Quantum Physics

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  • Created by: Uptona09
  • Created on: 21-03-15 13:59
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  • Quantum Physics
    • Electron volt
      • The kinetic energy gained by an electron when it is accelerated through pd 1V.
      • Energy (J) = charge (C) x voltage (V)
      • Charge on an electron = 1.6x10-19 C
        • Energy of 1eV
      • The energy of a photon is very small. eV doesn't require standard from
    • Energy of a photon
      • em radiation consists of photons.
      • A photon is a packet of energy or quantum of em energy.
      • The photoelectric effect is the emission of electrons from a metal surface when em waves above a certain frequency is shone on the metal surface.
        • Photoelectron
        • GOLD LEAF
          • Clean zinc plate on top of a gold leaf electroscope. Give it a negative charge  and the leaf deflects. Shine electro- magnetic radiation from a mercury discharge lampoon the plate an the leaf gradually falls.
            • Charging the electroscope give it an excess of electrons. The electro-magnetic radiation helps the electrons to escape from the surface (photoelectrons).
              • A sheet of glass between the lamp and plate, the radiation is no longer effective.
            • Electron at the surface of a metal gain energy from light. The electrons cannot leave is the light frequency is less than a threshold voltage. They need energy to leave the metal surface because they are loosely attached to the positive ions.
              • The minimum amount of energy required to free an electron from the surface of a metal is called  the work function of the metal.   (0 = hfo)
                • An electron near the surface absorbs an individual photon and gains energy = to the photon energy. Energy must be conserved in the interaction between a photon and electron.          hf > 0    leave the metal.       The incident rate of photons is doubled when the intensity is doubled.
                  • One electron absorbs the energy of one photon.
      • Energy of photon= work function + maximum Kinetic Energy of emitted electron.   hf= 0 + KEmax
        • Einsteins photoelectric equation
        • If the photon energy is less than 0, can't leave the metal. The electron requires the KE that is dispersed as heat to the rest of the metal.
          • No photoelectrons are emitted if the frequency of light is less than fo
          • If the photon is greater than 0 electrons are emitted from the metal surface with a range of KEs.
            • KEmax =hf-0   y=mx+c           m=h c=0         intercept with x axis is fo        y intercept is 0
        • If the photon energy is equal to 0 then the photoelectron is just removed from the metal. KE=zero so there is a threshold frequency of light which photons must exceed to emit electrons.
      • When light is absorbed by a metal surface it behaves as particles. Individual photons are absorbed by individual electrons in the metal. Geiger counter detects gamma radiation.
        • Dual nature of light                  Light interacts with matter as a particle (photoelectric effect)              Light travels through space as a wave. Diffraction and interference of light using slits show this.
    • Wave-Particle Duality
      • \= h/mv           m=mass     h=planck's constant       v= sped
        • The waves associated with moving particles are sometimes referred to as matter waves or deBroglie waves
      • Light-wave-Young's experiments with interference
        • Light-particle-reflection-photoelectric effect
        • Particles and waves are 2 views of the same thing. Electron microscope- a wavicle
      • Electron Diffraction
        • Fire a beam of electrons at a crystalline material, the electrons think it's a diffraction grating so you get the electrons at only a few angles.
        • Most crystals contain grains with the atoms lined up in different directions. So the bright spots where the electrons turn up become circles.
        • The electrons are boiled off the heater, which is a small metal filament. They are accelerated by the large voltage at the anode. These electrons travel through a thin layer of graphite and are thus diffracted and show diffraction rings on the fluorescent screen.
        • Separation of atomic nuclei-Arrangement of atoms in crystals.
          • If V increases v increase so \ decreases.
          • If \ is equal to the separation between nuclei diffraction will occur. Vary V so that \ matches the spacing between atoms.
          • If you increase the energy of the electrons then the rings get brighter (as the electrons have more KE) and they gets a smaller radius as n\=dsino           V ^ v ^               \ and o  decreases so the radius decreases.
        • Finding the size of the nucleus
          • v^, \ decreases so we need smaller objects in order for diffraction to be observed
            • Electrons have a diffraction pattern. dsino=1.2\ d=diameter
      • Electron Gun- Thermionic Emission
        • The cathode, a small metal filament, is heated by a low voltage electric heater. Electrons leave and are accelerated by the high positive voltage towards the + anode.
        • Emission of electrons when a wire is heated by voltage
      • X rays can be diffracted by matter because their \ is 10x-10m similar to the diameter of atoms and their spacing. Used to probe matter.
        • Slow-moving electrons (x10^7m/s) have a \ of about x10^-10m. Determine the diameter of atoms and the structure of complex molecules. Slow moving neutrons can also be used.
        • Fast-moving electrons (x10^8m/s) have a \ of x10^-15m. They can be used to determine the diameter of atomic nuclei and then internal structure of the nuclei. They need to be accelerated by voltages up to 10^9 V.
    • Energy Levels in Atoms
      • Line spectra
        • Used to identity elements in stars.
        • The electrons sit in the lowest energy level. If an electron is given enough energy it may jump up to n=infinity and become free. Energy of levels below zero KE are negative.
      • Emission spectra
        • The spectrum of light emitted hot gas consists of well-defined bright lines. It is produced by hot gases which provides direct evidence that the energy  of the electrons in the atoms is quantised.
        • Collisions from heated atoms cause the electron to jump up from E1 to E2.
          • The electron falls back to ground state later, giving out the energy change as a photon of light.
        • E1-E2=hf =hc/ \
        • The minimum energy needed to remove an electron from its lowest energy level so that it escapes from the atom is called the ionisation energy of the atom.
      • Absorption spectrum
        • Produced by cooler gases so energy can be absorbed
        • Light form a hot white source passes through a cooler gas and is observed using a diffraction grating.
          • White light consists of photons of many different energies. A photon is absorbed by an atom of the cool gas when its energy is the same as the difference between any 2 energy levels. It will not be absorbed if the energy level is too high or too low.
        • The spectrum is continuous with sharp dark lines.
          • Lines have the same wavelengths as the emission spectral lines.
        • A photon absorbed by an atom can be re-emitted in a multitude of directions- why it appears dark.
        • Electrons jump up levels.
        • Found when the light from stars is analysed.
          • Interior stars emits white light of wavelengths. This light has to pass through the cooler outer layer of the star. Certain wavelengths are absorbed so an absorption spectrum is produced. Happens at the same frequencies that  the hot atom gives out light.
      • Isolated atoms
        • Atoms are far apart so they don't interact with one another in a gas.
          • Gas atoms that exert negligible electrical forces on each other.
            • Give a simple line spectra.
              • Gemstones and coloured glass produce a similar spectrum. The basic material is colourless but it gains colour from impurity atoms.
            • In a solid or liquid the atoms are close together meaning the energy level diagram have a greater no. of closely spaced energy levels.
              • Usually produce a continuous spectra
  • E=hf=hc/ \
    • Planck's constant 6.63x-34 Js
      • LEDs- Red LED emits photons that are of low energy. It requires a low threshold voltage to make it conduct. A blue LED-higher energy photons- a higher threshold voltage.
        • The electrical energy lost by a single electron passing through the diode reappears as the energy of a single photon.
          • Several LEDs of different colours. Produce a graph of V against 1/ \. The gradient will be hc/e so h can be estimated.
        • An LED gives out light because an electron loses energy and gives it to make a photon
        • When an LED just starts to glow, the electron have just enough energy to make photons.
    • Energy of a photon
      • em radiation consists of photons.
      • A photon is a packet of energy or quantum of em energy.
      • The photoelectric effect is the emission of electrons from a metal surface when em waves above a certain frequency is shone on the metal surface.
        • Photoelectron
        • GOLD LEAF
          • Clean zinc plate on top of a gold leaf electroscope. Give it a negative charge  and the leaf deflects. Shine electro- magnetic radiation from a mercury discharge lampoon the plate an the leaf gradually falls.
            • Charging the electroscope give it an excess of electrons. The electro-magnetic radiation helps the electrons to escape from the surface (photoelectrons).
              • A sheet of glass between the lamp and plate, the radiation is no longer effective.
            • Electron at the surface of a metal gain energy from light. The electrons cannot leave is the light frequency is less than a threshold voltage. They need energy to leave the metal surface because they are loosely attached to the positive ions.
              • The minimum amount of energy required to free an electron from the surface of a metal is called  the work function of the metal.   (0 = hfo)
                • An electron near the surface absorbs an individual photon and gains energy = to the photon energy. Energy must be conserved in the interaction between a photon and electron.          hf > 0    leave the metal.       The incident rate of photons is doubled when the intensity is doubled.
                  • One electron absorbs the energy of one photon.
      • Energy of photon= work function + maximum Kinetic Energy of emitted electron.   hf= 0 + KEmax
        • Einsteins photoelectric equation
        • If the photon energy is less than 0, can't leave the metal. The electron requires the KE that is dispersed as heat to the rest of the metal.
          • No photoelectrons are emitted if the frequency of light is less than fo
          • If the photon is greater than 0 electrons are emitted from the metal surface with a range of KEs.
            • KEmax =hf-0   y=mx+c           m=h c=0         intercept with x axis is fo        y intercept is 0
        • If the photon energy is equal to 0 then the photoelectron is just removed from the metal. KE=zero so there is a threshold frequency of light which photons must exceed to emit electrons.
      • When light is absorbed by a metal surface it behaves as particles. Individual photons are absorbed by individual electrons in the metal. Geiger counter detects gamma radiation.
        • Dual nature of light                  Light interacts with matter as a particle (photoelectric effect)              Light travels through space as a wave. Diffraction and interference of light using slits show this.
  • Rate=total energy emitted per second/ energy of a single photon
  • eV=1/2mv^2
    • mass of electrons: 9.11x10-31kg
  • When gases are heated/have a high voltage across them, the electrons gain energy and the atoms give out light. (Neon lights)
    • The light is not a continuous range of colours but mainly no light and just a few narrow lines of colour which are unique for each element.
    • Line spectra
      • Used to identity elements in stars.
      • The electrons sit in the lowest energy level. If an electron is given enough energy it may jump up to n=infinity and become free. Energy of levels below zero KE are negative.

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