IB Physics

Quantum Theory and Nuclear Physics.

LNK2LRN™ 2015/16

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

"photoelectrons".

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

Principle.

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

model.

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

energy.

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.

View Quantum Theory Slides.

View Nuclear and Atomic Slides.

USEFUL LINKS AND WEBSITES TO VISIT:   

Photoelectric Effect

The Bohr Atom

Particle Accelerators

Radioactive Elements

 And Always Remember... 

"From  Newtonian Mechanics,

 Through Quantum Theory,  

Without Knowledge of Physics,  

Life Would Be Dreary."

 

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