AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
P-waves travel at different speeds through solids and liquids.
Full specification wording
Waves for detection and exploration (physics only) (HT only): P-waves travel at different speeds through solids and liquids.
The speed of a seismic wave depends on the properties of the material it is passing through, so when a P-wave crosses from the solid mantle into the liquid outer core its speed changes sharply.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 8 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The speed of a seismic wave depends on the properties of the material it is passing through, so when a P-wave crosses from the solid mantle into the liquid outer core its speed changes sharply. A change of speed at a boundary refracts the wave, bending its path, so P-waves do not travel in straight lines through the Earth; their paths curve as the speed changes gradually with depth and kink where it changes suddenly. The consequence is measurable at the surface: there is a band of the globe, roughly 105° to 140° away from the earthquake, where no direct P-waves arrive, called the P-wave shadow zone. Only a core of a particular size with a very different wave speed can produce a shadow of that width.
What earns the marks
- Stating that the speed of a P-wave changes when the material it travels through changes.
- Saying that a change of speed at a boundary causes refraction, so the path bends.
- Linking the refracted paths to a shadow zone where no direct P-waves arrive.
- Using the position of the shadow zone as evidence for the size of the core.
Common mistakes and how to put them right
Saying P-waves travel in straight lines through the Earth.
Claiming P-waves stop at the outer core, which is what S-waves do.
Saying the waves speed up in every new material, rather than that the speed simply changes.
Confusing the narrow P-wave shadow zone with the much larger S-wave shadow.
Exam tips
- Whenever a wave changes speed at a boundary, make so it refracts your next phrase.
- On a cross-section of the Earth, draw P-wave paths as curves that kink at each boundary.
- Use the shadow zone as your evidence when a question asks how we know the size of the core.
Try the lesson
The start of the Waves for detection and exploration (physics only) (HT only): P-waves travel at different speeds through solids and liquids. lesson, made from checked teaching notes for this specification point.
Have a go first
A P-wave from an earthquake travels through a solid rock layer and then into a liquid layer. Which statement describes the speed of the P-wave in the two layers?
Listen
The speed of a seismic wave depends on the properties of the material it is passing through, so when a P-wave crosses from the solid mantle into the liquid outer core its speed changes sharply. A change of speed at a boundary refracts the wave, bending its path, so P-waves do not travel in straight lines through the Earth; their paths curve as the speed changes gradually with depth and kink where it changes suddenly. The consequence is measurable at the surface: there is a band of the globe, roughly 105° to 140° away from the earthquake, where no direct P-waves arrive, called the P-wave shadow zone. Only a core of a particular size with a very different wave speed can produce a shadow of that width. A strong answer earns marks for these points. stating that the speed of a P-wave changes when the material it travels through changes. saying that a change of speed at a boundary causes refraction, so the path bends. linking the refracted paths to a shadow zone where no direct P-waves arrive. using the position of the shadow zone as evidence for the size of the core. Exam tip: Whenever a wave changes speed at a boundary, make so it refracts your next phrase.
Key words
Say what each word means before you turn the card.
speed depends on medium
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refraction inside the Earth
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curved ray paths
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P-wave shadow zone
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See every definition
- speed depends on medium
- The speed of a seismic wave depends on the properties of the material it is passing through, so it changes sharply when a P-wave crosses from the solid mantle into the liquid outer core.
- refraction inside the Earth
- A change of speed at a boundary refracts the wave, bending its path, so P-waves do not travel in straight lines through the Earth.
- curved ray paths
- P-wave paths curve as the speed changes gradually with depth and kink where it changes suddenly.
- P-wave shadow zone
- A band of the globe, roughly 105° to 140° away from the earthquake, where no direct P-waves arrive.
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