Review Topics

In phase / out of phase

Compressions and rarefactions

MODERN PHYSICS: III

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Light atoms tend to combine and release energy as they do so.

Heavy atoms tend to split and release energy as they do so. Uranium and Plutonium are particularly useful in this regard, and are the basis of nuclear fission.

Heavy nuclei break into lighter nuclei and energy is released.

Light nuclei fuse into heavy nuclei and energy is released.

Three basic types of radioactive decay

•Alpha decay – release of an alpha particle (helium-4 nucleus) from a nucleus

•One way for an atom to move from a heavier to a lighter atom to become more stable

•Beta decay – release of a beta particle (electron) •For a free neutron, decay into proton + electron + neutrino•For a nucleus, conversion of a neutron to a proton

•Gamma decay – release of a high energy wave (photon) •From change in energy inside a nucleus•From self-annihilation of a particle and its antiparticle

Three basic types of radioactive decay•Alpha decay – release of an alpha particle (helium-4 nucleus) from a nucleus

•One way for an atom to move from a heavier to a lighter atom to become more stable•Write an equation showing the change

• What is conserved?

•Beta decay – release of a beta particle (electron) •For a free neutron, decay into proton + electron + neutrino•For a nucleus, conversion of a neutron to a proton•Write an equation showing the change for each case

• What is conserved?

•Gamma decay – release of a high energy wave (photon) •From change in energy inside a nucleus•From self-annihilation of a particle and its antiparticle•Write an equation showing the change for the p(anti-p) case

• What is conserved?

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Let’s look at a neutron sitting by itself in space. After ~15 minutes, the neutron has a 50% probability of decaying to a proton plus an electron:

Neutrino

No charge

Momentum

Very tiny mass

v ~ c

Note that:

1)Charge is conserved

2)Mass + energy is (almost) conserved3)What is wrong with this picture?

4)With the addition of the neutrino, momentum is conserved.

Decay products

Neutrons look like they have internal “stuff” rather than just being a simple round blob…

6 Quarks6 leptons (electron, 3 neutrinos, two others)

Hadrons: Baryons (3 quarks) and Mesons (2)

Plus their antiparticles

Four Fundamental forces

Strong Force (gluons)Weak force (weird particles)Electromagnetic force (photons)Gravity (gravitons)

- They both have mass- They have opposite sign- If they meet, they self-annihilate and release energy

MODERN PHYSICS: III

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p+n

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Note that charge is unitary (+1, 0, -1) outside the nucleon and fractional (+/- 1/3 or +/- 2/3) inside it. Charge is quantized.

Light (not heavy)

These don’t live long

Heavy

Here there be nucleons

Which Fundamental Interaction/Force is responsible for:

• Friction?– Electromagnetic.

• Nuclear Bonding?– Residual Strong Nuclear.

• Orbiting Planets?– Gravity.

• Which force carriers have not been observed?– Gravitons (Gluons have been observed indirectly)

Wave Theory of Light• Christian Huygens (1629 – 1695): Light travels in wavelets• Huygen's Wavelets

Corpuscle Theory of Light: Sir Issac Newton (1642 –1727)

• Newton believed that bodies emitted energy in particles or corpuscles that traveled in straight lines.

• 1666: Performed an experiment with a prism that showed that the sun’s light is white light consisting of all of the colors of the spectrum.

Wave Theory of Light: Thomas Young (1773 – 1829)-revisited

• 1801: Through use of the Double-Slit Experiment, the wave properties of light were first experimentally shown to exist.

• Experiment demonstrated that light undergoes interference and diffraction in much the same way that water and sound waves do.

• Used source of monochromatic light to eliminate the problems with phase differences associated with incoherent light.

Young Double-Slit Experiment

Huygen’s Wavelets

www.src.wits.ac.za

Wave Theory of Light: James Clerk Maxwell (1831 – 1879)

• 1860: James Maxwell hypothesized that electric fields changing in time would create magnetic fields and vice-versa.

• These fields travel together in space as waves.• Electromagnetic Wave

www.hyperphysics.com

Let’s talk about photons….

So if light IS a wave, and if light strikes a surface, how will it impart energy?

- Based on its intensity (amplitude)- The energy will be gradually absorbed as the whole

material “heats up”

What actually happens is different

•Electrons are ejected from a metal surface as soon as certain frequencies of light strike it

• Frequencies of light lower than a threshold frequency will not eject electrons, regardless of the intensity

•The ejected electrons have an upper bound of energy

•All of this ends up looking suspiciously like light is striking the metal in discrete packets, not a diffuse wave

•Einstein got the Nobel prize for figuring out how this works•(I have heard it said that he really got it for Relativity, but no one could figure out how it worked so they gave it to him for something people could understand)

Photons

•Massless particles that carry no charge•They carry energy•They have momentum

• The energy of a photon is proportional to its frequency

•The energy of a beam of light is given by the intensity of the beam times the energy of the photons in the beam

• Photon energy E = hf = hc/

How do we get photons?

•In most (all?) cases they are created by a change in the energy of some piece of matter

•One source is the decay of atomic particles

•Another source is the annihilation of a particle with its antiparticle

•A typical source of light is the change in the energy of an electron in orbit around an atom

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We like to think of electrons as being in pretty “orbits”. We like to think of electrons as particles, but they also act like waves and spend part of the time inside the nucleus!

Electrons act like waves that move in specific resonant frequencies. There is a “fundamental state” (ground state) that is close to the atom.

As energy is added to the electron, it is added in discrete “chunks” – too little energy cannot be absorbed, too much energy and some of it goes into increasing the “harmonic” of the electron and some is thrown away.

Energy is quantized at the atomic level.