Y12 Explain thats

• Explain how the resistance of a filament lamp changes as temperature increases

  • As the temperature increases the metal lattice ions vibrate with an increased amplitude
  • The frequency of collisions between lattice ions and conduction electrons increases, so kinetic energy is transferred from conduction electrons to the lattice ions
  • The drift velocity of the conduction electrons decreases so from I = nAvQ the current will decrease
  • From V=IR if the current increases the resistance will increase
• How are temperature current and resistance related in a NTC thermistor

  • A thermistor is a semi-conductor
  • As the temperature heats up more conduction electrons are released
  • As I = nAvQ the current will increase
  • As R = V/I for a constant potential difference, the resistance will decrease
• How are light intensity on a LDR related to the current

  • As a lamp is moved towards an LDR the intensity of light falling on the LDR increases
  • In an LDR the electrons are charge carriers
  • Electrons gain energy from the light and are promoted to the conduction band
  • This increases the number of conduction electrons so as I = nAvQ the current increases
  • This reduces the resistance of the LDR so current increases
• How does sound travel through a metal

  • The sound waves pass through the metal as a longitudinal wave
  • The metal ions vibrate with oscillations parallel to the direction of energy transfer creating a series of compressions and rarefactions which are regions of high and low pressure
• Why do bats speed up their clicks when approaching their prey

  As the bat gets closer to its pray, the reflected pulses take less time to return so the bat reduces the time between emitted pulses to give itself more frequent updates
• How is light polarised

  • Unpolarised electromagnetic waves are transverse waves with oscillations in many planes which are perpendicular to the direction of energy transport
  • When waves pass through a polarising filter all the components perpendicular to the plane of the filter are absorbed
  • Polarised electromagnetic waves consist of oscillations in a single plane which is perpendicular to the direction of energy transport
• How do you test if light is polarised

  • Pass the light through a polarising filter
  • Rotate the axis of the filter
  • If the intensity of transmitted light drops to zero then the light is polarised in a plane perpendicular to the axis of the filter
  • If the light intensity decreases, the light is partially polarised
  • If the light intensity decreases to zero the light is fully polarised
• When does total internal reflection occur

  Total internal reflection occurs only at a boundary between a more optically dense medium and a less optically dense medium (SFA). If the angle of incidence is greater than the critical angle, the light ray will be totally internally reflected
• How is a standing wave on a wire formed

  • A wave travels along the wire and is reflected from both ends
  • The reflected waves superpose to form a standing wave
  • The standing wave has a number of nodes and antinodes
  • There is a node at each end
  • At the nodes, there is destructive superposition because the reflected waves are in anti phase
  • At the antinodes, there is constructive superposition because the waves are in phase
• Explain how dark and light bands are formed by light reaching the screen from two slits of a double slit

  • The light from the two slits reaches the screen
  • Where light from both slits is in phase, bright fringes are formed due to constructive superposition
  • In constructive superposition path difference = n(lander)
  • Where light is in anti phase, dark fringes are formed due to destructive superposition
  • In destructive superposition = (n + 0.5)(lander)
• How is atomic absorption spectrum formed

  • An electron absorbs a photon
  • The electron is excited and moves to a higher energy level
  • This is because each photon has a discrete energy, E = hf
  • Only specific energy changes are possible
  • The photon energy equals the difference in energy levels
• How is atomic emission spectrum formed

  • When an atom is in an excited state an electron can move to a lower energy level
  • As it does so, the electron emits a photon of electromagnetic radiation
  • Only specific energy changes are possible
  • The frequency of the photon depends on the difference in discrete energy levels
• How does the particle theory explain the photoelectric effect

  • Light is incident on a metal surface
  • Each photon has energy proportional to the frequency (E=hf)
  • One photon interacts with one electron on the surface
  • The electron is emitted instantly only if the energy of the photon is greater than the work function of the metal. The work function is the energy needed for an electron to break free from the metal surface
  • When photon energy is just enough to cause electrons to be emitted from the surface, photon energy = the work function of the metal so the kinetic energy of the electrons is zero
• How does the wave model fail to explain the photoelectric effect

  • The wave model predicts that the energy in the energy of the electron would build up and eventually be emitted
  • This doesn't explain the observation that if the radiation is below a threshold frequency electrons are never emitted. Threshold frequency is constant for a metal
  • The wave model predicts that the kinetic energy of emitted electrons would depend on the intensity of the wave. This is not observed
  • The wave model doesn't explain the observation that the maximum kinetic energy depends on the frequency of the incident light.