6.7 Ultrasound, echoes and seismic waves
9 exam questions worth 23 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What type of wave is an S-wave?
Choose one answer.
Correct Not quite: the answer is Transverse
Mark scheme
B: Transverse
Model answer
Transverse (B).
A ship uses sonar to find the depth of the sea. The echo takes 0.80 s to return. The speed of sound in sea water is 1500 m/s. Calculate the depth.
Mark scheme
distance there and back = 1500 × 0.80 = 1200 m (1); divide by 2 (1); 600 m (1)
Model answer
Distance there and back = 1500 × 0.80 = 1200 m, so depth = 1200 / 2 = 600 m.
A pulse of ultrasound in a body travels at 1540 m/s. An echo from an organ returns after 0.000 065 s. Calculate the depth of the organ.
Mark scheme
distance = 1540 × 0.000 065 = 0.10 m (1); divide by 2 (1); 0.050 m (5.0 cm) (1)
Model answer
Distance there and back = 1540 × 0.000 065 = 0.10 m, so the depth is 0.050 m (5.0 cm).
Explain why a gel is used between an ultrasound probe and the skin.
Mark scheme
to remove the air between the probe and the skin (1); because ultrasound would be reflected at an air boundary, so the gel allows it to enter the body (1)
Model answer
The gel removes the air between the probe and the skin. Ultrasound would be reflected by air instead of entering the body.
Give two uses of ultrasound.
Mark scheme
any two from: medical scanning of a fetus / organs (1); breaking up kidney stones (1); detecting cracks in metal (1); sonar (1); cleaning (1)
Model answer
Scanning an unborn baby, and finding cracks in metal.
Explain why ultrasound is safer than X-rays for scanning an unborn baby.
Mark scheme
ultrasound is non-ionising (1); so it does not damage cells / cause cancer, unlike X-rays (1)
Model answer
Ultrasound is non-ionising so it does not damage the cells of the baby, unlike X-rays, which are ionising.
State two differences between P-waves and S-waves.
Mark scheme
P-waves are longitudinal, S-waves are transverse (1); P-waves travel through solids and liquids, S-waves only through solids (1); P-waves are faster (1)
Model answer
P-waves are longitudinal and S-waves are transverse. P-waves can travel through liquids and S-waves cannot.
Explain how the observations of S-waves after earthquakes led scientists to conclude that the Earth's outer core is liquid.
Mark scheme
S-waves are not detected on the far side of the Earth (1); S-waves cannot travel through a liquid (1); so there must be a liquid layer / core in the Earth that blocks them (1)
Model answer
S-waves are not detected on the opposite side of the Earth from an earthquake. They cannot travel through liquids, so there must be a liquid part of the core.
Explain how P-waves and S-waves can be used to find out about the structure of the Earth.
Mark scheme
waves are detected by seismometers around the Earth (1); S-waves do not pass through the core, which shows that it is (partly) liquid (1); P-waves change speed and direction (refract) at boundaries (1); so there are layers with different properties (1); the P-waves suggest a solid inner core (1)
Model answer
Seismometers around the world detect P-waves and S-waves from an earthquake. S-waves do not pass through the core, showing it is partly liquid. P-waves change speed and direction at boundaries, showing there are layers of different material, and the changes at the inner core suggest it is solid.
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