Welcome to Physics 7C! Lecture 8 -- Winter Quarter -- 2005 Professor Robin Erbacher 343 Phy/Geo...

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

Lecture 8 -- Winter Quarter -- 2005

Professor Robin Erbacher

343 Phy/Geo

[email protected]

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, DLM 14.

• Block 14: The Fundamental Forces of Nature.

• Lecture 10 will be a review for the final.

• Turn off cell phones and pagers during lecture.

http://physics7.ucdavis.edu/

What Holds Our World Together?

What Holds Our World Together?

FourFundamental

Forces

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

In block 14 we willconcentrate on

these two forces

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.

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

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

So, if the nucleus is loaded with positively charged protons which repel each other, then why doesn’t it blow apart?


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).


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 are focusing on the former: protons and neutrons.



ppRadiusproton ~ 10-15

Using our field model of forces, we can 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: PEnuclear = PEelectric + PEstrong

Total change PEnuclear = 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 amu 1.674929 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

The mass decreased by m=-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.

Where does the energy mc2 go?

€

211p + 20

1n + 2e-

€

24 He

Eenvironment!



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:

The Weak ForceThe Weak Force



Interaction processes due to the weak nuclear force occur when a neutron changes to a proton….…. Or a proton changes to a neutron.

PEweak for neutron proton reactions

PEweak for proton neutron reactions

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Weak processes involve or anti-

€

Example : 01n → 1

1p +e- +ν

Complete energy conservation: Enuclear = PEelectric + PEstrong + PEweak









Nuclear Fission, or…Nuclear Fission, or…Things that go boom!

If a nucleus has too many protons compared to neutrons Unstable!

If a nucleus has too few protons compared to neutrons Unstable!

We could have many different combinations of protons and neutrons to make up a nucleus with A=62, for example. . But only has the right number to be stable.

Stable nuclei have right mixture: strong force balances EMMetastable: Decays after a period of time (“half-life”).

€

2862Ni

€

2862Ni, 26

62Fe, 2762Co, 29

62Cu

A nucleus that breaks apart undergoes fission.

Nuclear Fission ReactionsNuclear Fission Reactions

€

88226Ra →2

4 He + 86222Rn + energy

Evaluate this nuclear reaction:Rn

86+

He2+

7.266 × 10m

–15

Ra

88+

fission

reaction

products" "unstable reactant

Rn86+

He2+

What does energy environment tell you about the mass of the products versus mass of original nucleus?

What happens to PEelectric?

Does PEstrong increase or decrease?

The repulsive force between protons for heavy elements (>Z=83) is too much: no amount of neutrons can make it stable. They undergo spontaneous fission.

(Half-lives range from nano-seconds to billions of years!)

€

N(t) = N0e−λt Half - life T1/ 2 = ln(2)

λ


are needed to see this picture.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.



Nuclear Radioactive DecayNuclear Radioactive Decay









QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Alpha particles (helium nuclei)



beta particles (electrons/positrons)


are needed to see this picture.Gamma rays (or x-rays)

•Decays with and change nuclear species.•Decays with are for heavy elements.•Decays with emission do not change A or Z but occur during nucleus restructuring.

Chain Reactions: Nuclear Energy

Chain Reactions: Nuclear Energy

Controlled fission: Nuclear Reactors

Heat released can do work.

A heavy element can undergo induced fission by absorbing a stray neutron, releasing energy, lighter elements, then more stray neutrons.

These are momentarily absorbed by other heavy atoms, which undergo fission due to unfavorable # neutrons to protons.





Enrico Fermi

Chain reaction!Controlling the cascade: Absorb the stray neutrons using inert “mediators”.

First controlled nuclear reaction:

U. Of Chicago, 1942

My particle research:Fermilab, near Chicago

Problem: nuclear wasteAfter time, not enough heavy unstable nuclei for efficient reaction. Product nuclei also unstable: Spent fuel rods! (Iran!)

The A-Bomb and the H-BombThe A-Bomb and the H-BombAtom Bomb: A heavy element, even if stable, can absorb neutrons and undergo induced fission. A chain reaction: an

increasing cascade of stray neutrons.

Hydrogen Bomb: Light elements can spontaneously undergo fusion if temperatures are high enough, as occurs in the sun. Lots of energy is released.

H-Bombs: Energy comes from Eenv due to fusion of lighter elements.

€

01n + 92

235U → 56141Ba + 36

92Kr + 301n + energy (lots!)



A-Bombs: Energy comes from Eenv due to fission of heavy elements.

A-bombs can act as H-bomb triggers!Strap a tank of light-nuclei (H, Li) to an A-bomb to make an H-bomb.

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Welcome to Physics 7C! Lecture 8 -- Winter Quarter -- 2005 Professor Robin Erbacher 343 Phy/Geo...

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