topic 4 waves

• Infrasound waves

  • sound waves so low in frequency that humans cannot hear
  • frequency less than 20 Hz
  • travels further than higher frequency sound waves before they become too faint to detect
  • animals e.g. elephants and whales can hear infrasounds
• Natural events

  • volcanic eruptions, earthquakes produce infrasound in the local area
  • we can monitor infrasound to predict events
• Earthquakes

  • causes vibrations = seismic waves
  • energy released by an earthquake travels through the Earth as longitudinal P waves and as transverse S waves
• Waves from earthquakes

  • produces seismic waves at a range of frequencies which travel out of the Earth
  • we detect using seismometers
  • produces waves that travel through different layers of the Earth
• How to detect waves all over surface of planet?

  using seisometers
• Seismologists

  • works out the time it takes for the waves to reach each seismometer
  • also note which parts of the Earth don't receive the waves at all
• What happens when seismic waves reach a boundary between different layers of material?

  • inside the Earth
  • some waves will be absorbed and some will be refracted
• What happens to the waves when the properties change suddenly?

  • most of the time, if the waves are refracted they change speed gradually resulting in a curved path
  • but when properties change, the wave speed changes abruptly and the path has a kink
• S wave shadow zone

  • places where there is large area of the Earth on the opposite side to the earthquake were no S waves are detected
  • occurs because part of the interior of the Earth is liquid
  • there is also a band around the Earth called the P wave shadow zone
• P wave shadow zone

  band around the Earth that prevents any S waves from earthquakes to be detected
• Longitudinal waves

  • can be transmitted through solids, liquids and gases
• Transverse waves

  transverse waves that need a medium to travel through can only be transmitted by solids
• What can scientists work out by observing how seismic waves are absorbed and refracted?

  • where the properties of the Earth change dramatically
  • understanding of the internal structure of the Earth
  • size of the Earth's core
• P-waves inside the Earth

  • P-waves pass through core and are detected on the opposite side too
  • longitudinal
  • travels through solids and liquids
  • travels faster than S-waves
• S-waves inside the Earth

  • no S-waves are detected on the opposite end as they cannot pass through the liquid outer core
  • transverse
  • only travels through solids
  • slower than P-waves
• What do all objects do all the time?

  continually emitting (radiating) and absorbing EM radiation over a range of wavelengths
• What does the distribution and intensity of the wavelengths depend on?

  • only depends on the object's temperature
• Intensity of radiation

  power / area

  • amount of radiation transferred to a given area within a certain amount of time
• What happens to the intensity of the emitted wavelength as temperature increases?

  as the temperature of an object increases, the intensity of every emitted wavelength increases
• Peak wavelength

  • the wavelength with the highest intensity
• What happens to the peak wavelength when the object gets hotter?

  • the peak wavelength decreases but intensity increases
  • objects appear brighter and less red (whiter)
• What happens to the intensity for shorter wavelengths / longer wavelengths?

  Shorter wavelengths = intensity increases more rapidly

  Longer wavelengths = intensity decreases
• Peak wavelength for cooler objects

  • peak for cooler object is near to the red end of the visible range, so object appears red
• How does temperature of objects increase?

  when the amount of radiation absorbed is larger than the amount of radiation emitted
• How does the temperature of an object decrease?

  when the amount of radiation absorbed is smaller than the amount of radiation emitted
• How can an object be at a constant temperature?

  when the object radiates and absorbs the same average power
• What does the overall temperature of the Earth depend on?

  • the amount of radiation it reflects, absorbs and emits
• What happens during the day to the Earth?

  • lots of radiation is transferred to the Earth from the Sun
• What happens to the some of the radiation from the Sun on the Earth?

  • some is reflected
  • most is absorbed
  • radiation is reflected and absorbed by the Earth's atmosphere, clouds and surface
• What reflects and absorbs the radiation on the Earth?

  the Earth's atmosphere, clouds and surface
• What causes an increase in local temperature?

  • when some radiation is reflected and absorbed by the Earth's atmosphere, clouds and surface during the day
• What causes a decrease in local temperature?

  • at night, when the radiation is emitted by the atmosphere, clouds and Earth's surface
• What happens to the temperature of the Earth overall?

  it stays constant
• Earth's overall temperature

  • changes to the atmosphere can cause changes to this
  • if atmosphere starts to absorb more radiation without emitting the same amount, the overall temperature will rise until absorption and emission are equal again (global warming)
• CP: Investigate how well different surfaces emit radiation

  1. wrap 4 identical test tubes with material
  2. material covering each test tube must be same, but each should have a different surface or colour
  3. boil water in kettle and fill each test tube with same volume of water
  4. use thermometer to measure temperature of water every minute
  5. seal test tubes with bungs
• CP: What will happen to the temperature of the water surrounded by good emitter surfaces?

  the temperature of water will decrease quicker
• What are the better emitters?

  • matte surfaces are better emitters than shiny ones
  • black surfaces emit radiation better than white ones