OCR A Physics A level

• What is a planet?

  Object in orbit around a star

  Has a mass large enough for a round shape

  No fusion reactions

  Cleared orbit of other objects
• What is a planetary satellite?

  Body in orbit around a planet
• What is a comet?

  Irregular bodies made up of ice,dust and rock

  Elliptical orbits
• What is a solar system?

  Contains Sun and objects that orbit it
• What is a galaxy?

  A collection of stars,dust and gas
• What is the universe?

  Everything
• How is a star formed from dust and gas?

  Dust and gas get close together and gravitational collapse accelerates

  Denser regions form

  Denser regions pull in more dust and gas and get hotter as gravitational energy is converted to thermal

  energy

  Protostar forms

  Nuclear fusion needs to start

  Repulsion needs to be overcome so high pressure and temperature needed
• How is star equilibrium maintained?

  Gravitational forces try to compress but radiation pressure from photons emitted during fusion and gas pressure from nuclei push outwards
• What does the stability of star depend on?

  How much power it releases
• What stars evolve into red giants?

  0.5-10 solar masses
• What happens when stars evolve into red giants?

  Reduction in energy due to fusion means gravitational force is greater than radiation and gas pressure

  Core begins to collapse

  Core shrinks so pressure increases to start fusion in a shell around core
• What does fusion in shell cause?

  Periphery of star to expand as layers move away from core
• What is formed from the layers of the red giant?

  Planetary nebula
• What is left behind from the red giant?

  White dwarf
• What is the electron degeneracy pressure?

  Core of a star begins to collapse under gravity,electrons are squeezed together and this pressure prevents core from further gravitational collapse
• What is the Chandrasekhar limit?

  Limit for electron degeneracy pressure

  1.44 solar masses
• What happens to a star bigger than 10 solar masses?

  Changes in core cause star to expand and form a red supergiant

  Fusion occurs until iron core formed

  This makes star unstable and star dies and layers bounce off and form supernova
• What happens to more bigger stars?

  Fusion become unable to stand gravitational force so star collapses

  Core turns into neutron star or black hole
• When does the star turn into a neutron star?

  If mass is greater than 1.44 solar masses, gravitational collapse continues
• When does the core turn into a black hole?

  Greater than 3 solar masses

  Gravitational collapse continues to compress the core

  Results in a gravitational field so strong that nothing can escape
• What is the Hertzsprung-Russell diagram?

  Luminosity against temperature

  Temperature increases from right to left
• Where is everything located on the H-R diagram?

  White dwarfs= bottom left

  Main sequence=bottom right

  Sun=middle

  Giants=top right

  Supergiants= at the top
• What are energy levels in electrons?

  Set of energies that they can exist at
• Why are the energy levels negative??

  Energy is required to remove an electron from an atom
• What happens when an electron is excited?

  It moves from a lower energy level to a higher energy level
• What happens when an electron is de-excited?

  It moves from a high energy level to a lower energy level and emits a photon as energy is conserved
• What equations can you use for energy of a photon emitted?

  hf or hc/lambda
• What is an emission line spectrum?

  Each element produces a unique one because of its unique set of energy levels
• What is a continuous spectrum?

  Visible frequencies or wavelengths are present

  Produced by the atoms of a heated solid metal
• What is an absorption line spectrum?

  Dark lines against the background of a continuous spectrum

  Dark lines have exactly the same wavelengths as bright emission spectral lines for the same gas atoms
• What produces an emission line spectrum?

  Photons are emitted with set of discrete frequencies specific to element

  Each line=photons with a specific wavelength

  Can be observed from heated gases
• How is an absorption spectrum produced?

  Light from a source that produces a continuous spectrum passes through a cooler gas

  Some photons are absorbed by the gas and excite electrons

  Only absorbed if their energy= difference in energy levels

  This creates dark lines in spectrum
• What do the dark lines in an absorption spectrum show?

  Show which photons have been absorbed by gas atoms
• What is a transmission diffraction grating?

  Large number of lines ruled on a glass or plastic side

  Produces a clearer and brighter interference pattern
• What is the grating equation?

  dsintheta=n lambda
• What is Wien's displacement law used for?

  To estimate the peak surface temperature of a star
• What is Wien's displacement law?

  The inversely proportional relationship between the wavelength of maximum emission and the temperature of a black body.
• What happens as the temperature increases?

  Peak wavelength reduces and peak becomes sharper
• What is Stefan's law?

  The total power radiated per unit surface area of a black body is directly proportional to the fourth power of temperature
• What is the luminosity of a star directly proportional to?

  Radius squared

  Surface area

  Absolute temperature quadrupled