Topic 4 - Waves Flashcards
Waves basics
What do waves do?
Waves transfer energy from one place to another without transferring any matter.
Waves basics
What happens when a wave travels through a medium?
The particles of the medium vibrate and transfer energy between each other e.g. if you drop a twig in a stagnant pool the ripples don’t carry the twig away as the water only moves up and down.
Waves basics
Define amplitude
Maximum displacement from rest position of a wave that measures the amount of energy the wave transfers
Waves basics
Define trough
The lowest point of the wave
Waves basics
Define crest
The highest point of the wave
Waves basics
What is the rest position of a wave
The point of zero displacement on a wave (undisturbed)
Waves basics
What is the wavelength of a wave
The distance from trough to trough or crest to crest measured in metres
Waves basics
Define frequency
Number of oscillations a wave completes every second (Hz). 1 Hz is 1 wave per second
Waves basics
Define period
The time it takes for one oscillation to occur. (seconds)
Period = 1 / frequency
Waves basics
Define wave speed
Wave speed (m/s) = distance (m) / time (seconds)
How far the wave travels every second
Transverse and longitudinal
What is a transverse wave?
Give examples
A wave where the oscillations are perpendicular to the direction the wave travels/perpendicular to the direction of energy transfer.
Examples in include:
* All electromagnetic waves
* S-waves
* Water waves
Transverse and longitudinal
What is a longitudinal wave?
Give examples
A wave where the oscillations are parallel to the direction the wave travels
Examples include:
* Sound waves
* P-waves
Measuring wave speed
How do you measure the speed of sound using on oscilloscope?
- Set up the oscilloscope so the detected waves at each microphone are shown as separate waves.
- Start with both microphones next to the speaker then move one away until the 2 waves are alinged again but 1 wavelength apart.
- Measure distance between 2 microphones which = wavelength.
- Use formula velocity = frequency * wavelength (the frequency = frequency set in the signal generator)
Measuring speed of water waves
How do you measure the speed of water ripples?
- Using a signal generator attached to the dipper of a ripple tank you can create water waves at a set frequency.
- Dim the lights in the room and turn on a strobe light above the ripple tank which will create a wave pattern shadow below.
- Alter the frequency of the strobe light so the shadows appear to be still
- Distance between 2 shadows = wavelength (λ)
- Measure to find average
- Use velocity (m/s) = frequency (Hz) * wavelength (λ) to calculate the speed
Waves at boundaries
What are the 3 possible outcomes when a wave reaches a boundary between 2 materials?
- Absorbed - The wave transfers energy into the material’s energy stores (often thermal)
- Transmitted - The wave carries on travelling through the material (leads to refraction)
- Reflection - The wave is ‘sent back’
Why would different substances and waves do one of the three?
(Absorption, transmission, reflection)
Depends on the wavelength and the properties of the material
Refraction
What affects a waves speed?
Density of the substance it is travelling through
Refraction
What happens when a wave hits a boundary at an angle?
Its change in speed causes a change in direction
Refraction
As the change in speed increases, how does this impact the change in direction?
Increases change in direction
Refraction
What is a normal?
An imaginary perpendicular line to the boundary
Refraction
Fill in the gaps:
The wave bends ??? the normal if it slows down and ??? from the normal if it speed up
The wave bends towards the normal if it slows down and away from the normal if it speed up
Refraction
What type of wave usually goes slower in denser materials?
EM (electromagnetic) waves such as light
Refraction
Pick the correct answer:
Short/long wavelengths bend more when crossing a boundary
Short wavelengths bend more when crossing a boundary
Refraction
What happens to the wavelength of a wave as it is slowed down?
The wavelength decreases
Critical angles
What is total internal reflection?
When a wave crosses a boundary it is reflected back into a material
Critical angles
When does total internal reflection occur?
When the angle of incidence > the critical angle
Critical angles
What is the critical angle
The angles where a ray travelling in a tranparent medium can hit the boundary without being reflected in the medium
Sound waves
What are sound waves caused by?
Vibrating objects
Sound waves
Are sound waves transverse or longitudinal
Longitudinal waves
Sound waves
What happens when a sound wave passes through a solid?
Particles in the solid vibrate
Sound waves
From slowest to fastest, state the speed of sound waves in the three states of matter
(solid, liquid,gas)
- Gas
- Liquid
- Solid
This is because longitudinal waves travel through particle collision, and because solids have the highest density, sound travels faster in solids, making gas slowest due to lowest density of particles.
Sound waves
What happens to the frequency when it travels through a different medium?
The frequency of the sound wave doesn’t change
Sound waves
What is the range of frequencies that the human ear can detect (hear)?
20 - 20,000Hz
Anything below 20 Hz = Infrasound
Anyhting above 20,000 Hz = Ultrasound
The human ear
How does the ear work?
Sound waves that reach the eardrum cause it to vibrate. The vibrations are passed through your earbone to the semicircular canals and into the cochlea.
The cochlea converts these vibrations into electrical signals that are sent to the brain and are interpreted by it as sound.
Below is a link that contains an annotated diagram of the ear. Copy it into google to view:
[https://www.savemyexams.com/a-level/physics/aqa/17/revision-notes/10-medical-physics/10-2-physics-of-the-ear/10-2-1-structure-of-the-ear/]
Using Infrasound and Ultrasound
How can ultrasound be used to look inside things?
Ultrasound waves can pass through the body but wehn they reach a boundary between 2 medias some of the wave is refected back which is processed by a computer to form an image.
Using Infrasound and Ultrasound
How can ultrasound be used for sonar to locate objects in water
A pulse of ultrasound is sent downwards by the submarine (travelling at 1500m/s) and it takes 4 seconds to get to the sea floor and back. 4 / 2 = 2 seconds. 2 * 1500 =3,000 3000m - So the waiter is 3000m deep.
Core Practical
Describe a practical to measure wave speed on water
- Set up a ripple tank with a ruler above the water
- Change the voltage at the motor until 2 waves can be seen at once (around half the length of the tank
- Count how many waves are formed in 10 seconds and record it
- Estimate wavelength using ruler. Use wavelength and frequency to find out the wave speed
Core Practical
Describe an experiment to measure wave speed in solids?
- Suspend a metal rod hoprizontally using clamp stands and rubber bands
- Hit one end of the rod using a hammer and hold a frequency meter and **note down the frequency.
- Measure the length of the rod - The wavelength will be 2 * the rod
- Use the frequency and wavelength to find the speed of sound in a rod**
