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

Announcements Course policy and regrade forms on the web: 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.

What Holds Our World Together? Four Fundamental Forces Electromagnetic: Photon  Weak Force: W/Z Bosons Strong Force: Gluons * * * Gravitational Force: Graviton(?) In block 14 we will concentrate on these two forces

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? 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 Force Never 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. p p Radius proton ~ 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 Notation for atomic elements: 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 PE strong wins over the increase in PE electric We can quantify this! Example: carbon-14 has 14 nucleons, 6 protons, 8 neutrons

 Potential Energy of Nucleus How do we know if the total energy increases or decreases? Define: PE nuclear = PE electric + PE strong Total change  PE nuclear = observed change in the nucleus mass:  mc 2 (  E=  mc 2 … Does this look familiar?) Initial State: Final State: x kg = M p amu x kg = M n (1 amu = x kg) x kg = M e Mass( 2p+2n+2e ) = x kg => x kg The mass decreased by  m= x kg in putting the subatomic particles together to make helium. Hence E nuclear decreased. Then the decrease in PE strong is larger than the increase in PE electric. Where does the energy  mc 2 go? E environment !

Energy Interaction Diagrams To calculate PE electric : When a positively charged proton is brought closer to another positively charged proton, r final is smaller than r initial, and  PE electric is positive. Work must be done to bring them together. For PE strong : Quantify PE strong using knowledge of PE electric and  mc 2 Evaluate this nuclear reaction:

The 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. PE weak  for neutron  proton reactions PE weak  for proton  neutron reactions Weak processes involve or anti- Complete energy conservation:  E nuclear =  PE electric +  PE strong +  PE weak

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 EM Metastable: Decays after a period of time (“half-life”). A nucleus that breaks apart undergoes fission.

Nuclear Fission Reactions Evaluate this nuclear reaction: What does energy environment tell you about the mass of the products versus mass of original nucleus? What happens to PE electric ? Does PE strong 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!)

Nuclear Radioactive Decay Alpha particles  (helium nuclei) beta particles  (electrons/positrons) 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 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 waste After time, not enough heavy unstable nuclei for efficient reaction. Product nuclei also unstable: Spent fuel rods! (Iran!)

The A-Bomb and the H-Bomb Atom 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 E env due to fusion of lighter elements. A-Bombs: Energy comes from E env 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.