AP Physics B – Ody“NOTES”ki
MODERN PHYSICS
MODERN PHYSICS
1) Dual Nature of Light
a) Wave Nature – Interference, Polarization & Diffraction support it
b) Particle Nature – Reflection, Refraction & Photoelectric Effect support it
c) Photoelectric Effect – The emission of electrons when a material is exposed to EM Radiation
i) Intensity of light controls the quantity of photoelectrons released
(1) More photons = High Intensity
ii) The frequency of the light controls the energy of the photoelectrons
(1) More energy = Higher frequency
iii) Photoelectric Equation
(1)
(a)
(b)
(c)
(i)
1.
a. Minimum Frequency to eject photoelectrons
iv) Stopping Potential (Voltage)
(1)
(a)
(b)
-
2) Quantum Theory
a) Light travels in energy packets called photons
b) Energy of a Photon
i)
c) Momentum of a Photon
i)
d) De Broglie’s Wavelength – Matter Waves
i) Stemming from the Wave-Particle Duality of Light, all objects emit a certain wavelength
ii)
e) The Compton Effect
i) When a photon collides with an electron, knocking it off its atom; the
scattered photon has less energy and a longer wavelength. This is
due to the Law of Conservation of Momentum since the electron
gains some of the energy from the photon.
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3) Models of the Atom
a) Plum Pudding Model
i) In the 1890’s scientists generally believed that an atom consisted of a positively charged material
(pudding/Jell-O) that contained negatively charged electrons (plums/fruit) within)
b) Rutherford’s Model
i) Experiment – Gold foil being bombarded by alpha (He atoms with a +2e charge)
particles.
ii) Most of the alpha particles passed through the foil as if nothing was there;
which disproves the Plum Pudding Model.
iii) Some alpha particles were deflected at large angles, sometimes even
backwards. This also provided evidence that the atom must have a dense,
positively charged nucleus.
iv) Rutherford’s Model of the atom consisted of a nucleus with an electron orbiting
around it, similar to that of a miniature solar system.
c) Bohr’s Model
i) A continuation of Rutherford’s model but further postulated that the electron only moves within certain
orbits without emitting energy.
ii) The closest orbital an electron can have is called the ground state (n=1).
iii) For EM radiation to be emitted the electron jumps from a higher orbital to a lower orbital giving off the
difference in energy.
(1)
iv) Conversely, for radiation to be absorbed, it must be the correct value to jump to a save orbital with no extra
energy left over.
d) Cloud Model – Current Accepted Model
i) There are no specific orbits
ii) Each electron occupies a state or a region where the probability of finding the electron is greatest
iii) Each state has a maximum of 2 electrons
NUCLEAR PHYSICS
1) The Nucleus
a) Nucleons – Particles found in the nucleus (Protons and Neutrons)
b) Atomic Number (Z) – Number of protons found in the element
c) Mass Number(A) – Number of protons & neutrons
i)
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d) Nuclear Force – The force that holds the nucleus together
i) Considered to be extremely strong but affective on a very small range
e) Binding Energy – Energy equal to the amount of mass defect
i) The particles in the nucleus are less massive than their parts outside the nucleus, hence the difference in
mass is converted to energy to hold the nucleus together
ii)
f)
Atomic Mass Unit (AMU)
i)
ii)
iii)
iv)
g) Nuclides – A Single nuclear species
h) Isotopes – Same Atomic Number but different Mass Number
2) Nuclear Reactions
a) Alpha Decay
i) A Helium Nucleus with a +2 charge is emitted from the nucleus of a different element
ii)
b) Beta Decay
i) An electron with a –1 charge is emitted from a nucleus because a neutron changes into a proton
ii)
-
(1)
-
iii) Some elements have a surplus of protons in comparison to neutrons and emit a POSITRON
(1) A positron has the same mass as an electron but has a positive charge
(2)
c) Gamma Radiation
i) EM waves emitted by the nucleus
ii) No mass
iii) No Charge
iv) Very short wavelength
v) High Energy
d) Artificial Transmutation
i) Transmutations occur due to the gain of protons or neutrons
(1)
ii) Some results could be radioactive
(1)
(Positron Emission)
(2)
-
(Beta Emission)
iii) Electron Capture
(1) A nucleus may capture an electron, changing a proton into a neutron
(2)
-
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iv) Neutron Emission
(1)
(a)
3) Nuclear Fission
a) The splitting of an atom is usually done by neutron capture
b) The mass on the left side of the equation is always greater than the mass on the right
i) The “missing” mass is converted to energy
c) Nuclear Fuels consist of Uranium-235 and Plutonium-239
d)
e) Nuclear Reactor
4) Nuclear Fusion
a) Two light elements combine to produce heaver elements
b) The mass on the left side is greater than the mass on the right
i) The “missing” mass is converted to energy
c)
i)
ii)
iii)
d)
i)
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