Topic 6 - Waves Flashcards
What do waves do?
- Waves transfer energy from one place to another wihtout transferring any matter.
What is the AMPLITUDE of a wave?
- The maximum displacement of a point on the wave from its undisturbed position.
What is the WAVELENGTH of a wave?
- The distance between the same point of two adjacent waves.
What is the FREQUENCY of a wave?
- The number of complete waves passing a certain point per second.
- It is measured in hertz (Hz).
What is the PERIOD of a wave?
- The amount of time it takes for a full cycle of the wave.
How to calculate frequency?
Frequency = 1 / Period
What are the two types of wave?
- Transverse and longitudinal
What kind of vibrations do transverse waves have?
- Sideways vibrations
Describe transverse waves?
- The oscillations are perpendicular (90 degrees) to the direction of energy transfer.
Types of transverse wave?
- Electromagnetic waves
- Ripples and waves in water
- A wave on a string
What kind of vibrations do longitudinal waves have?
- Parallel vibrations
Describe longitudinal waves?
- The oscillations are parallel to the direction of energy transfer.
Types of longitudinal wave?
- Sound waves in air, ultrasound.
- Shock waves, e.g. some seismic waves.
What is the formula for wavespeed?
- Wave speed = frequency x wavelength.
What are sound waves?
- Sound waves are caused by vibrating objects. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions.
- Sound is a type of longitudinal wave.
- Sound generally travels faster in solids than in liquids, and faster in liquids than in gases.
- When a sound wave travels through a solid it does so by causing the particles in the solid to vibrate.
Can sound travel through space?
- No. Sound cannot travel through space because it is mostly a vacuum (there are no particles to vibrate).
How do you hear sound?
- You hear sound when your eardrum vibrates. Sound waves cause your eardrum to vibrate.
- These vibrations are passed on to tiny bones in your ear called ossicles, through the semicircular canals and to the cochlea.
- The cochlea turns these vibrations into electrical signals which get sent to your brain and allow you to sense the sound.
What is the range of sound for humans?
- 20Hz - 20kHz
What is echosounding?
- Echo sounding uses pulses of high-frequency sound waves to detect objects in deep water and to measure water depth.
- An echo is the reflection of soundwaves from a smooth surface.
- The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object.
How to calculate depth of water?
- 0.5 x vt (1/2 x the wave speed x the time)
What is ultrasound?
- Ultrasound is sound with frequencies higher than 20,000 Hz
What happens to ultrasound at boundaries?
- Ultrasound waves get partially reflected at boundaries. When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two media, and some is transmitted (and refracted). This is partial reflection.
- This means that wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back.
- The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is.
What are uses of ultrasound?
- Medical imaging. Ultrasound waves can pass through the body, but whenever they reach a boundary between two different media (like the fluid in the womb and the skin of the foetus) some of the wave is reflected back and detected.
- The exact timing and distribution of these echoes are processed by a computer to produce a video image of the foetus.
- Industrial imaging. Ultrasound can also be used to find flaws in objects such as pipes or materials like wood and metal. Ultrasound waves entering a material will usually be reflected by the far side of the material. If there is a flaw such as a crack inside the object, the wave will be reflected sooner.
What are seismic waves?
- The energy transferred from an earthquake produces shock waves called siesmic waves.
What provides information about the structure of the earth?
- The study of seismic waves.
Where are earthquakes generated?
- The earths crust. This is a solid layer of rock about 50km thick that surrounds a much thicker layer of molten rock called the mantle.
Where is the point that an earthquake originates from?
- The focus
What are primary waves (P-waves)?
- Primary waves cause the initial tremors lasting about one minute.
- P-waves are longitudinal.
- They can travel through liquids and solids.
- They travel faster than S-waves.
What are secondary waves (S-waves)?
- S-waves cause more tremors a few minutes later.
- S-waves are transverse.
- They can’t travel through liquids or gases, so they can’t travel through the liquid outer core.
- They are slower than P-waves.
What happens to P and S wave as they travel through the mantle?
- They bend as they travel through the mantle. This is because their speed changes gradually with depth, and so their direction changes, resulting in a curved path.
What happens to P-waves when they reach the mantle and outer core boundary?
- They refract because their speed changes abruptly at the boundary.
What happens to S-waves when they reach the mantle and outer core boundary?
- S-waves are transverse - they cannot travel through the liquid outer core.
What are shadow zones?
- Areas on the earth’s surface where seismic waves are not detected after an earthquake.
Where are shadow zones?
- Usually from around 105 degrees to 142 degrees from the earthquakes epicenter.
Why can P-waves not reach the shadow zone?
- P-waves are at the boundary between the mantle and the outer core when the waves enter the core and leave the core.
- This refraction bends the waves away from the shadow zone.
Why can S-waves not reach the shadow zone?
- S-waves cannot travel through the other core because they are transverse waves and can’t travel through a liquid.
- The S-waves are completely blocked by the outer-core.
