Chapter 17- Probing Deep Into Matter

A summary of chapter 17 from OCR A2 Advancing Physics

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  • Created by: R_Hall
  • Created on: 15-04-14 16:11

Scattering to Determine Structure

  • Geiger and Marsden studied scattering of alpha particles by thin gold foils. A stream of alpha particles was fired at the gold foil, and the number of alpha particles scattered at different angles were recorded. They observed that occasionally particles scatter at an angle greater than 90- only possible if they're striking something more massive than themselves. Conclusions-
  • Most particles passed straight through the foil= the atom is mostly empty space
  • Some particles are deflected back through significant angles= the centre of the atom must be tiny but contain a lot of mass
  • The alpha particles were repelled= the nucleus must be positively charged
  • Atoms are neutral overall= the electrons must be on the outside of the atom
  • An alpha particles that bounces back and is deflected through 180 will have stopped a short distance from the nucleus. At this point electrical potential = initial kinetic energy
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Classification of Particles

  • Hadrons (eg. protons and neutrons) feel the strong interaction. They are not fundamental particles, as they are made up of quarks. There are 2 types of hadron- baryons and mesons
  • Protons and neutrons are baryons, and can be thought of as two versions of the nucleon
  • All baryons (expect protons) decay to protons. Protons are not thought to decay
  • Baryon number is the number of baryons. Protons and neutrons have the baryon number +1, and the total baryon number in any particle reaction does not change
  • Leptons are fundamental particles which don't feel the strong interaction. They only interact with other particles via the weak interaction and gravity.
  • Electrons (e-) are very stable, but there are 2 more leptons called the muon (μ-) and the tau (Τ-)
  • Muons and taus are unstable, and will decay into electrons. The electron, muons and taus each have their own neutrino
  • Neutrinos have zero or almost zero mass and zero electric charge
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  • Each particle type has a corresponding antiparticle with the same mass but with opposite charge
  • Energy and mass are interchangeable. When energy is converted into mass you have to make equal amounts of matter and antimatter
  • Fire two protons at each other at high speed and lots of energy will be released, which can form more particles. If an extra proton is created, there must be an antiproton created also. This is pair production.
  • Pair production only happens if one gamma ray photon has enough energy to produce that much mass. Also tends to happen near a nucleus, which helps conserve momentum. You usually get electron-positron pairs produced as they have a relatively low mass
  • Annihilation occurs when a particle meets its antiparticle. All of the mass of the particle and antiparticle gets converted to energy
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  • Quarks are the building blocks for hadrons. To make protons and neutrons you need the up quark and the down quark.
  • Antiparticles of hadrons are made from antiquarks, which have the opposite properties to quarks
  • Evidence for quarks came from hitting protons with high-energy electrons. The way the electrons scattered showed that there were three concentrations of charge (quark) inside the proton
  • Even if a proton is blasted with lots of energy, the quarks cannot be separated. The energy gets changed into more quarks and antiquarks, a quark-antiquark pair. This is quark confinement.
  • When two particles interact, they must exchange particles called gauge bosons, which are virtual particles that only last for a short time
  • All the forces in nature (strong, weak, electromagnetic and gravity) have their own gauge boson.
  • The exchange particles that cause the strong force which 'glues' hadrons together is called the gluon.
  • As they cause a force, you can think of them as fields as well as particles. As you try to separate quarks, you increase the energy of the gluon field, increasing the attraction between them
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Particle Accelerators

  • Linear accelerator- A long straight tube containing a series of electrodes. An alternating current is applied to the electrodes so their charge continuously changes between +ve and -ve. The AC is timed so that the particles are always attracted to the next electrode and repelled by the previous one.
  • A particle's speed will increase each time it passes an electrode, and the high-energy particles leaving the linac collide with a fixed target at the end of the tube.
  • Cyclotron- Two semicircular electrodes to accelerate protons (or charged particles) across a gap. An alternating pd is applied, and the particles are attracted from one side to the other as their energy increases. A magnetic field is used to keep the particles moving in a circular motion. The combination of fields (electric and magnetic) makes the particles spiral outwards
  • Synchrotron- Produce very high energy collisions. Electromagnets keep the particles moving in a circular path, and magnets focus and deflect the beam
  • Particles are accelerated to such high speeds that the effects of relativity become noticeable and important. No particles that has a mass can move equal to or greater than the speed of light.
  • As you increase the kinetic energy of a mass, the more massive it gets. Particle accelerators have to alter their fields to compensate for the relativistic mass of particles
  • Relativistic factor= the total energy of a particle divided by the rest energy. Low speed= close to 1
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Electron Energy Levels 1

  • Electrons in an atom can only exist in well-defined energy levels. Each level is given a number, and n=1 is the ground state
  • Electrons move down an energy level by emitting a photon, and the photon can only take certain values as the transitions are between definite energy levels
  • Electron-volt- the kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt
  • All electrons that are bound to the atom have negative energies. The higher the energy level, the more energy the electron has and the less negative the energy. An electron is 'free' (no longer bound to the nucleus) when it has a pd of 0- it has been ionised
  • The energy carried by a photon is equal to the difference between energy levels.
  • Electrons are fermions, which means they obey the Pauli exclusion principle. This states that no two fermions can be in the same quantum state at one time eg. no more than two electrons can be in the same energy level at the same time
  • You get a line absorption spectrum when light with a continuous spectrum passes through a cool gas. Photons of the correct wavelength are absorbed by in electrons in the gas (in their ground state) to excite them to higher energy levels. The wavelengths are then missing from the gas when it emerges, You see a continuous spectrum with black lines
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Electron Energy Levels 2

  • When an electron falls down to a lower energy level, it emits a photon. Emission spectra show the wavelengths of photon emitted, and are made up of a series of bright lines on a black background
  • As light has both particle and wave characteristics, De Broglie suggested electrons should be waves.
  • When they orbit around a nucleus, they behave like standing waves, as they only exist in well-defined frequencies
  • The wavelength of the electron should fit into the circumference a whole number of times
  • The principle quantum number = number of complete waves
  • Electrons are essentially trapped in the potential well made by the nucleus
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