Unit 5: Nuclear and Thermal Physics

Michael Priest
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Flashcards for AQA A Level Physics Unit 5: Nuclear and Thermal Physics

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Michael Priest
Created by Michael Priest over 4 years ago
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Question Answer
Define binding energy The energy released when a nucleus is formed or The energy required to separate the nucleons from a particular nucleus
Describe the nature and properties of alpha radiation 2 protons and 2 neutrons, +2 relative charge, highly ionising, relatively slow (10^7 m/s), low penetrating ability (stopped by paper / skin)
Describe the nature and properties of beta radiation electron emitted from the nucleus, -1 relative charge, medium ionising ability , fast speed but variable (~10^8 m/s), medium penetrating ability (stopped by 5 mm of aluminium)
Describe the nature and properties of gamma radiation high energy EM radiation, 0 relative charge, low ionising ability, speed of light (3x10^8 m/s), highly penetrating (absorbed by 10cm of lead / metres of concrete)
Describe the principles of Rutherford scattering Alpha particles fired at gold foil, most particles pass straight through undeflected or with very small deflections. 1 in 8000 suffer a large deflection (greater than 90 degrees). Atom is mostly empty space with a tiny, dense positive nucleus.
Compare the dangers of different radiation inside and outside the body Gamma - least dangerous, most passes through body without absorption, least ionising also. Beta - dangerous inside and outside, as beta penetrates the skin, damages DNA Alpha - only dangerous inside the body, highly ionising and damages DNA
Describe how background radiation should be measured and taken account of in experiments Use G-M tube to record counts over 5 minutes without source present. Calculate count rate for background. Subtract background count rate from all readings taken during experiment.
Describe the tracks of alpha, beta and gamma radiation in cloud and bubble chambers Alpha - bold and straight Beta - fainter tracks, may be tortuous if travelling slowly Gamma - no tracks, but may cause secondary radiations (Alpha and beta will be curved if magnetic field is applied)
Describe the inverse square law for gamma radiation Intensity is proportional to the inverse of the distance to a source squared. If the distance is doubled, the intensity decreases by a factor of 4.
Define the Becquerel An activity of 1 Becquerel means 1 nucleus decays each second
Define radioactive decay constant The probability of a nucleus decaying in a certain time period (usually per second)
Define half life The time for half the radioactive nuclei in a sample to reach half the original level or The time for the intensity of radiation from a source to reach half the original level
Carbon-14 has a half life of 5700 years Why is radiocarbon dating only useful if an artefact is older than a few hundred years old, but not older than 60 000 years? 200 years - little change in the activity of the carbon-14 in an artefact 60 000 years - too few carbon-14 nuclei remain in sample to differentiate between sample and background radiation
Describe the features of the N against Z graph for nuclear stability N = Z up to approx. 20 Line curves upwards and passes through points Z=80, N=120 Alpha emitters - heavy nuclei Beta minus (and neutron) emitters - above line Beta positive emitters - below line
How do atomic number and mass number change in alpha particle emission? Atomic number decreases by 2, mass number decreases by 4
How do atomic number and mass number change in beta minus particle emission? Atomic number increases by 1 Mass number doesn't change
How do atomic number and mass number change in beta plus particle emission? Atomic number decreases by 1 Mass number doesn't change
How do atomic number and mass number change in gamma ray emission? No change to either number Gamma rays are emitted by a nucleus in an excited state as it returns to ground level
Carbon-14 decays by beta minus decay What are the products of the decay? A neutron changes to a proton An electron (beta minus particle) An anti-electron neutrino
Oxygen-15 decays by beta plus decay What are the products of the decay? A proton changes to a neutron A positron (beta plus particle) An electron neutrino
Describe electron capture An atomic electron from an inner orbital is absorbed by a proton in the nucleus. The proton decays to a neutron, emitting a neutrino.
Technetium-99m is used in radiotherapy Describe why Technetium 99-m is a gamma emitter (gamma rays can be detected outside the body) and has a half life of 6 hours 6 hours is long enough for the procedure to take place but not too long that the patient remains radioactive for extended time periods
Describe two methods to determine nuclear radius and the basic principles involved Electron diffraction - electrons fired at thin slice of material, electrons behave like waves forming a diffraction pattern Rutherford scattering - alpha particles are scattered by gold nuclei and the angle measured to determine closest approach
Describe how nuclear radius depends on mass number As mass number increases so does nuclear radius, the density of the nucleus remains constant
Define the mole The number of atoms in 12g of carbon-12
How is the mass defect used to calculate the energy released in fission and fusion reactions? The mass defect in amu is multiplied by 931.5 MeV/c^2 or the mass defect is converted to kg and E = mc^2 is used
Explain how the binding energy curve shows that energy is released in both fission and fusion Energy is released when binding energy (per nucleon) increases Movement towards the peak of the curve involves fusion (joining) of lighter nuclei and fission (breaking up) of heavier nuclei The peak is at a mass number of 56 (iron)
In a nuclear power station, what is meant by chain reaction? Each fission releases one neutron (on average) which goes on to cause another successful fission reaction
State one material that is used as a moderator in a nuclear power station graphite or water
State one material that is used as a coolant in a nuclear power station water
State one material that is used to make control rods in a nuclear power station boron
How is the chain reaction controlled in nuclear power stations Control rods are lowered to decrease the number of neutrons released for further fission reactions (and decrease energy output) The opposite happens to increase energy output
Uranium used in nuclear reactors needs to be enriched. What does this mean? Uranium ore dug out of the ground contains 1% U-235. Uranium fuel needs to contain 2-3% U-235. The uranium is therefore enriched to increase the proportion of U-235.
U-235 is fissile, U-238 is not. What happens to U-238 in a nuclear reactor? U-238 absorbs neutrons and (after two beta decays) becomes Pu-239
What is the function of the moderator in a nuclear reactor? To reduce the speed of neutrons to thermal speeds. Only slow moving neutrons are absorbed by U-235 and cause fission. Fast moving neutrons are scattered elastically.
What is the function of the coolant in a nuclear reactor? Transfer energy from the reactor vessel to the heat exchanger which uses the energy to boil water, steam turns a turbine which turns a generator
How is different levels of radioactive waste dealt with? Low Level - Buried in sealed containers Intermediate Level - Solidified in concrete and buried in sealed containers High Level - Placed in cooling ponds until activity decreased (a few months), then vitrified and buried in sealed containers
Describe the shielding around a nuclear reactor Thick walled steel cylinder to absorb gamma radiation Several metres of concrete to absorb neutron radiation (and further reduce gamma)
Define specific heat capacity The energy required to raise the temperature of 1 kg of a substance by 1 degree Celsius (or Kelvin) with no change of state
Define specific latent heat of fusion / vaporisation The energy required to change the state of 1 kg of a substance from solid to liquid (fusion) or from liquid to gas (vaporisation) with no temperature change
Define the Avogadro constant The no. of atoms in 1 mole of a substance (specifically 12g of carbon-12)
State Boyle's Law For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume
State Charles' Law For a fixed mass of gas at constant pressure the volume is directly proportional to temperature in Kelvin.
State the pressure law For a fixed mass of gas at constant volume, the pressure is directly proportional to the temperature in Kelvin.
What is the equation of state for an ideal gas pV = nRT
What are the properties of an ideal gas? High temperature Identical molecules Low pressure
What assumptions are made in the derivation of: pV = 1/3Nmc^2 The molecules are identical. Collisions between molecules and with the container are perfectly elastic. The motion is random. There are a very large no. of molecules so that statistics can be applied. Molecules are small compared to the distances between them.
How does increasing the temperature of a gas affect the movement of the molecules (at constant volume)? The rms speed increases, so mean kinetic energy increases, which means the frequency of collisions increases, which raises the pressure within the container