Today: Radioactivity Introduction Some of our goals for the next few lectures Learn about the physics of radioactivity and nuclear reactions. Learn about the hazards and safety mechanisms Be able to apply the physics to real-life debates related to nuclear physics
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Today: Radioactivity Introduction
Some of our goals for the next few lectures
Learn about the physics of radioactivity and nuclear reactions.
Learn about the hazards and safety mechanisms
Be able to apply the physics to real-life debates related to nuclear physics
A soap bubble or film is a thin layer of water stabilized by detergent molecules
http://www.funsci.com/fun3_en/exper2/exper2.htm
Water: speed of light slower
Air: speed of light fast
Air
Incident light will reflect and pass throughinterfaces.
Reflections off the top surface and bottom surface will interfere with each other
KEY POINT: 180 degree phase shift when reflecting off interface going from fast to slow (top interface in this case)
Detergent layer
180 degree phase shift explains the black color in the thinnest part of the soap film
Thinnest part of film
http://www.funsci.com/fun3_en/exper2/exper2.htm
Water: speed of light slower
Air: speed of light fast
Air
When film is very thin, path length difference is negligible. So interference is destructive.
Appears black because I used black mug.
As film gets thicker, there is some path length difference, so interference is not completely destructive – white color.
Wikipedia.org
Detergent layer
Rachelle Regan
Jocelle B.
Rachelle: “I do not agree with the book because when I look at my picture of my bubbles I do see a lot of green”
John Z.: “I will say that the predominant colors will be the green and magenta. Yellow and cyan dot appear as much.”
Laura F.: “I saw greens, blues, reds, and violets. I think the author should explain how varied angles of light striking the bubble can have an effect on color. “
(click on movie)
If we were to learn and apply the mathematical physics,we could very well predict the pattern of colors!
You know which colors are missing / present (e.g. magenta = blue + red)It is a really nice way to apply your knowledge of physics!
Today: Radioactivity Introduction
Some of our goals for the next few lectures
Learn about the physics of radioactivity and nuclear reactions.
Learn about the hazards and safety mechanisms
Be able to apply the physics to real-life debates related to nuclear physics
First! Cloud Chamber Demo
We will see the trails of particles from radioactive decay!
Andrew Foland / Cornell University / Wilson Lab /
Slide Projector
Alcohol-soaked side
Dry Ice
Super-saturated alcohol vapor
Alpha particlesource
Let’s observe the demo first, and then learn more about what we saw.
http://www.youtube.com/watch?v=chxv5G6UFl0&feature=relatedYou can build one yourself!
One slide about charge (electrical charge)
Particles in the universe can have positive (+), negative (-) or neutral (zero) charge
Opposite charges attract each other Same charges repel each other
Force Charge1 * Charge2
distance2
“is proportional to”
Physics can exactly predict the force between two charged objects:
Increase charge = increase forceIncrease separation = decrease force
The main reason I wanted to show you this slide:Charge is conserved; Charge is quantized (+1, +2, etc. -1, -2, etc.)
“Conservation of Charge” … akin to “conservation of energy”
Clicker Question—Conservation of charge
Only one of these hypothetical events is allowed by the law of “conservation of charge.” Which one?
A) Two electrons (-1 charge each) combine to form a neutron (0 charge)
B) Two protons (+1 charge each) annihilate with two neutrons to form four gamma ray photons (0 charge)
C) An electron and a positron (+1 charge) annihilate to form two gamma rays (0 charge)
Clicker Question—Conservation of charge
Only one of these hypothetical events is allowed by the law of “conservation of charge.” Which one?
A) Two electrons (-1 charge each) combine to form a neutron (0 charge)
B) Two protons (+1 charge each) annihilate with two neutrons to form four gamma ray photons (0 charge)
C) An electron and a positron (+1 charge) annihilate to form two gamma rays (0 charge) This actually happens!
OK, one more thing about charge
Charged particles move in curved pathswhen in a magnetic fieldNeutral particles move in straight paths
Path of a charged particle
“B” is magnetic field
Summary of charge characteristics
Particles in the universe can have positive (+), negative (-) or neutral (zero) charge
Opposite charges attract each other Same charges repel each other
Force Charge1 * Charge2
distance2
“is proportional to”
Charge is conservedCharge is quantized (+1, +2, etc. -1, -2, etc.)
Charged particles move in curved pathswhen in a magnetic fieldNeutral particles move in straight paths
Path of a charged particle
Refresher on the atom
Atoms are composed of electrons (-1), protons (+1) and neutrons (zero)
The type of element is determined by the number of protons.
Two atoms with same number of protons, but with differing number of neutrons are different isotopes of the same element
Helium Atom
Proton
Neutron
Electron “Cloud”
Refresher on the atom
Because the electron “clouds” surround the nucleus, most interactions are governed by the physics of electron-electron interactionsi.e. the electrical force … “Electronic reactions” = Chemistry
The electrons have been involved in most of what we’ve been talking about: atomic spectra; fluorescence; interaction of matter and electromagnetic waves
When an atom loses or gains an electron, it is still the same element!(It is called an ion)
Helium Atom
Proton
Neutron
Electron “Cloud”
X-rays discovered in 1895, Wilhelm Röntgen
Cathode Ray Tube (CRT) Röntgen noticed fluorescence across the room (through books and papers)
While experimenting with putting objects in the unknown beam, he saw images of his (wife’s) bones appear!
Shortly thereafter, radioactivity discovered by Henri Becquerel (1896) (1903 Nobel prize with the Curies)
Uranium Salt
Photographic film
Expose to sunlight
Cloudy day /Dark drawer for a couple days
Much, much larger effect!
The uranium itself was emitting radiation!
This sparked the exciting ageof nuclear physics, led by Rutherford
Clicker question—Invention of Radioactivity
When did Physicists invent radioactivity?A) ~1900B) World War IC) World War IID) None of the above
Clicker question—Invention of Radioactivity
When did Physicists invent radioactivity?A) ~1900B) World War IC) World War IID) None of the above
Radioactivity has existed since the infancy of the universe!
For example, the uranium that Roentgen used was naturally occurring.Uranium is about as abundant as arsenic.
Molecules in the atmosphere are radioactive.
*** The interior of the Earth is heated by radioactive decay!
Three kinds of radiation emitted via radioactive decaynamed before they were understood…alpha, beta, gamma
Alpha particles stopped by paperalpha particles are helium nuclei
Beta particles stopped by aluminumbeta particles are electrons (or positrons)
Gamma rays stopped by leadgamma rays are high energy photonsx-rays similar, but lower energy than gamma
Let’s say by examination, physicists and chemists discovered that one type of the radiation was not deflected by magnetic fields. Which of the following would be true?
A) The un-deflected radiation must be charged.B) The un-deflected radiation must be uncharged.
Clicker question…charge of radiation
Let’s say by examination, physicists and chemists discovered that one type of the radiation was not deflected by magnetic fields. Which of the following would be true?
A) The un-deflected radiation must be charged.B) The un-deflected radiation must be uncharged.
Charged particles travel in curved paths when in magnetic fields!
Path of a charged particle
Alpha, beta and gamma radiation also distinguished by charge behavior
Alpha particles deflected by magnet
Gamma rays unperturbed by magnet
Beta particles deflected opposite alpha
It was noticed early on that beta particles behaved exactly like cathode rays (electrons)Alpha particles were discovered to be helium nucleiGamma rays were very high energy x-rays (photons)