Modern Physics

1. Modern Physics NCEA AS 3.5 Text Chapters:20,22

2. The Photoelectric Effect • The photoelectric effect occurs when shining light (usually UV) onto a piece of metal causes electrons to be given off. • This effect can be used in a photoelectric cell to produce small electric currents. • Photoelectric cells are used in • Lightmeters • Burglar alarms • TV cameras etc

3. LIGHT Thin glass tube (evacuated) Emitter – curved metal plate Collector A Photoelectric Cells

4. Photoelectric Experiments • When the photoelectric effect was studied in detail, the experimental results were very different to what was expected. A new theory about the nature of light was needed to explain what happened. • Scientists at the time considered light to behave like a wave……

5. Photoelectric Experiments • What was expected: • Brighter light would cause electrons with more kinetic energy to be emitted • What actually happened: • Brighter light caused more electrons to be emitted, but there was no change in the amount of energy they had

6. Photoelectric Experiments • What was expected: • If very dim light was used, it would take some time before any electrons had absorbed enough energy to escape from the metal • What actually happened: • With UV light, even the faintest light caused some electrons to be emitted instantly

7. Photoelectric Experiments • What was expected: • The frequency (or colour) of the light used would not affect the energy of the emitted electrons. • What actually happened: • The higher the frequency, the higher the energy of the electrons. Below a certain frequency, no electrons were emitted.

8. Photoelectric Experiments • Einstein explained these results, using an idea suggested by Max Planck, that said electromagnetic radiation comes in fixed “packets” or quanta of energy called photons • The amount of energy each photon has depends on the frequency of the radiation.

9. Photoelectric Experiments • Each photon has a fixed amount of energy given by: • h=Planck’s Constant = 6.63x10-34Js • This suggested that light behaved like a moving particle, rather than a wave

10. LIGHT V Photoelectric Experiments • The power supply provides an opposing voltage to the p.e. cell. • The variable voltage is adjusted until the current in the circuit is zero

11. Photoelectric Experiments • When the current was zero, the supply voltage was equal to the cut-off voltage of the cell • Different frequencies of light were tried, and the cut-off voltages measured:

12. V f f0 Photoelectric Experiments • These were the results: • Below a certain threshold frequency f0 no electrons were emitted

13. Photoelectric Experiments • The maximum Ek of the electrons can be found from the voltage: • Where e= electron charge = -1.6x10-19

14. Ek Gradient = h f f0 Intercept= Work function Φ (or B) Photoelectric Experiments • Another way of looking at that last graph:

15. Photoelectric Experiments • By equating to y=mx+c: • Ek = max kinetic energy of emitted electrons • hf = energy of incoming photons • Φ = The work function of the metal – the minimum amount of energy required for the electron to escape from the metal surface.

16. Ek Pb Cu f f0 f0 Photoelectric Experiments • Different metals have different f0’s and work functions depending on how tightly they hold onto their electrons

17. The Conclusion • So the photoelectric effect could be explained by thinking of light as a stream of incoming particles that collided with electrons in the metal. If the photon had enough energy, it could knock the electron free of the metal and send it across the cell to the collector. • If photon was too small, it couldn’t hit electrons hard enough (overcome work function) so no electrons emitted.

18. Atomic Spectra • 2 types • Emission – certain frequencies of light given off by low pressure gases excited by heat or electricity • Absorption – certain frequencies absorbed from a continuous spectrum by low pressure gases • Spectra are unique to each element and can be used to identify unknown elements

19. The Hydrogen Spectrum • Balmer studied the emission spectrum lines of Hydrogen, as it is the simplest atom. • He was limited by the fact that he could only observe visible frequencies – we now know there are UV and IR spectral lines • The spectral lines are caused by the movement of electrons between different energy shells in the atom

20. The Hydrogen Spectrum • In Balmer’s case he was looking at spectral lines caused by electrons jumping from higher energy level (shells) down into the 2nd shell. • They would release their extra energy as a photon of light. • Other Scientists later found series of spectral lines corresponding to jumps into the 1st, 3rd, 4th, 5th etc

21. Paschen Series S=3 (IR) Bracket Series S=4 (IR) Balmer Series S=2 (visible) Pfund Series S=5 (IR) Lyman Series S=1 (UV) Nucleus ∞ 1 2 4 3 5 Shell no. / Energy level The Hydrogen Spectrum

22. The Hydrogen Spectrum • A formula was worked out to calculate the wavelengths of these lines: • R=Rydberg’s Constant=1.097x10-7 • S=Series no. (the shell jumped into) • L=Line no. (the shell jumped from)

23. The Hydrogen Spectrum • The formula worked perfectly for Hydrogen, but started to get more inaccurate the bigger and more complex the atom got • Absorption spectra are produced by electrons absorbing photons of energy which allows them to jump up energy levels

24. Bohr’s Model of the Atom • Rutherford’s student Niels Bohr proposed that: • Electrons in H could only exist in stable orbits with certain fixed amounts of energy, called energy levels • An electron moves from one energy level to another by either emitting or absorbing a photon of light equal in energy to the difference between the two energy levels

25. Bohr’s Model of the Atom • The energy levels in the Hydrogen atom are given by : • h=Plancks constant = 6.63x10-34 • c=speed of light = 3x108 • R=Rydbergs constant = 1.097x107 • n=energy level = 1,2,3,4…… (quantum number)

26. Bohr’s Model of the Atom • All energy values are negative – this represents the fact that it is an energy which binds the electron to the nucleus • The lowest energy state n=1 is called the ground state • As n∞, E0. This represents the energy required to ionise the atom by removing the electron completely.

27. Bohr’s Model of the Atom n=∞ 0 n=4 n=3 Energy (x10-18J) n=2 -1 -2 n=1

28. Electron Volts • Sometimes an alternative unit for energy is used called the electron volt • 1eV is the energy gained by 1 electron when accelerated by a potential of 1 Volt • 1eV=1.6x10-19J • Using this unit:

29. Nuclear Reactions 3 types: Radioactive Decay – the spontaneous emission of particles from the nucleus of an atom Nuclear Fission – splitting one large nuclei into two smaller ones Nuclear Fusion – combining two small nuclei into one large one.

30. Radioactivity • 3 types: • Alpha a • Beta b • Gammag • Named in order of their discovery. • Alpha and beta decay don’t usually occur by themselves, there is usually some gamma that occurs with them.

31. The Nucleus • In small atoms, the number of protons and neutrons are usually the same (roughly) • In larger atoms, there are usually many more neutrons than protons, in order to keep the nucleus stable. • If a nucleus is unstable, it may spontaneously decay to something more stable by emitting alpha, beta or gamma radiation

32. Alpha Particles • Helium nucleus • Charge of +2 • Mass of 4 (a.m.u) • Travel slowly ie. 10% of light speed • Don’t travel very far ie. A few cms in air • Low penetration power – can be stopped by a piece of paper • Very good ionising power – because they’re big and slow.

33. Beta Particles • An electron from the nucleus • Charge of -1 • Same mass as an electron (effectively 0) • Travel relatively fast – up to 95% of light speed • Travel about 30 cms in air • Average penetration power – can be stopped by a few mm of Aluminium • Average ionising power

34. Gamma Radiation • A wave of electromagnetic radiation (energy) • No charge • No mass • Travels at light speed • Travels several metres in air • High penetration power – Several cms of lead needed to stop it • Low ionising power – because no mass

35. b g a Radiation • One way that the different types of radiation can be distinguished is by observing their behaviour in a magnetic field:

36. The Nucleus • Writing nuclei X = element symbol A = mass number or nucleon number (the number of p+n) Z = atomic number (the number of protons)

37. Isotopes • Atoms with the same atomic number but different mass numbers • Eg:

38. Alpha Decay • Example: Radium 226 decays to Radon 222 by alpha decay: • Note: Both mass and charge must be conserved (ie 226=222+4, 88=86+2

39. Beta Decay • Cobalt 60 decays by beta decay to Nickel 60 • Again, mass and charge are conserved • NB. the a or b symbols can be used instead of He or e

40. Half-life • The time it take for the decay rate to have halved, or…. • The time taken for half of the original atoms to have decayed • Usually shown on a graph

41. Half-life

42. Geiger-Muller tube filled with low pressure Ar +Cathode: metal cylinder 400V DC Supply Counter or speaker End: thin mica window - Anode: central wire Detecting Radioactivity • Geiger Counter – detects electrical current caused by the ionisation of atoms in a gas

43. Uses of Radioactivity • Radiation therapy to treat cancer • Sterilisation • Carbon dating • Nuclear medicine eg tracers • Smoke detectors

44. Binding Energy • If we put together a nucleus from individual protons and neutrons, we would find that the mass of the resulting nucleus is less than the total mass of the individual nucleons. • This reduction in mass is called a mass deficit

45. Binding Energy • In order to break up a nucleus into separate nucleons the mass deficit must be restored by adding extra energy. • This energy changes into mass according to Einstein’s famous equation:

46. Binding Energy • This energy shortage has the effect of holding the nucleus together so it is called the binding energy. • Binding energy represents the amount of “glue” holding the nucleus together. • The more binding energy per nucleon, the more stable an atom will be

47. B.E per nucleon (MeV) 56Fe 8 238U 4He 6 7Li 4 2 Fusion Fission Mass number 200 100 150 50 Binding Energy

48. Nuclear Fission • Breaking large unstable nuclei into smaller ones. • Lots of possible combinations of fragments from one initial nucleus • Eg:

49. Nuclear Fission • When a large nucleus is split into smaller fragments, the fragments have less mass per nucleon • The lost mass is released as energy in the form of kinetic energy of neutrons and gamma rays

50. U U U U n n n n n n n n n n n n n Ba Ba Ba Ba Kr Kr Kr Kr Nuclear Fission • Only one neutron is needed to start the reaction, but several are produced • This starts a “chain reaction”