Online Physics Tutor 'A** Questions'

• • The travelling wave generated by the vibrator meets the wave reflected by the pulley
  • If the wave speed and frequency are such that the two waves interfere constructively and destructively at fixed positions
  • then a standing wave is formed

  
• • Diffraction illustrates the wave aspect of light.
  • Diffraction is the spreading out of a wavefront when passing through a gap or obstacle.
  • The wavefront acts as a series of secondary sources.
  • A stream of particles passing through a gap would not spread out in this manner.
  • Light striking a metal surface can lead to emission of an electron.
  • That electron’s maximum energy is directly related to the frequency of the incident light and not the intensity.

  
• • If the tension increases, then effectively a mass element in the wire will experience a greater acceleration leading to a greater velocity.
  • If the mass / length is lower, then the acceleration experienced will also be greater leading to a greater velocity.
  • Overall the velocity will be proportional to the tension and inversely proportional to the mass / length.

  
• • Beats require slightly different frequencies.
  • If the wind is to produce the frequency, then the wires have the same radius then this will not result.
  • So one possibility is to slightly alter the radius of one of the wires.

  
• • A displacement node.
  • The centre of the star will not move.
  • As a place of zero displacement, it must be a node

  
• • A standing wave is formed by two travelling waves with the same frequency, amplitude, wavelength and speed, travelling in opposite directions.
  • This is created in this situation by a reflection from the surface (open end of the tube).
  • At the resonant frequency, the reflected wave will meet the incoming wave in such a way that the two wave displacements will constructively sum (form an antinode) at the open end of the tube.

  
• • The mechanism for the reflection can be explained by considering, a) the particle collisions (some must reflect) or equally validly via a mechanism
  • where the external pressure (atmospheric pressure) is higher than the pressure node that exists at the boundary – this effectively creates a mechanism for particles to be “pulled” down the tube.

  
• • The dark lines correspond to light that has the correct energy to excite transitions between the quantised energy levels of electrons of the atoms (or molecules) in the gas.
  • The light is absorbed by the gas and re-emitted (in all directions), and so there is a strong dip in the intensity of the spectrum at these energies, causing dark lines.

  
• • Diffraction illustrates the wave aspect of light.
  • Diffraction is the spreading out of a wavefront when passing through a gap or obstacle.
  • The wavefront acts a series of secondary sources.
  • A stream of particles passing through a gap would not spread out in this manner.
  • Light striking a metal surface can lead to emission of an electron.
  • That electron’s maximum energy is directly related to the frequency of the incident light and not the intensity

  
• • To achieve fusion, the two nuclei would have to just touch (i.e. approach within at least one nuclear radius), i.e. 10^–15 m.
  • For two deuterium nuclei that were originally well apart, the kinetic energy needed to make them approach within one nuclear radius will be very high due to electrostatic repulsion.
  • At room temperature very few nuclei would have the required kinetic energy, making fusion unlikely.

  
• • Each electron comes with its own proton.
  • The 1 gram of hydrogen is almost entirely single protons, so there are about as many electrons as there are protons.
  • In all the other light elements, half the mass is composed of neutrons so half the mass is protons, meaning half the number of electrons in the 1 gram mass.
  • The number of electrons is only approximately half because both hydrogen and the light elements have isotopes with differing numbers of neutrons which usually serve to reduce the number of protons in any given mass. (But not in the case of He3).

  
• • The photoelectric effect is the observed release of electrons when sufficiently energetic photons impact on the metal surface.
  • Below the energetic threshold, no electrons are released.
  • At the energetic threshold, electrons of zero kinetic energy are produced.
  • The incident photon delivers a fixed amount of energy (E = hf ).
  • The incident energy of the photon equals the kinetic energy of the liberated electron plus the energy required to liberate the electron (work function).

  
• • The classical explanation should have resulted in a time delay of the release of the electrons – no time delay was measured.
  • No lower limit would exist for the incident frequency to eventually release electrons.
  • Idea of the photon is required to explain the phenomenon.

  
• Similarities:

  • Both experience a centripetal force, and both forces are inversely proportional to the square of the orbital radius.
  • Both orbit about the centre of mass of the system (not about the centre of mass of the more massive object in the system).

  
• Differences:

  • The centripetal force is supplied by electrostatic attraction in the hydrogen atom and by gravitational force in the Earth / Moon system.
  • The electron orbits are obviously quantised (restricted to specific possible values), while the possible orbital radius of the Moon is effectively continuous.
  • (The gap between the quantum levels of the Moon’s orbit is unmeasurably small.)

  
• • At maximum, both waves arrive in phase.
  • As the frequency increased, the wavelength decreases (constant velocity) and therefore, as the path difference is constant, at some frequency the waves will be completely out of phase (minima).