PHOTOELECTRIC EFFECT. Photoelectric History In 1839, Alexandre Edmond Becquerel discovered the...

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."


• 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.


• In 1905, Albert Einstein formulated the wave-particle duality by describing light as composed of discrete quanta, now called photons, rather than continuous waves.


• 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.


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


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


• 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.


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


• 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.



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



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


• 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.


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


• 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

•


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


• This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge from nearby plasma,


and the imbalance can discharge through delicate electrical components.


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


• 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.”


• 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


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


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


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

http://www.google.com/imgres?imgurl=http://www.ecologic-cars.com/wp-content/uploads/solar-cars.jpg&imgrefurl=http://www.ecologic-cars.com/solar-cars/&usg=__B_ydUEnaswHZNsUfH7Ck9oT8UB8=&h=470&w=500&sz=39&hl=en&start=4&zoom=1&tbnid=lnbXjuXfDAepIM:&tbnh=122&tbnw=130&ei=BvDBTZ-AJsegtweph-XGBQ&prev=/search%3Fq%3Dsolar%2Bcars%26hl%3Den%26biw%3D1276%26bih%3D735%26site%3Dsearch%26tbm%3Disch&itbs=1


• 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.

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http://hyperphysics.phy-astr.gsu.edu/hbase/mod5.html

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 fluorersnece 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 SpectrumA 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 SpectrumThe 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

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

neutrons

238

92

Mass Number (A)Nucleons (protons

+neutrons)

Atomic Number (Z)Just protons

Radioactivity

• 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 radiationAlpha ()

Beta ()

Gamma ()

Antoine-Henri Becquerel

(1852 - 1908)

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.

He2

4

Beta Radiation

• 234 Th 234Pa + 0e 90 91 1

beta particle• Beta () 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 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 Penetration

Power

2+ Low, paper stops it

1- Medium, clothes stop it

None 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 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

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

initial

1 half-life 2 3

4 mg2 mg1 mg

What is up with E=mc2?

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)

E=mc2

• 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.

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.

This causes instability in the nucleus, and sometimes the nucleus will decay.

Nuclear Fission

When a larger nucleus breaks into smaller nuclei. The nucleons loose 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. nBaKrU 1

014156

9236

23592 2 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

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

One of the many, many problems that came up from the explosion was the release of massive amounts of radioactive Cesium-137.

Cesium-137 has a half-life of 30 years and is easily absorbed by trees and other plants as a salt.

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.

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.

Nuclear Fusion

This nuclear reaction occurs when smaller atoms smash into one another and fuse together. This process makes the nucleons loose 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 + 3H 4He + 1n +

1 1 2 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

While the moral, ethical, and political aspects of nuclear weapons can be argued, their devastating power and destructive capabilities cannot.

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.

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.

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

• 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

PHOTOELECTRIC EFFECT. Photoelectric History In 1839, Alexandre Edmond Becquerel discovered the...

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