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Flashcards

Chapter 7: Quantum Behaviour

Matt Clancy Physics (AS) AS - Level Inglês 122 flashcards

Flashcards do conjunto (122)

  1. 17.1; QUANTUM BEHAVIOUR.
  2. 2What 3 things can you explain by modelling light as a wave?-Refraction
    -Diffraction
    -Interference.
  3. 3What can this model not explain however?Some other phenomena.
  4. 4What happens when you take a photograph in progressively dimmer light? What does this suggest?Photograph breaks up into randomly arranged exposed patches on the film/
    Suggesting light is arriving randomly in small packets.
  5. 5What happens when you turn up the brightness?The packets of light arrive at a greater rate, produces smooth-looking picture
  6. 6What are these packets, or quanta of light called?Photons.
  7. 7So what does a dimmer photograph mean there are?Fewer photons.
  8. 8So what is the case where the picture is: -bright? -dim?-Bright: Good chance of photons arriving at that location.
    -Dim: Lower chance.
  9. 9What are these however?Probabilities, not certainties.
  10. 10When is the random nature of the arrival of photons much more obvious?When the rate of arrival is low.
  11. 11What did Planck suggest?The idea that electromagnetic radiation can be emitted in energy quanta (discrete packets of energy).
  12. 12How did Einstein extend this concept?Electromagnetic radiation is not only emitted in energy quanta, but is also absorbed as quanta (photons).
  13. 13What happens to electromagnetic radiation of frequency f? What is this relationship?It is emitted and absorbed in quanta (of energy E) where:
    Energy E (J) = Planck Constant h (Js) x Frequency f (Hz).
    E=hf
  14. 14Since the speed of light c=fλ, what can we write?Can write f=c/λ, and substitute this into the photon-energy equation to give:
    E = hc/λ
  15. 15Why is the Joule not a convenient energy unit to use when discussing the energy of photons or sub-atomic particles?Because the energy values considered are so small.
  16. 16What is often used as an alternative unit?The electronvolt (eV)
  17. 17What is one electronvolt?The energy transferred when an electron moves through a potential difference of one volt.
  18. 18How many joules is one electronvolt (1 eV)?(Charge of an electron) J.
  19. 19What is the photoelectric effect?Emission of electrons (photoelectrons) when light of a sufficiently high frequency strikes a metal surface.
  20. 20What is the intensity of light?The amount of energy transferred per m^2 per second.
  21. 21What do we think of intensity as in everyday language?The brightness of a light source.
  22. 22What 2 things did the wave model of light suggest?-Would be a delay in the emission of photoelectrons from the metal surface using low intensity light to strike it.
    -More intense light = greater kinetic energy of photoelectrons.
  23. 23However, what 3 things were found?-Kinetic energy of the ejected electrons not affected by the intensity of light.
    -Kinetic energy depends on frequency of light
    -Intensity effects the number of electrons produced (higher intensity = greater numbers).
  24. 24Is there a measurable delay in the emission with low intensity light?No.
  25. 25What happened if the frequency was lower than a certain threshold frequency 'fo'?No electrons were released, no matter how high intensity was.
  26. 26What does it take to remove an electron from the metal surface? What is this known as?A certain amount of energy- known as the work function of the metal, ϕ
  27. 27What does Einstein's equation state that?The maximum kinetic energy of the ejected electrons =
    The energy of an individual photon -
    The work function of the metal.
    Ek(max) = hf - ϕ
  28. 28Why must the energy of the electron be described as the maximum kinetic energy?Some photons interact with electrons a little below the surface (Require more energy to be released than at surface).
    -Cannot be sure where the electrons emitted came from.
  29. 29What happens when one photon strikes the surface of the metal?-It transfers energy, hf, to an individual electron.
    -Some of this energy, ϕ, goes to releasing the electron from the surface.
    -Any remaining energy is transferred as the kinetic energy of the electron.
  30. 30What is the gradient of a frequency-Ek(max) graph?The Planck constant
  31. 31How can the work function be calculated? Therefore what can we say?By finding the threshold frequency (intercept on frequency axis) and multiplying it by the Planck constant (the gradient).
    ϕ=hfo.
  32. 32What do electrons in atoms have?Different energies.
  33. 33How can these electrons gain and lose energy?By changing energy levels.
  34. 34When do atoms release photons?When an electron falls from a higher energy level to a lower energy level.
  35. 35What is the energy of this released photon?Equal to the difference between the energy levels.
  36. 36How could this be shown in a diagram?(See diagram).
  37. 37What would be the case for a photon emitted by an electron moving from level 4 to level 1?High energy, so short wavelength and higher frequency.
  38. 38What would be the case for a photon emitted by an electron moving from level 4 to level 3?Low energy, so long wavelength and lower frequency.
  39. 39What 2 things will higher energy photons have?-Higher frequency
    -Shorter wavelength.
  40. 40What happens when a compound or element is heated?Electrons in the atoms move into higher energy levels, and photons are released as the electrons fall back to lower levels.
  41. 41What is an example of this?Materials 'glowing' red-hot when heated to a sufficiently high temperature.
  42. 42What do atoms of each element have?Their own set of energy levels.
  43. 43What does this mean they will emit?Emit photons of different energies, so light of different wavelengths.
  44. 44What can a diffraction grating be used to do?Spread incident light into its component wavelengths.
  45. 45What do line spectra resemble?Multi-coloured bar codes- each line produced by electrons dropping from a higher energy level to a lower one within the visible spectrum.
  46. 46So what do atoms of each element have?Their own pattern of lines.
  47. 47For example, what if yellow-orange light was the only pattern of lines on a spectra?Then this is the colour of that particular light.
  48. 48What are LEDs?Semiconducting devices that only allow current to travel in one direction.
  49. 49What happens as an electron crosses from one side of the diode to the other?It falls to a lower energy level and emits a photon.
  50. 50How can the energy change between levels be calculated?By multiplying the p.d. across the diode when it just begins to glow (strike voltage) by the charge on an electron.
  51. 51What does this value also depend on? Why?Depends on the semiconducting material-because diodes made from different semiconducting materials will release photons of different energy (so light of different wavelengths).
  52. 52What do semiconductors have?A band gap.
  53. 53What happens if enough voltage is applied?Electrons can move across the gap, so a photon is emitted.
  54. 54Why does the energy of the photon have a set value?Because the gap has a set value.
  55. 55What do LEDs requite to light? What is this given by?Requite a minimum voltage (strike voltage) to light:
    eVo = hf
  56. 56Since power is energy per second, what is the power of a lamp in terms of photons?(Number of photons emitted per second x energy per photon) / Time.
  57. 577.2; QUANTUM BEHAVIOUR AND PROBABILITY..
  58. 58How do quantum objects such as photons behave? Is it wave or particle behaviour?Have own way of behaving- not wave or particle behaviour.
  59. 59What is the rule for quantum behaviour?Explore all possible paths.
  60. 60What 2 types of effects does this quantum behaviour produce?- Wave-like effects- such as interference
    - Particle-like effects- such as the way images build up.
  61. 61What is a photon emitted by and how it is detected?Emitted by a source and is detected at a certain place and time.
  62. 62What must we imagine?The photon taking every possible path from the source to arrive at that place and time.
  63. 63What does a longer path mean?The photon will have to be emitted at an earlier time.
  64. 64What does the path include?The emission, travel and detection of a photon.
  65. 65What can be calculated for each path?A phasor arrow.
  66. 66What can the phasor arrow be thought of as?Rotating with frequency.
  67. 67What does f=E/h for?The time it takes the photon to travel the length of the path- the trip time
  68. 68What can be determined for each path?The angle at which the phasor ends up.
  69. 69How do you get the resultant phasor?You add the phasor arrows for all possible paths, tip-to-tail.
  70. 70What 2 things determine trip time?Trip distance and speed.
  71. 71What do longer paths lead to? Why?More phasor rotation, as it takes longer to explore each path.
  72. 72How do you find the chance of a photon hitting the detector?By squaring the amplitude from the resultant phasor.
  73. 73What produces more accurate calculations?Repeating for more paths.
  74. 74What does the amplitude of the resultant phasor arrow at a particular location give?A measure of the probability of a photon arriving at that point.
  75. 75What is this relationship?The larger the amplitude, the higher the probability of arrival.
  76. 76Why is this an important statement?Because it shows that where photons end up is not a certainty, but a probability.
  77. 77What does this explain?The random nature of the arrival of protons.
  78. 78What does 'probability' in this sense mean?How likely it is that a photon will arrive at a particular place during a given time interval.
  79. 79What is the rate at which energy arrives at a point on a screen?The intensity of the light at that point.
  80. 80What is intensity proportional to?The probability of the arrival of a photon.
  81. 81What is this intensity of light also proportional to?The square of the length of the resultant phasor arrow at that point.
  82. 82Therefore, what relationship can we form?Probability of the arrival of a photon ∝ Square of the length of the resultant phasor arrow.
  83. 83What can we use to turn this relationship into an equation?A constant of proportionality (k).
  84. 84What law do photons obey?The law of reflection.
  85. 85So, what angle do they come off the surface of a mirror at?The same angle at which they reach it.
    (See diagram).
  86. 86How is the angle through which a phasor turns related to the trip time?Longer the trip time, larger the angle.
  87. 87Where is the smallest rotation?For the path in the middle.
  88. 88What happens if the resultant phasor is found?(See diagram).
  89. 89What happens if this is repeated for many significant paths?-Phasors from the end paths curl up
    -Phasors from the centre paths line up
    (See diagram).
  90. 90What does a longer trip lead to?More rotations.
  91. 91What do the phasors from the ends do?Reach the detector at very different phases and tend to curl up.
  92. 92Do these paths contribute much to the resultant amplitude?No.
  93. 93What do the paths taken near the equal-angled path (path that follows the law of reflection)?They have little phase difference and line up.
  94. 94What comes from these paths?Most of the probability of photons arriving at the detector.
  95. 95What happens to photons in glass?They slow down.
  96. 96So, what will a phasor arrow for a photon travelling a given distance in glass make?More rotations than the arrow for a photon travelling the same distance through air.
    (See diagram)
  97. 97What happens to photons travelling straight through the lens to the detector?They take the shortest possible route but are slowed down in the glass.
  98. 98What happens to photons taking the longest route (to the tip of the lens)?They take the longest route but are not slowed down by the glass.
  99. 99What is the point at which the lens focuses the light?The point at which all the phasor arrows have made the same number of rotations.
  100. 100What must be remembered?V = s/t,
    and the velocity of photons in air is c.
  101. 1017.3; ELECTRON DIFFRACTION.
  102. 102What wave-like behaviour do electrons show?They can diffract.
  103. 103What did Davisson and Thomson demonstrate?That a thin layer of atoms can act as a natural diffraction grating.
  104. 104What does the 2-dimensional array of atoms produce?A diffraction pattern of concentric circles.
  105. 105What does this show?That electrons show wave-like behaviour.
  106. 106What would particle behaviour result in?A central bright region with no concentric circles.
  107. 107What do experiments show?That electrons diffract through slits in the same manner as light.
  108. 108What did de Broglie develop?An equation which suggested that an electron has a wavelength associated with it.
  109. 109What is this wavelength related to?The Planck constant, h, and the momentum of the electron, p.
  110. 110What is de Broglie's equation?λ = h/p
  111. 111Because momentum is given as mv (mass x velocity) of the electron, what can this be rewritten as?λ = h/mv
  112. 112Why are electrons not exactly like protons?Electrons have mass, so cannot travel at the speed of light like a photon- they travel at slower speeds.
  113. 113However, how are electrons similar to protons?They explore all possible paths between 2 points.
  114. 114What can therefore be done?A phasor can be used to track the phase change along every path.
  115. 115What is similar about phasors for photons and electrons?They superpose in the same way.
  116. 116When an electron is emitted, what does a longer path mean?The electron will have to be emitted at an earlier time.
  117. 117What does this depend on?The speed of the electron.
  118. 118What does a path include?The whole process of emission, travel and detection of an electron.
  119. 119What can the phasors arrows be thought of as doing?Making one turn for each distance h/p along the path.
  120. 120What can be calculated from the square of the resultant phasor?The probability of detection of an electron.
  121. 121What would modern electron microscopes not work without?The quantum behaviour of electrons.
  122. 122What is built up as electrons arrive at a detector?An electron interface pattern.

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Descrição
AS - Level Physics (AS) FlashCards sobre Chapter 7: Quantum Behaviour, criado por Matt Clancy em 22-04-2016.
Criado por
Matt Clancy · abr 2016
Atualizado
23 mai 2016
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