2 0
• Occurs in the sun and other stars
• A clean and powerful source of energy

101
(No Transcript)
102
(No Transcript)
103
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.

104
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)

105
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.

106
Little Boy
Death from radiation exposure can take anywhere
from days to years. The actual causes range from
immediate organ collapse to cancers decades after
the exposure.
The release of massive amounts of alpha, beta,
and gamma radiation by both fission and fusion
bombs can cause horrific burns to the human body.
While the moral, ethical, and political aspects
of nuclear weapons can be argued, their
devastating power and destructive capabilities
cannot.
107
(No Transcript)
108
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.

109
(No Transcript)
110
Thermonuclear Device

• 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

111
(No Transcript)
112

113
(No Transcript)
114

115

116

117

118