AQA quantum

Mainly directed towards AQA physics students but could be used for other exam boards (maybe).

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  • Created by: StudentKZ
  • Created on: 28-02-23 08:52
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  • Quantum physics (AQA)
    • Photoelectric effect
      • Light is shined onto a metal and photoelectrons are emitted.
        • E = hf = hc / lamba
          • E=      Energy (J)
          • h = Planck's constant (6.63x10^-34)
          • f = Frequency of the light (Hz)
          • c = Speed of light    (3x10^8m/s)
          • lamba = Wavelength   of the light. (m)
      • No photoelectrons are emitted if the light has a frequency that is below the threshold frequency (f0)
      • Maximum kinetic energy of the emitted photoelectrons increases with the frequency of the radiation and is unaffected by the intensity of the radiation
      • Number of photoelectrons emitted per second is proportional to the intensity of the radiation.
      • The threshold frequency (f0) is the minimum frequency required to release photoelectrons
        • f0 = phi / h
          • f0 = Threshold frequency (Hz)
          • phi = Work function (J)
          • h = Planck's constant (6.63x10^-34)
      • The work function is the minimum energy an electron needs to be released from the metal.
        • hf = phi + Ek(max)
          • h = Planck's constant (6.63x10^-34)
          • f = Frequency (Hz)
          • phi = Work function (J)
          • Ek(max) = Maximum kinetic energy (J)
      • The maximum kinetic energy is a maximum because we are only concerned about the electron being released from the surface of the metal. Electrons deeper in the metal lose more energy than the electrons on the surface.
      • The stopping potential is the p.d needed to stop the electrons moving between plates.
        • eVs= Ek(max)
          • e = Charge of an electron (1.6x10^-19 C)
          • Vs = Stopping potential (V)
          • Ek(max) = Maximum kinetic energy (J)
      • The intensity of the light is the amount of energy per second hitting the metal.
    • Energy levels and spectra
      • To go from eV to J: Multiple by   1.6x10^-19  (eVJom)                              To go from J to eV: Divide by 1.6x10^-19 (JeVd)
      • Excitation is when a photon is absorbed and an electron goes up an energy level
      • Ionisation is when a photon is absorbed and an electron leaves the atom.
      • De-excitation is when a photon is emitted and an electron goes down an energy level.
      • The ground state is the lowest energy level an electron can be in.
      • Fluorescent         tubes
        • 1. High voltage is applied to mercury vapour. This high voltage accelerates fast-moving free electrons that ionises some of the      mercury           atoms.
        • 2. When this flow of free electrons collides with the electrons in the mercury atoms, the electrons in the mercury atoms excite to higher energy             levels.
        • 3. When these excited electrons return to their ground states, they emit UV       photons.
        • 4.           A phosphor coating absorbs these UV photons, exciting its electrons to much higher orbits. These electrons then cascade down the energy levels and produce visible light photons.
      • hf = E1 - E2
        • h = Planck's constant (6.63x10^-34)
        • f =   Frequency         (Hz)
        • E1 = Energy of initial energy level           (J)
        • E2 = Energy of final energy level (J)
      • Spectra
        • Emission spectra
          • Set of discrete wavelengths represented by coloured lines on a black background
        • Absorption spectra
          • White light passes through a cool, low pressure gas
          • Continuous spectrum contains all colours with dark lines at certain wavelengths which correspond to the differences in energy levels in an atom.
    • Wave-particle duality
      • Light can behave as a particle or a wave. This is called wave-particle duality
        • Light as a particle
          • Photon model of light explains that EM waves carry energy in discrete packets called photons
          • Only frequencies above the threshold frequency will emit a photoelectron.
          • Energy is absorbed instantly. Photoelectrons are either emitted or not emited
          • If the intensity of light is increased then more photoelectrons are emitted per second
          • The photoelectric effect explains the particle nature of light
        • Light as a wave
          • Any frequency of light can emit photoelectron if exposure time is long enough
          • Energy absorbed will increase gradually with each move
          • Kinetic energy should increase with intensity
          • Diffraction explains the wave nature of light
      • Electron diffraction
        • Demonstrates wave nature of electrons
        • 1. Electrons are accelerated in an electron gun to a high p.d and then directed towards a thin film of graphite
        • 2. Electrons diffract and produce a pattern of concentric rings on a fluorescent screen made from phosphor
      • de Broglie wavelength
        • lamba = h/p = h/mv = h/sqrt2mE
          • Lambda = de Broglie wavelength (m)
          • h = Planck's constant (6.63x10^-34)
          • p = Momentum (kgms^-1)
          • m = Mass (kg)
          • v = Speed (ms^-1)
          • E = Kinetic energy (J)

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