Topic 6 - Waves Flashcards
Waves
> Waves carry energy from one place to another and can also carry information.
When waves travel through a medium, the particles of the medium oscillate and transfer energy between each other.
Only energy is transferred, the particles stay in the same place.
Types of wave
- Transverse
2. Longitudinal
Amplitude - definition
> The amplitude of a wave is the maximum displacement of a point on the wave from its undisturbed position.
Wavelength - definition
> The wavelength is the distance between the same point on two adjacent waves.
E.g. trough to trough.
Frequency
> Frequency is the number of complete waves passing a certain point per second.
It’s measured in Hertz.
1 Hz = 1 wave per second.
Period of a wave
> The period of a wave is the amount of time it takes for a full cycle of the wave to pass a point.
Period = 1 divided by Frequency.
Seconds = 1 divided by Hz.
Wave Speed
> The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.
Transverse Waves
> In transverse waves, the oscillations are perpendicular to the direction of energy transfer.
Most waves are transverse, inc. all electromagnetic waves (e.g. light), ripples and waves in water and a wave on a string, spring wiggled from side to side.
Mechanical Wave - definition
> A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium.
Water waves, shock waves and waves in springs and ropes are all examples of mechanical waves.
Longitudinal waves
> In longitudinal waves, the oscillations are parallel to the direction of energy transfer.
If you push a spring you get a longitudinal wave.
Examples: sound waves in air, ultrasound, shock waves e.g. seismic waves.
Longitudinal waves show areas of compression and rarefaction.
Rarefaction - definition
> A rarefaction is a region in a longitudinal wave where the particles are furthest apart.
Compression - definition
> A compression is a region in a longitudinal wave where the particles are closest together.
The wave equation
>Applies to all waves. >wave speed = frequency × wavelength v = f λ >wave speed, v, in metres per second, m/s >frequency, f, in hertz, Hz >wavelength, λ, in metres, m
Experiments with waves - measuring the speed of sound - steps
> By attaching a signal generator to a speaker you can generate sounds with a specific frequency.
You can use 2 microphones and an oscilloscope to find the wavelength of the sound waves generated.
1. Set up the oscilloscope so the detected waves at each microphone are shown as separate waves.
2. Start with both microphones 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.
3. Measure the distance between the microphones to find one wavelength.
4. You can then use the wave speed formula to find the speed of the sound waves passing through air - the frequency is whatever you set the signal generator to.
Experiments with waves - measuring the speed of water ripples practical - steps
> Using a signal generator attached to the dipper of a ripple tank, you can create water waves at a set frequency.
- Dim the lights and turn on the lamp - you’ll see a wave pattern made by the shadows of the wave crests on the screen below the tank.
- The distance between each shadow line is equal to one wavelength. Measure the distance between the shadow lines that are 10 wave lengths apart, then divide this distance by 10 to find the average wavelength. This is a suitable method for measuring small wavelengths.
- If you’re struggling to measure the distance, you could always take a photo of the shadows and ruler, and find the wavelength from the photo instead.
- Use the wave speed formula to find the speed of the waves.
- This set-up is suitable for investigating waves, because it allows you to measure the wavelength without disturbing the waves.
Experiments with waves - finding the speed of a wave on string - steps
> You use a signal generator, but this time you attach it to a vibration transducer which converts the signals to vibrations.
- Set up the equipment shown in book, then turn on the signal generator and vibration transducer. The string will start to vibrate.
- You can adjust the frequency setting on the generator to change the length of the wave created on the string. You should keep adjusting the frequency of the signal generator until there appears to be a clear wave on the string. This happens when a whole number of half-waves fit exactly on the string (you want at least 4 or 5 ideally). The frequency you need will depend on the length of string between the pulley and the transducer, and the masses you’ve used.
- You need to measure the wavelength of the wave. Best way to do so accurately is to measure the length of all the half-wavelengths on the string in one go, then divide by the total number of half-wavelengths to get the mean half-wavelength. You can then double this value to get the full wavelength.
- The frequency is whatever you set the signal generator to.
- Find speed using formula.
Experiments with waves - finding the speed of a wave on string - set-up
> This set-up is suitable for investigating waves on a string because it’s easy to see and measure the wavelength and frequency.
Signal generator attached to vibration transducer which is attached to a string and pulley with masses on end of strong.
All on a bench with pulley and masses coming off edge of bench.
Waves - the three actions
> When waves arrive at a boundary between 2 different materials, 3 things can happen:
- Absorbed
- Transmitted
- Reflected
What can happen when waves arrive at a boundary between 2 different materials?
> When waves arrive at a boundary between 2 different materials, 3 things can happen:
1. The waves are absorbed by the material the wave is trying to cross into - this transfers energy to the material’s energy stores. This is how microwaves work.
2. The waves are transmitted - the waves carry on travelling through the new material. This often leads to refraction.
3. The waves are reflected.
What actually happens depends on the wavelength of the wave and the properties of the materials involved.
Rule of reflection
> Angle of incidence = angle of reflection.
>See diagram in book for ray diagram.
Angle of incidence - definiton
> The angle of incidence is the angle between the incoming wave and the normal.
Angle of reflection - definition
> The angle between the reflected wave and the normal.
The normal - defintion
> The normal is an imaginary line that’s perpendicular to the surface at the point of incidence (point where the wave hits the boundary).
Usually shown by a dotted line.
Types of reflection
> Waves are reflected at different boundaries in different ways:
- Specular
- Diffuse