Topic 4 - Waves Flashcards
Waves transfer energy and information without transferring ……
matter
What is the amplitude of a wave?
The displacement from the rest position to a crest or trough
What is the wavelength of a wave?
The length of a full cycle of the wave
What is the frequency of a wave?
The number of complete cycles of the wave passing a certain point per second
What is the period of wave and what is the formula to work it out?
This is the number of seconds it takes for one full cycle.
period = 1/frequency
Use the example of a twig in water to show that it is only the wave and not the water that travels?
If you drop a twig in water, ripples form on and move across the waters surface. The ripples dont carry the water (or twig) away with them though
Use the example of a guitar string to show that is is only the wave and not the air that travels?
If you strum a guitar string and create a sound wave, the sound wave travels to your ear but it doesnt carry the air away from the guitar - or it would create a vacuum.
What are transverse waves?
Waves with vibrations that are perpendicular to the direction of energy transfer
Give 3 examples of transverse waves?
- all electromagnetic waves
- S-waves
- Ripples and waves in water
What are longitudinal waves?
Waves with vibrations that are parallel to the direction of energy transfer. These waves squash up and stretch out the arrangement of particles to make compressions (high pressure, lots of particles), and rarefactions (low pressure, fewer particles)
Give 2 examples of longitudinal waves?
- Sound waves
2. P-waves
What are the 2 formulas to work out wavespeed?
- wavespeed = distance/ time
2. Wavespeed = frequency * wavelength
Describe how you could use an oscilloscope to measure the speed of sound
- Attach a signal generator to a speaker to generate sounds (set at specific frequency)
- set up oscilloscope so that detected waves at each mics are shown as separate waves
- Start with both mics next to the speaker, then slowly move one away until the two waves are aligned on the display, but have moved exactly one wavelength apart
- measure distance between mics to find one wavelength
- use v = f*wavelength to find the speed of sound passing through air.
How could you measure the speed of water ripples using a strobe light?
- Attach a signal generator to a dipper of a ripple tank to create water waves at a set frequency
- Dim the lights and turn on strobe light. Wave pattern appears on screen below tank
- Alter frequency of strobe light until wave pattern appears to freeze on the screen
- Distance between each shadow line is equal to one wavelength. Measure distance between waves that are 10 wavelengths apart, then find average. use v = f*wl
how could you work out the frequency by using a regular light (ripple tank)
Count how many waves pass a mark on the screen in a given time. Then Divide this by the time in seconds to find frequency
What happens when a wave is absorbed by a second material? (Energy)
The wave transfers energy to the materials energy stores. Often, energy is transfered to thermal energy store, causing heating
What happens when a wave is transmitted through a second material and how is this useful?
The wave carries on travelling through the second material. This often leads to refraction. This can be used in communications as well as in lenses of glasses and cameras
What happens when a wave is reflected from a hard flat surface?
The incoming ray is sent back away from the hard flat surface. This is how echoes are created
When does refraction happen and how does it affect speed?
When a wave crosses a boundary between materials at an angle and so the wave changes speed
The …… a change in speed, the more the wave bends (refraction)
greater
Why is white light a spectrum?
Shorter wavelengths bend more. This can lead to wavelengths spreading out (dispersion)
What causes a change in speed in waves? Is it the frequency or the wavelength
The wavelength.
What is the angle of incidence?
The angle between the incident ray and the normal
What is the angle of refraction?
The angle between the refracted ray and the normal
What are sound waves caused by?
Vibrating objects
Why can’t all frequencies of sound be transferred through an object?
An objects size, shape and structure determines which frequencies it can transmit
Does sound travel fastest in solids, liquids or gases?
Solids
Why can’t sound travel in space?
Because Space is mostly a vacuum (there are no particles to move or vibrate)
How do we hear sound? (reference to our ears)
- Sound waves that reach our eardrum cause it to vibrate
- Vibrations are passed onto ossicles, to semicircular canals and then to the cochlea
- Cochlea turns vibrations into electrical signals which get sent to our brain
- Brain interprets signals as sounds of different pitches and volumes , depending on their frequency and intensity
A higher frequency sound wave has a ………. pitch
Higher
What is human hearing limited by?
The size and shape of our eardrum, and the structure of all the parts within the ear that vibrate to transmit the sound wave
What is ultrasound?
Sound with frequencies higher than 20,000 Hz
Explain how ultrasound waves get partially reflected at boundaries?
When a wave passes into a different medium, some of the wave is reflected and some is transmitted. (partial reflection)
How is ultrasound used in medical imaging?
1) ultrasound can pass through body but when it reaches a different medium (from fluid in the womb to the skin of the foetus), some of the wave is reflected back and detected
2) the exact timing and distribution of these echoes are processed by a computer to produce a video image of the foetus
How is ultrasound used in industrial imaging?
- can be used to find flaws in objects e.g. pipes, wood or metal
- Ultrasound waves entering material are usually reflected by far side of material
- If there is a flaw e.g. crack, waves are reflected sooner
What does ultrasound allow us to do in relation to sonar?
- find out distance to sea bed
2. locate objects in deep water
What is infrasound waves?
Sound with frequencies lower than 20 Hz
Why is detecting infrasound important? (3)
- Allows us to track animals that use infrasound to communicate for conservation purposes
- Scientists can monitor infrasound to try to predict events that produce infrasound in local areas e.g. volcanoes
- Some infrasound waves, produced by earthquakes, travel through different layers of the earth. They can be used to explore the structure of the earth
What type of wave do earthquakes and explosions cause?
Seismic waves
What device can be used to detect these waves?
Seismometers
What do seismologists do
they work out the time it takes for waves to reach seismometers. They also note which parts of the Earth dont receive the waves at all
What happens when seismic waves reach a boundary between different layers of material in the earth?
Some waves will be absorbed and some will be refracted
What happens if the seismic waves are refracted?
They change speed gradually, resulting in a curved path
What happens when the properties (material) change suddenly?
The wave speed changes abruptly and the path has a kink
What is the main difference between P-waves and S-waves?
P-waves can travel through the earth’s core whilst S-waves can’t
How can we use peak frequency to find the speed of waves in solids?
- Measure and record the length of a metal rod
- set up apparatus
- Tap rod with hammer. write down peak frequency displayed on the computer
- repeat 3 times to get average peak frequency
- use v = f*wl where wl is equal to twice the length of the rod
How can we use sound waves around the rod to find the frequency of the waves IN the rod
Hitting rod causes waves to be produced along rod. These make the rod vibrate and produce sound waves in the air around the rod. These sound waves have the same frequencies as the waves in the rod
