ap physics b - atomic and nuclear physics

7/28/2019 AP Physics B - Atomic and Nuclear Physics

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Atomic & Nuclear Physics

AP Physics B


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Life and AtomsEvery time you breathe you are takingin atoms. Oxygen atoms to beexact. These atoms react with theblood and are carried to every cellin your body for various reactionsyou need to survive. Likewise,every time you breathe out carbondioxide atoms are released.

The cycle here is interesting.

TAKING SOMETHING IN.

ALLOWING SOMETHING OUT!


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The AtomAs you probably already know an

atom is the building block of allmatter. It has a nucleus withprotons and neutrons and anelectron cloud outside of thenucleus where electrons are

orbiting and MOVING.

Depending on the ELEMENT, theamount of electrons differs aswell as the amounts of orbitssurrounding the atom.


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When the atom gets excited or NOT

To help visualize the atom think of it like a ladder. The bottom ofthe ladder is called GROUND STATE where all electrons wouldlike to exist. If energy is ABSORBED it moves to a new rung onthe ladder or ENERGY LEVEL called an EXCITED STATE.This state is AWAY from the nucleus.

As energy is RELEASED the electron can relax by moving to anew energy level or rung down the ladder.


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Energy LevelsYet something interesting happens asthe electron travels from energylevel to energy level.

If an electron is EXCITED, that meansenergy is ABSORBED andtherefore a PHOTON is absorbed.

If an electron is DE-EXCITED, thatmeans energy is RELEASED andtherefore a photon is released.

We call these leaps from energy levelto energy level QUANTUM LEAPS.

Since a PHOTON is emitted thatmeans that it MUST have a certainwavelength.


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Energy of the PhotonWe can calculate the ENERGYof the released or absorbed

photon provided we know theinitial and final state of theelectron that jumps energylevels.


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Energy Level DiagramsTo represent these

transitions we canconstruct an ENERGYLEVEL DIAGRAM

Note: It is very important to understanding that these transitions DO NOT

have to occur as a single jump! It might make TWO JUMPS to get back toground state. If that is the case, TWO photons will be emitted, each with adifferent wavelength and energy.


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ExampleAn electron releases energyas it moves back to itsground state position. As aresult, photons are

emitted. Calculate thePOSSIBLE wavelengths ofthe emitted photons.

Notice that they give us theenergy of each energylevel. This will allow us tocalculate the CHANGE inENERGY that goes to theemitted photon.

This particular sample will release three

different wavelengths, with TWO beingthe visible range ( RED, VIOLET) andONE being OUTSIDE the visible range

(INFRARED)


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Energy levels Application: Spectroscopy

Spectroscopy is an optical technique by which we canIDENTIFY a material based on its emissionspectrum. It is heavily used in Astronomy and

Remote Sensing. There are too many subcategoriesto mention here but the one you are probably themost familiar with are flame tests.

When an electron gets excited inside

a SPECIFIC ELEMENT, the electronreleases a photon. This photonswavelength corresponds to the

energy level jump and can be usedto indentify the element.


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Different Elements = Different Emission

Lines


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Emission Line SpectraSo basically you could look at lightfrom any element of which theelectrons emit photons. If youlook at the light with a diffraction

grating the lines will appear assharp spectral lines occurring atspecific energies and specificwavelengths. This phenomenonallows us to analyze theatmosphere of planets orgalaxies simply by looking at thelight being emitted from them.


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Nuclear Physics - RadioactivityBefore we begin to discuss the specifics of radioactive

decay we need to be certain you understand the

proper NOTATION that is used.To the left is your typical radioactive

isotope.

Top number = mass number = #protons

+ neutrons. It is represented by theletter "A

Bottom number = atomic number = # ofprotons in the nucleus. It is represented

by the letter "Z"


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Nuclear Physics Notation & Isotopes

An isotope is when you havethe SAME ELEMENT, yetit has a different MASS.This is a result of haveextra neutrons. SinceCarbon is always going to

be element #6, we canwrite Carbon in terms of itsmass instead.

Carbon - 12Carbon - 14


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Einstein Energy/Mass EquivalenceIn 1905, Albert Einstein publishes a 2nd majortheory called the Energy-MassEquivalence in a paper called, Does the

inertia of a body depend on its energycontent?


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Einstein Energy/Mass EquivalenceLooking closely at Einsteins equation we see that he

postulated that mass held an enormous amount ofenergy within itself. We call this energy BINDING

ENERGY or Rest mass energy as it is the energythat holds the atom together when it is at rest. Thelarge amount of energy comes from the fact that thespeed of light is squared.


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Energy Unit Check

2

2

2

2

2

2

2

s

mkgm

s

mkgWE

s

m

kgNmaF

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net

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Mass Defect

The nucleus of the atom is held together by a STRONG NUCLEAR

FORCE.

The more stable the nucleus, the more energy needed to break it apart.Energy need to break to break the nucleus into protons and neutrons iscalled the Binding Energy

Einstein discovered that the mass of the separated particles is greaterthan the mass of the intact stable nucleus to begin with.

This difference in mass (m) is called the mass defect.


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Mass Defect - Explained

The extra mass turns into energy

holding the atom together.


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Mass Defect

Example


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Radioactivity

When an unstable nucleus releases energy and/orparticles.


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Radioactive DecayThere are 4 basic types of

radioactive decay

Alpha Ejected Helium Beta Ejected Electron

Positron Ejected Anti-

Beta particle Gamma Ejected Energy

You may encounter protonsand neutrons being emittedas well

n

p

ee

He

1

0

1

1

00

0

1

0

1

4

2


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Alpha Decay

HeUPu4

2

236

92

240

94+


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Alpha Decay Applications

?

4

2

241

95

A

ZHeAm +

Americium-241, an alpha-emitter, is used in smoke detectors. The alpha

particles ionize air between a small gap. A small current is passed through thationized air. Smoke particles from fire that enter the air gap reduce the currentflow, sounding the alarm.


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Beta Decay

AceRa228

89

0

1

228

88 +

There arent really any

applications of beta decay

other than Betavoltaics which

makes batteries from betaemitters. Beta decay, didhowever, lead us to discover

the neutrino.


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Beta Plus Decay - Positron

ThePa230

90

0

1

230

91 +

Isotopes which undergo this decay

and thereby emit positrons includecarbon-11, potassium-40,nitrogen-13, oxygen-15, fluorine-

18, and iodine-121.


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Beta Plus Decay Application - Positron

emission tomography (PET)Positron emission tomography

(PET) is a nuclear medicine

imaging technique whichproduces a three-dimensionalimage or picture of functionalprocesses in the body. Thesystem detects pairs of gammarays emitted indirectly by apositron-emitting radionuclide(tracer), which is introducedinto the body on a biologicallyactive molecule. Images oftracer concentration in 3-

dimensional space within thebody are then reconstructed bycomputer analysis.


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Gamma Decay

0

0

240

94

240

94 + PuPu


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Gamma Decay ApplicationsGamma rays are the most dangerous type of radiation

as they are very penetrating. They can be used tokill living organisms and sterilize medical equipment

before use. They can be used in CT Scans andradiation therapy.

Gamma Rays are used to view stowaways inside of a truck. This

technology is used by the Department of Homeland Security at many portsof entry to the US.


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Significant Nuclear Reactions - Fusion

nHeHH 1042

31

21 ++

nuclear fusion is the process by which multiple like-charged atomic nucleijoin together to form a heavier nucleus. It is accompanied by the release orabsorption of energy.


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Fusion Applications - IFEIn an IFE (Inertial Fusion Energy) power plant, many (typically

5-10) pulses of fusion energy per second would heat a low-activation coolant, such as lithium-bearing liquid metals or moltensalts, surrounding the fusion targets. The coolant in turn wouldtransfer the fusion heat to a power conversion system to produceelectricity.


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Significant Nuclear Reactions - Fission

Nuclear fission differs from other forms of radioactive decay in that it can be

harnessed and controlled via a chain reaction: free neutrons released byeach fission event can trigger yet more events, which in turn release moreneutrons and cause more fissions. The most common nuclear fuels are 235U

(the isotope of uranium with an atomic mass of 235 and of use in nuclearreactors) and 239Pu (the isotope of plutonium with an atomic mass of 239).

These fuels break apart into a bimodal range of chemical elements with

atomic masses centering near 95 and 135 u (fission products).

energynKrBaUn ++++1

0

92

36

141

56

235

92

1

0

3


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Fission BombOne class of nuclear weapon, a fission

bomb(not to be confused with thefusion bomb), otherwise known asan atomic bombor atom bomb, is a

fission reactor designed to liberateas much energy as possible asrapidly as possible, before thereleased energy causes the reactorto explode (and the chain reaction to

stop).


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ap physics b - atomic and nuclear physics

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