P:6 Part 3

P:6 Part 3

1. Ultrasound

   • ultrasonic waves have a frequency of 20kHz or higher so cannot be heard by humans
   • when they meet boundaries between two materials, they are slightly reflected
   • it is possible to measure how far away a boundary is by measuring the time taken for the reflected wave to return to the detector
   • uses in industry include detecting defects in materials without cutting into them. These could be manufacturing faults or damage
   • Uses in medicine include prenatal scanning, detection of kidney stones and tumors and producing images of damaged ligaments and muscles

2. Echo Sounding

   • used for detecting objects in water and measuring the depth of the water
   • involves sending an ultrasound pulse into the water, which is reflected when it hits a surface
   • the time between the pulse being sent and the reflection being detected is used to calculate the distance travelled by sound wave:
   • speed of sound in water = 1500 m/s

3. Seismic Waves

   • two types of seismic wave are produced during an earthquake:
     • P-waves (primary)
     • S-waves (secondary)
   • P-waves:
     • are longitudinal
     • travel at the speed of sounds (2x as fast as S-waves)
     • travel at different speeds through solids and liquids
   • S-waves:
     • are transverse
     • not able to travel through liquids

4. The Structure of the Earth

   • During an Earthquake:
     • seismic waves travel outwards from the earthquake and are capable of travelling all the way through Earth
     • seismic waves travel in a curved path through Earth, due to the Earth increasing in density with depth
     • detectors placed around the Earth measure when and where the different waves arrive
   • there are two key pieces of evidence that come from the P-wave shadow zone and the S-wave shadow zone
   • S-wave shadow zone:
     • unable to travel through the liquid outer core
     • this makes a large shadow zone on the opposite side to where the earthquake happened
     • this shadow zone provides evidence of the size of the Earth's core
   • P-wave shadow zone:
     • able to travel through the liquid outer core
     • however, are refracted at the boundary between the semi-solid mantle and liquid outer core
     • are refracted again at the boundary between the liquid outer core and solid inner core
     • these result in P-wave shadow zones

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