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 • Light meters • 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 • But what actually happened?

6. Photoelectric Experiments • Brighter light caused more electrons to be emitted, but there was no change in the amount of energy they had. • 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

7. Photoelectric Experiments • What actually happened: • With UV light, even the faintest light caused some electrons to be emitted instantly • What was expected: • The frequency (or colour) of the light used would not affect the energy of the emitted electrons.

8. Photoelectric Experiments • What actually happened: • The higher the frequency, the higher the energy of the electrons. Below a certain frequency, no electrons were emitted.

9. What did you take in?Try and answer these without looking back at yesterdays notes • What is a photo electron? • Which theory of light does the photo electric effect support? • What quantity dictates the amount of energy a photo electron contains? • What is an electron-volt? • Does dull light produce photo-electrons? What happens when the same light is brightened up? All type 1 questions

10. Usually 2 achieved type 1 ticks are given away per paper for knowing the correct units and significant figures Imagine being called Plank? • What are the units for magnetic flux? • ………………………………………… impulse? • ………………………………………… rotational inertia? • 3.00 x 108 how many s.f.? • 0.003450 how many s.f.? Some of you could have passed with 2 extra type 1 marks so get into the habit of using the correct units and s.f.s

11. 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.

12. 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

13. 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

14. 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:

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

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

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

18. 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.

19. 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

20. 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.

21. Type 1 training – Part 2It is important that you are able to explain what relatively straight forward words and concepts What do the following mean? (don’t yell out – write it down) • Capacitor An electrical device that stores charge • Transverse wave A wave where the particles move at right angles to the wave direction • Angular Momentum is conserved The angular momentum of a system will remain the same provided there is no net external torque.

22. Some More • Electric field A region of space in which a charged particle experiences a force. • Resonance When the frequency of a forced motion like SHM equals a natural frequency, resulting in a large amplitude. • Doppler Effect A change in observed frequency caused by relative motion between the observor and source of the waves.

23. Task to do at home. Make a spelling list of physicswords, and terms. Then see how many you can write a proper definition for. Check the answers with the glossary in the back of your text. Do it tonight and again a few times before the 20th November. Eg. Simple Harmonic Motion, standing waves, path difference ……………..

24. More energy is required to remove an electron from iron than from calcium. Which of the metals has a higher work function? Which of the metals has a higher threshold frequency? Iron Iron

25. A beam of 640nm red light shines onto a • polished copper disc. • Calculate the energy of each photon • 2.Write this energy in eV • The work function of copper is 1.28x10-18J • 3.Figure out whether an electron will be emitted • 4.What about 160 nm light? • 5.What is 160 nm Light? E=hf=hc/=3.11x10-19J 1eV=1.6x10-19J  1.9eV That’s 12.8 x 10-19J. 3.11x10-19J is about ¼ of what's needed NO 12.4x10-19J. Still not quiet enough UV

26. Atomic Spectra • The spectral lines are caused by the movement of electrons between different energy shells in the atom

27. 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

28. Type 1 training – Part 3You have to be able to draw fully label accurate diagrams • Draw a force diagram of an aeroplane (1500kg) banked at 450. Show components of the lift force. • Draw a labeled phasor diagram and graph for a SHM. Y=-3.0sin0.31t • Add the phasors for velocity and acceleration • Draw a diagram of an ambo’s siren sound waves as it’s speeds along the street.

29. Another One • A laser is shone through a double slit onto a screen. Then the double slit is replaced with a diffraction grating of many slits. Draw two labeled diagrams to show the different patterns produced.

30. Type 1 training – Last Part The last and most common type 1 question is figuring out and explaining a physics situation. Discuss what happens when- (don’t yell out – write it down) • Two cars of very similar mass and speed hit head on. • Current is being passed through an LED incorrectly. • A flat stone hits the surface of a smooth lake at a very small angle to the water, traveling reasonably fast

31. A Few More • A spring has a very large spring constant • Your radio when in behind a hill only receives an AM signal and not FM

32. Try This The diagram above shows the mode of vibration of the string. Describe the type of wave formed on the guitar string.

33. A Humdinger • The frequency of the note produced by a guitar string is usually altered by changing the length of the string that vibrates. State two other factors that can alter the frequency of notes produced by guitar strings. Explain how they affect the frequency

36. A first order bright fringe is seen at a point on a screen. State the type of interference Constructive Explain, in terms of path difference, how the first order bright fringe is formed. Diffracted waves from the grating meet at the point P in phase. Since P is the first order bright fringethe waves meeting at this pointhave path difference of one wavelength. And that was a type 1 merit !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

37. 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

38. 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

39. Quick verbal recapConcentrate and answer theseMaking an educated guess is better than “ I dunno” • So what is an atomic absorption spectra? • And an emission spectra? • They are useful because…………………………… • Which spectra were discovered first? • Because…………………………………………… • What level do the elctrons end up at for the visible spectra? • What is this series called?

40. 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

41. 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)

42. 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

43. 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

44. 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)

45. 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.

46. Recap What energy level transition doesan electron make to produce a visible emission? Higher level2 Why do electron transitions to level 1 produce UV? It’s the largest energyjump and UV in a largeramount of energy than visible or IR What were Bohrs 2 assumptions about the H atom Nip back 3 slides Use Bohrs Energy level formula to work the energy of the first level.

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

48. 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:

49. Balancing Nuclear Equations • The atomic numbers are conserved. That means they have to add up to the same number on both sides of the equation. • The same applies to the atomic masses. ? Note: Both mass and charge must be conserved

50. Try this (ie 226=222+4, 88=86+2)