WEBSITE NOTES: Quantum Theory and Nuclear Physics.
1. The photoelectric effect involves electrons acquiring energy by absorbing
particles of light (photons). These energetic electrons are then termed
2. The "200-Year debate" (Light. What is it? Waves or particles?) was revived
when it was observed that a metal plate emitted electrons when struck with
light from a spark discharge.
3. The photoelectric effect contradicts classical physics, but can be explained
assuming that energy is quantized, or occurs in discrete bundles or units.
4. In the visible range, the frequency of light determines its color. Red light
is at the lower end of the frequency scale, and violet is at the upper.
5. The energy of a photon, or light quantum, depends on the frequency of the
light, E = hf . The frequency, f, is related to the wavelength, λ , through the
wave equation for light, c = fλ .
6. Max Planck (1858-1947), determined the value of h, which came to be known
as Planck's Constant, h = 6.6x10-34 Js.
7. Heinrich Hertz (1857-1894) experimented with electromagnetic waves and
confirmed the work of the theoretical physicist, James Maxwell (1831-1879)
who postulated their existence. The unit of frequency is the Hertz (Hz).
8. The minimum energy required for an electron to escape from a metal depends
on the threshold frequency of the metal. This is known as the work function
which is Wo = hfo , with fo, this time, being threshold frequency.
9. The maximum kinetic energy of photoelectrons is then found by using the
equation KEmax = E - Wo = hf - hfo . Another way to determine is by using a
"stopping potential", KEmax = qVo . The unit that we use for this energy is the
electron-Volt, or eV, with 1 eV = 1.6x10-19 J.
10. Light now has a dual nature, wave and particle, but each particle has no
mass. We call these mass-less particles, photons.
11. The wave equation for light still applies, c = f·λ which is the velocity of a
wave equals the product of its frequency and wavelength, with, as we should
already know, c = 3.0x108 m/s , the speed of light.
12. Ernest Rutherford (1871-1937) performed the famous scattering experiment
(Gold-Foil) revealing that all of atom's positive charge and almost all of the
mass is at the center, or nucleus. Most of the atom is empty space.
13. Robert Millikan (1868-1953) determined the charge on the electron with his
famous "Oil-Drop" Experiment. He found that charge, q, is quantized, or only
occurs in multiples of the elementary unit of charge, 1.6x10-19 C. His equation
was qE = mg.
14. Sir Joseph J. Thomson (1856-1940) found the mass-to-charge ratio for the
electron by experimenting with the CRT. His equations were Bqv = mv2/r and
qE = Bqv , which produced v = E/B and m/q = Br/v .
15. Since each gas has a unique emission and absorption spectrum, the Danish
Physicist, Niels Bohr (1885-1962) proposed a Quantum-Mechanical Atomic
Model instead of Rutherford's Planetary Model.
16. He proposed that electrons can move from one energy level to another by
absorbing or emitting photons. His equations were rn = 5.2x10-11 m x n2 ,
and En = -13.6 eV x 1/n2 , with n = 1,2,3,... , the energy level. As stated
above, the electron-volt (eV) is the energy unit for electrons, with,
1 eV = 1.6x10-19 J .
17. Count Louis Victor de Broglie (1892-1987) postulated that, "if waves can
have particle properties, why can't particles have wave properties?" His
equation was λ = h/mv for the wavelength of a matter particle.
18. Albert Einstein (1879-1955) explained this, as well as the photoelectric
effect, to get the Nobel Prize in 1921 for his work in 1905. This is why,
100 years later, 2005 was the "World Year of Physics". His famous equation
is E = mc2 .
19. Werner Heisenberg (1901-1976) determined that it is not possible to
know the exact position and momentum of the electron, the Uncertainty
20. Arthur Compton (1892-1962) bombarded a graphite block with X-rays
demonstrating the momentum of photons (The Compton Effect ). The
equation is mv = p = h/λ .
21. James Chadwick (1891-1974) an original member of Rutherford's research
team proved the existence of neutrons in 1932.
22. Light Amplification by Stimulated Emission of Radiation (LASER), which
was explained by Einstein in 1917, was invented in 1960. Laser light is very
directional, powerful, monochromatic, and coherent, making it very useful.
23. Early atomic models were: (i) single indivisible particle, (ii) "plum pudding"
model, (iii) planetary model. Today we have model (iv) the planetary-quantum
24. Henri Becquerel (1852-1908) accidentally found that all compounds
containing uranium emitted rays that penetrate and fog photographic plates,
after examining a mysterious rock.
25. Ernest Rutherford (1871-1937) identified alpha, beta, and gamma radiation
and used alpha particles to bombard gold foil. He found that most of an atom
is empty space but contains a massive positively charged nucleus.
26. The Curies, Pierre and Marie, were the first to discover other radioactive
elements, for example, Polonium and Radium.
27. Atoms having the same number of protons but different amounts of
neutrons are called isotopes.
28. The nucleus of an atom contains most of the mass, consists of protons
and neutrons, and number of protons is the atomic number.
29. The nucleus can be characterized by a mass number, A, an atomic number,
Z, and a neutron number, N, with A = Z + N.
30. The change, transmutation, in an atomic nucleus can be natural or artificial.
Enrico Fermi (1901-1954) successfully produced artificially radioactive elements
in the laboratory.
31. Radioactive decay produces three kinds of particles: alpha, helium nuclei;
beta, high-speed electrons; and gamma ray photons.
32. Bombardment of nuclei by protons, neutrons, alpha particles, electrons,
gamma rays, or other nuclei can produce a nuclear reaction.
33. Linear accelerators, synchrotrons, and super-colliders produce high-energy
protons and electrons which can collide with each other or an atomic nucleus.
34. Particle detectors include photographic plates, the Geiger-Muller tube,
scintillation screens, and the cloud chamber.
35. Alpha can be stopped by thick paper, beta by thick aluminum foil, and a
few centimeters of lead will stop gamma.
36. During positron decay a proton changes into a neutron with the emission
of a positron and a neutrino.
37. When matter and antimatter combine, all matter is converted into energy,
or lighter matter-antimatter particle pairs. By pair production, energy is
converted into a matter-antimatter particle pair.
38. The weak interaction operates in beta decay while the strong force binds
the nucleus together. During beta decay a neutron changes into a proton and
the nucleus emits a beta particle and a mass-less antineutrino.
39. The binding energy is the energy equivalent of the mass defect. The
assembled nucleus has less mass than its constituent parts due to mass-to-
energy conversion, E = mc2 .
40. Nuclear reactors use the energy released in fission as heat to boil water,
which produces steam, that turns turbine blades to run a generator.
41. The binding energy of the nucleus is the difference in energy between its
nucleons when bound and its nucleons when unbound. Energy-mass equivalent
can be computed using 1 amu = 931 MeV.
42. The half-life is the time required for half the original nuclei of a radioactive
substance to undergo radioactive decay.
43. The decay constant, lambda, indicates the rate of radioactive decay. Half-
life can be calculated by dividing .693 by the decay constant, λ, lambda.
44. Nuclear reactions involve a change in the nucleus and can be given by
equations. In equations for nuclear reactions, subscripts and superscripts must
agree on both sides.
45. In a nuclear equation the sums of the subscripts (atomic number or particle
charge) on both sides of the equation are equal and the sums of the
superscripts (mass number) on both sides of the equation are equal.
46. In fission, heavier nuclei split to form lighter nuclei and energy is released.
In fusion, lighter nuclei combine to form heavier nuclei with more binding
47. And still, we need these steps to solve any problem in Physics:
(i) read the problem and identify the given variables
(ii) determine what you are asked to solve for
(iii) find the correct equation to use
(iv) use Algebra, Trigonometry, and/or Calculus to isolate the unknown.
(v) substitute-in your known values and solve.