HSC Physics Quanta to Quarks

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With the old Quanta to Quarks elective being examined for the last time this November a list of true-false questions on concepts that are often misunderstood is given below. This list will be updated during the next six weeks.

Rutherford Model of the Atom

  1. Rutherford's model of the atom has a central charge concentration made of protons and neutrons.
  2. The alpha particles that bounce back from the gold foil hit the nuclei of the atoms in the foil.
  3. Rutherford's model did not have electrons orbiting the nucleus in rings
  4. In Rutherford scattering the number of alpha partices scattered through 60 degrees is one-half the number scattered through 30 degrees.
  5. If the thickness of the gold foil is doubled the number of alpha particles scattered through each angle is halved.
  6. In the Geiger and Marsden 1910 scattering experiment the number of alpha particles scattered through an angle greater than 90 degrees was 1 in 800.

Radioactivity

  1. Alpha particles released in the decay of Ra-226 have different kinetic energies.
  2. Beta particles given off in the decay of Sr-90 have the same kinetic energies.
  3. Gamma rays are released when an electron makes a transition to a lower energy level in Co-60.
  4. Gamma rays can be detected using a cloud chamber.
  5. Alpha particle tracks in a cloud chamber are thicker and longer than those of beta particles.
  6. In beta minus decay an electron is ejected from the nucleus.
  7. Half-life is the time for exactly one half of the original number of nuclei to decay.
  8. Technetium-99m is a radioisotope of half-life 6 h that emits gamma radiation and is used as a tracer in nuclear medicine.

Hydrogen Spectrum

  1. The line spectrum of hydrogen is evidence for quantised energy levels in the hydrogen atom.
  2. The longest wavelength in the Balmer series for hydrogen is 656.1 nm.
  3. The shortest wavelength in the Balmer series for hydrogen is 364.5 nm.
  4. The wavelengths in the Balmer series become closer together when placed in order from longest to shortest.
  5. The four longest wavelengths in the Balmer series can be seen by the human eye.
  6. The energy required to remove an electron from the ground state of the hydrogen atom is 13.6 eV.
  7. The energy of the photon released when an electron makes a transition from n i =5 to n f =3 is 1.0 eV.
  8. The shortest wavelength in a certain spectral series for hydrogen is 2.28 μm. The longest wavelength in this series is 7.46 μm.

Modifications of the Bohr Model of the Atom

  1. The Bohr postulates can be applied to an atom with more than one electron.
  2. The energy required to remove the electron from the ground state of a helium atom with one electron only is 54.4 eV.
  3. The difference in intensity of spectral lines is explained using additional quantum numbers to n. This predicts extra states for the electron to exist in. This allowed the development of selection rules and the calculation of transition probabilites.
  4. The state of the electron in the hydrogen atom is now described by 4 quantum numbers. These are n (1,2,3..), l (0,1,2,..n-1), m (-l, -2,-1,0,1,2,...l) and s (-1/2 or 1/2).
  5. The Zeeman effect in hydrogen is the splitting of spectral lines that occurs when a magnetic field is applied to the gas. The magnetic field interacts with the orbiting electron creating extra energy levels allowing more electron transitions.In the Balmer series, this creates 2 extra spectral lines equally spaced on either side of the spectral line observed with no magnetic field.

Neutrons

  1. When beryllium is bombarded with alpha particles a highly penetrating radiation is given off that can eject protons with considerable velocities from matter containing hydrogen.
  2. Neutron scattering is a non-destructive technique to probe materials such as turbine blades for defects as neutrons have no charge and have a wavelength comparable to the spacing between atoms and so are diffracted producing an interference pattern from which the arrangement of atoms can be determined.

Heisenberg

  1. The Heisenberg uncertainty principle limits us measuring the exact position of an electron.
  2. Matrix mechanics uses matrices to represent observable quantities and the eigenvalues of the matrix are the observable values.
  3. The uncertainty principle is a consequence of the wave nature of an electron.

Pauli

  1. Pauli proposed in 1930 the existence of an uncharged particle of very small mass that carried energy and momentum to explain the continuous spectrum of beta decay. This was later called the neutrino.
  2. The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers.

Fermi

  1. Fermi found that slower moving neutrons could produce more radioactivity in silver than faster moving neutrons. He deduced that slower moving neutrons, produced by collisions in paraffin wax, were more easily captured than faster moving neutrons.
  2. Fermi was in charge of the group that carried out the first sustained controlled fission reaction of U-235 using graphite blocks as a moderator and cadmium control rods on December 2 1942.

Particle Accelerators

  1. The drift tubes in a linear accelerator increase in length to allow allow an alternating voltage of constant frequency to be be applied across each tube so that the speed of the beam increases.
  2. The LHC uses superconducting magnets as these produce very strong magnetic fields using low currents.
  3. A cyclotron uses magnetic fields to increase the speed of the charges.
  4. A synchrotron uses a time varying magnetic field to allow for the increase in relativistic mass of a charge as it travels faster so that it can cross the dees at regular time intervals.

Standard Model

  1. The mediator of the strong nuclear force is a gluon.
  2. In beta decay the quark composition of the particles does not change.
  3. The strong nuclear force holds protons and neutrons together.
  4. The strong nuclear force holds quarks together.

HSC Physics Ideas to Implementation

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With the old Ideas to Implementation topic being examined for the last time this November a list of true-false questions on concepts that are often misunderstood is given below. This list will be updated during the year.

Cathode Rays

  1. A paddle wheel spins showing that cathode rays have momentum.
  2. A paddle wheel will spin if the pressure in the tube is zero.
  3. Cathode rays are not emitted when positive ions in a discharge tube impact on the cathode.
  4. Cathode rays cause glass to phosphoresce.
  5. Heinrich Hertz found that cathode rays could not travel through a metal foil.
  6. Heinrich Hertz observed that cathode rays were deflected by an electric field.
  7. When the pressure in a gas discharge tube is reduced the order of formation of structures in the tube is Faraday dark space, positive column, striated positive column, Crookes dark space, negative glow.
  8. Dark spaces in a discharge tube are caused by destructive interference.
  9. Cathode rays have their greatest speed in the bright areas.

Electric and Magnetic Fields

  1. In J J Thomson's original experiment he used perpendicular electric and magnetic fields to give equal magnitude forces on a cathode ray causing them to move in a straight line.

Blackbody Radiation Curve

  1. An example of a blackbody is a piece of iron painted black.
  2. In the blackbody spectrum there are not many photons released at low frequencies.
  3. In the blackbody spectrum there are many photons released at high frequencies.

Photoelectric Effect

  1. The frequency of light is the number of photons passing a point in one second.
  2. If the frequency of the light increases there are more photons passing a point in one second.
  3. If the frequency of the light increases the intensity of the light remains constant.
  4. The photoelectric current does not change if the intensity of the light is kept constant and the frequency is increased.
  5. Heinrich Hertz found that when ultraviolet light was directed on a spark gap a spark was not produced when the width of the gap was increased.
  6. Photoelectrons ejected by exposure to bright light have a greater maximum kinetic energy than those ejected by dimmer light of the same frequency.

Conduction in Metals

  1. The crystal lattice is composed of positively charged ions. Conduction electrons are attracted to the lattice ions and this causes resistance in a metal.
  2. Resistance in a copper wire is due to collisions between the conduction electrons and the stationary copper atoms in the wire.

Band Theory

  1. The valence band in an insulator is not full.
  2. Intrinsic silicon has no electrons in the conduction band at room temperature.
  3. Conduction in intrinsic silicon is due to electrons moving in the valence band.

Bragg Experiment

  1. The x-ray diffraction pattern produced by a crystal is a series of alternating bright and dark fringes.

Superconductors

  1. Magnetic fields cannot pass through all superconductors.