Welcome to Physics 7C!Welcome to Physics 7C!

Lecture 7 -- Winter Quarter -- 2005

Professor Robin Erbacher

343 Phy/Geo

erbacher@physics.ucdavis.edu

AnnouncementsAnnouncements

• Course policy and regrade forms on the web: http://physics7.ucdavis.edu

• If you received rubric code 4 on part b) of Quiz 2, please hand in your quiz for a possible regrade.

• Quiz today on Block 13, DLMs 9 through 12.

• Block 14: The Fundamental Forces of Nature.

• Lecture 10 will be a review for the final.

• Turn off cell phones and pagers during lecture.

Understanding our Universe:A brief introduction…

Understanding our Universe:A brief introduction…

What is the World Made Of?What is the World Made Of?

In ancient times, people sought to organize the world around them into fundamental elements

Aristotle:

–Earth

–Air

–Fire

–Water

What Else Did They Think?What Else Did They Think?

“By Convention there is color,

by convention sweetness,

by convention bitterness,

but in reality there are atoms and space.”

-Democritus (400 BC)

Atom = Mushy Ball (c. 1900)

Where We Were ~100 Years Ago…

Where We Were ~100 Years Ago…

Rutherford’s Scattering ExptRutherford’s Scattering Expt

Apparatus

Hypothesis

Results (data)

AnalysisConclusion: A Nucleus!

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New Types of Matter!New Types of Matter!

Fermilab: BubbleChamber Photo

Then…More Mysteries!

The Quark IdeaThe Quark Idea

The StanfordLinear Accelerator

Center

EndStation A: Beam of Electrons onto Target

Quarks Are Found!Quarks Are Found!‘Three Quarks for Muster Mark!’

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are needed to see this picture.1990 Nobel Prize in Physics: Quarks Revealed! Structure Inside Protons and Neutrons

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Quarks found: 1968!

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The Nature of MatterThe Nature of MatterWhat, then, is fundamental?

Quark Discoveries… Top!Quark Discoveries… Top!

•Quarks (u,d,s) were postulated in 1964by Gell-Mann and Zweig, discovered in 1968

ud

•The charm quark c was discovered in1974 by Brookhaven and SLAC

cs

•The bottom b quark was discovered In 1977 at Fermilab

…b

The bottom quark needed a partner… => top!

The top quark was finally found in 1995 at Fermilab!

Physics of the Top QuarkPhysics of the Top Quark

Top physics is one of the more sexy things to study at the Fermilab Tevatron…

Quarks and ScaleQuarks and Scale

It is also possible that quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles.

As far as we know now, however, they are fundamental!

What Holds it All Together?What Holds it All Together?•Electromagnetic: Xrays, radio, infrared, magnets•Weak Force: Nuclear radioactive decay•Strong Force: Binds quarks and nuclei

You will learn aboutthese two forces in

block 14…

The Fundamentals ForcesThe Fundamentals Forces4 Fundamental

Forces•Electromagnetic: Photon •Weak Force: W/Z Bosons •Strong Force: Gluons * * *•Gravitational Force: Graviton(?)

The New Periodic TableThe New Periodic Table

…of Particles and Forces

Standard Model of Particle Physics

Standard Model of Particle Physics

The SM states that:

The world is made up of quarks andleptons that interactby exchanging bosons (force carriers).

A Higgs field interactsas well, giving particlestheir masses.

Lepton Masses: Me<M<M M~0.*

Quark Masses: Mu ~ Md < Ms < Mc< Mb << Mt

Matter as we know it

Nature is Fundamentally Simple

Nature is Fundamentally Simple

The Standard Model -- Lots of Mysteries Still!Our current theory of how elementary particles interact does not make predictions

Example: We knew that the top quark existed but the S.M. could not tell us what its mass was.

•What makes the pattern of particles masses?•Why are there only 3 families of quarks?•Do protons decay?•Why is there so little antimatter in the universe?•What is Dark Matter? Dark energy?•Do we live in a universe that has “extra dimensions”?•What about gravity?•Is there a Grand Unified Theory that merges the 4 forces?(A Theory of Everything?)

Many Puzzles Remain…Many Puzzles Remain…

Summary of the Universe

Summary of the Universe

The world is made up of quarks and leptons

These particles interact through 4 different forces

The Standard Model works - but remains limited

We are still missing several pieces of the puzzle

Work is underway to extend understanding-- watch the nextdecade for discoveries…

Pretty cool stuff! On to Block 14…

The Funny Thing About the Nucleus

The Funny Thing About the Nucleus

As you know from chemistry, the nucleus of the atom is made up of protons and neutrons.

What did we just learn about the electric force between two positive charges?

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So, if the nucleus is loaded with positively charged protons, why doesn’t it blow apart?

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The force that counteracts the electromagnetic repulsion in the nucleus is called the force, and is mediated by the gluon (it’s carrier particle is the gluon, like the photon is for electromagnetism).

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The Strong ForceThe Strong ForceNever heard of the strong force???The strong force is responsible for binding nucleons (protons and neutrons) together inside of the nucleus, and for binding quarks together inside of the nucleon.We will focus on the former-- protons and neutrons.

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ppRadiusproton ~ 10-15

We’ll use our field model of forces, and extend our energy conservation model to explain some atomic behaviors.

other nucleon (n or p)

exerts force

nucleon strong field(n or p)

creates

Field Model of Forces

Making Helium: Nuclear Fusion

Making Helium: Nuclear Fusion

Notation for atomic elements:

€

ZA X

€

X = name of element

A = number of neutrons and protons

Z = number of protons (atomic number)

€

211p + 20

1n + 2e- → 24He + energy

To make helium, we need 2 protons, 2 neutrons, 2 electrons:

When the nucleons combine, strong bonds are formed, which decreases the potential energy in the system. However, the electric potential energy increases since the protons are now closer.

How do we know if the total energy increases or decreases?Because energy is released (our sun!) and helium is stable, we know that the decrease in PEstrong wins over the increase in PEelectric

We can quantify this!

Example:carbon-14has 14 nucleons, 6 protons, 8 neutrons

€

614 C

Potential Energy of NucleusPotential Energy of NucleusHow do we know if the total energy increases or decreases? Define: Enuclear = PEelectric + PEstrong

The total change Enuclear = observed change in the nucleus mass: mc2

(E= mc2… Does this look familiar?)

Initial State: Final State:1.672673 x 10-27 kg = Mp 4.00260 amu1.674929 x 10-27 kg = Mn (1 amu = 1.660540 x 10-27 kg)0.000911 x 10-27 kg = Me

Mass(2p+2n+2e) = 6.697026 x 10-27 kg => 6.46477 x 10-27 kg

Therefore, the mass decreased by 0.050549 x 10-27 kg in putting the subatomic particles together to make helium. Hence Enuclear decreased.

Then the decrease in PEstrong is larger than the increase in PEelectric.

Energy Interaction DiagramsEnergy Interaction Diagrams

More p-p

repulsion

PE ↑electric

nucleus system

E ↓ , mass ↓

Strong

bonds form

PE ↓strong

Environment

energy system

E ↑env

nuclear

To calculate PEelectric:

€

PE electric = kQqΔ1

r

⎛

⎝ ⎜

⎞

⎠ ⎟ = kQq

1

rfinal

-1

rinitial

⎛

⎝ ⎜

⎞

⎠ ⎟

When a positively charged proton is brought closer to another positively charged proton, rfinal is smaller than rinitial, and PEelectric is positive. Work must be

done to bring them together.

More p-p

repulsion

PE ↑electric

- Less p p

repulsion

PE ↓elec

XE XE

For PEstrong:

Strong

bonds form

PE ↓strong

Strong

bonds break

PE ↑strong

XE XE

Quantify PEstrong using knowledge of PEelectric and mc2

€

1327 Al+2

4 He → 01n+15

30 P

Evaluate this nuclear reaction:

Next Time: Nuclear Fission, or…

Next Time: Nuclear Fission, or…

What makes things go boom!