Cosmology

Question 1

Nuclear Fusion of hydrogen

Answer 1

• Occurs at high temperatures and pressures that can overcome the electrostatic repulsion of the particles
• 2 hydrogens -> Deuterium + positron + electron neutrino
• deuterium + hydrogen -> Helium-3 + gamma radiation
• 2*Helium-3 -> Helium + 2 Hydrogens
• Overall equation: 4 Hydrogen -> Helium + 2 positrons + 2 electron neutrinos + 2 gamma rays

Question 2

Nuclear Binding energy

Answer 2

• E = mc^2, u (unified atomic mass) = 930 MeV
• Mass deficit = difference in mass between the reactants and products in a nuclear reaction = energy given off in fusion = nuclear binding energy
• Nuclear binding energy = energy required to split the nucleus into its components or the energy given off in forming a nucleus its products

Question 3

Binding energy per nucleon

Answer 3

• Binding energy against RAM gets B/A
• highest B/A is Iron-56, so it requires the most energy per nucleon to break apart.
• Up to iron-56 B/A is greater in product than reactants so energy is released in fusion of smaller nuclei
• After iron-56 B/A is smaller in product than reactants so energy is given off in fission of larger nuclei

Question 4

Fission

Answer 4

• Nuclei with large mass have higher proportion of neutrons to protons, so they are less stable.
• Slow moving neutrons are absorbed by the nucleus and causes them to be too unstable and split
• e.g. Uranium-235 + Neutron -> Barium-144 + Krypton-89 + 3 Neutrons
• Binding energy of the new nuclei is greater so energy is released

Question 5

Chain Reactions of fission

Answer 5

• 3 Neutrons found in uranium fission must be slowed down with a graphite moderator so they can set off another fusion
• control rods absorb extra neutrons so on average only one is released per reaction. If it isn’t controlled the reaction can be explosive

Question 6

Nuclear Fission problems

Answer 6

• Uranium-238 doesn’t undergo fission so it must be enriched to U-235
• Radioactive waste has a long half-life so is hard to dispose of, some reactors can use it as power, and it can be used in nuclear weapons

Question 7

Stellar Fusion

Answer 7

• Stars exert huge temperatures and pressures that overcomes the repulsive force of coulomb’s law
• on earth we simulate these conditions using magnetic fields to exert force in the particles

Question 8

Expansion

Answer 8

• Black lines present in absorption spectra correspond to wavelengths of light that are absorbed by the elements in a star
• As stars move away or towards us the Doppler effect causes these lines to shift up or down from where they normally would be
• Change in wavelength/Wavelength = velocity of star/speed of light

Question 9

Hubble constant

Answer 9

• Velocity of galaxy is directly proportional to its distance from earth, with gradient of Hubble Constant
• 1/Hubble Constant = rough age of the universe
• measurements of such huge distances have large uncertainties, so gradient of V/D graph has lots of uncertainty and is a very rough value, so age of universe isn’t that accurate