Transcript Longitudinal waves
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AQA 2011 Physics Unit 1
This PowerPoint supports section P1.5 of the 2011 AQA Physics Unit 1 module W Richards
An introduction to Waves
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Some definitions…
25/04/2020 1) Amplitude – this is “how high” the wave is:
2) Wavelength (
) – this is the distance between two corresponding points on the wave and is measured in metres: 3) Frequency – this is how many waves pass by every second and is measured in Hertz (Hz)
“Wave behaviour”
Anything that travels as a wave demonstrates wave behaviour – in other words, it can be reflected, refracted and diffracted:
Reflection
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Refraction Diffraction
Transverse waves are when the the waves)… displacement is at right angles to direction of the wave (e.g. light and other electromagnetic Direction Displacement
Longitudinal waves
are when the displacement direction is parallel to the of the wave (e.g. sound waves)… Direction Where are the compressions and rarefactions?
Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency: High frequency, _____ wavelength Gamma rays γ X-rays Ultra violet Visible light Infra red Low frequency, _____ (high) wavelength Microwaves Radio/TV Each of these types travels at the same speed through a _______ (300,000,000m/s), and different wavelengths are absorbed by different surfaces (e.g. infra red is absorbed very well by ___________ surfaces). This absorption may heat the material up (like infra red and _______) or cause an alternating current (like in a __ _______).
Words – black, microwaves, long, short, TV aerial, vacuum
Type of radiation Gamma rays X rays Ultra violet Visible light Infra red Microwaves TV/radio Uses Treating cancer, sterilisation Medical Sun beds Seeing things Remote controls, heat transfer Satellites, phones Communications Dangers Cell mutation Cell mutation Skin cancer None (unless you look at the sun) Sunburn Very few Very few
The Wave Equation
All E-M waves obey the Wave Equation: 25/04/2020 Wave speed (v) = frequency (f) x wavelength (
)
in m/s in Hz in m V f
1) A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?
0.6ms
-1 2) A water wave travels through a pond with a speed of 1ms -1 and a frequency of 5Hz. What is the wavelength of the waves?
0.2m
3) The speed of sound is 330ms -1 (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
0.5m
4) Purple light has a wavelength of around 6x10 -7 m and a frequency of 5x10 14 Hz. What is the speed of purple light?
3x10 8 ms -1
How sound travels…
But how does the sound reach our ears?
25/04/2020 As we know, sound waves are formed when something vibrates. Air molecules 1) An object makes a sound by vibrating 2) The vibrations pass through air by making air molecules vibrate 3) These vibrations are picked up by the ear
Transmitting information
25/04/2020 Although E-M radiation travels in straight lines, we can send infra-red and light signals around a curved path using an optical fibre: Optical fibres have two main advantages: they can send more information compared to electrical cables of the same diameter and with less signal weakening.
Microwaves are used by satellites because they can pass through the Earth’s atmosphere: Microwaves are also used in mobile phone networks.
Reflection
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Angle of incidence = Angle of reflection
Normal
Incident ray Reflected ray
Angle of incidence Angle of reflection
Mirror
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Evidence about the origins of the universe…
Source of light
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“Spectra”
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If you pass the light through a gas something different is seen…
helium
Some wavelengths of light are absorbed by the gas – an “absorption spectrum”.
If the light source is moving away the absorption spectra look a little different…
Before
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After
The absorption lines have all been “shifted” towards the longer wavelength end (red end)…
Before
This is called red shift. The faster the light source moves the further its light will be “shifted” 25/04/2020
After
A similar effect happens with sound – this is called “The Doppler Effect”
Red Shift simplified
25/04/2020 Basically, if I walk towards you I’ll look slightly more blue. Then, if I walk away from you, I’ll look slightly more red!!
Let’s try it…
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25/04/2020 Light from different stars and from the edge of the universe also shows this “red-shift”. This suggests that everything in the universe is moving away from a single point.
This is the BIG BANG theory
Red shift summary
25/04/2020 Light from other galaxies has a longer _________ than expected. This shows that these galaxies are moving ____ from us very quickly. This effect is seen to a greater extent in galaxies that are _______ away from us. This indicates that the further away the galaxy is, the ______ it is moving.
This evidence seems to suggest that everything in the universe is moving away from a single point, and that this process started around 15 _____ years ago. This is the ____ ________ Theory. Further evidence of this theory is Cosmic Microwave Background Radiation (CMBR) – this radiation comes from the Big Bang and fills the _________.
Words to use – faster, away, universe, big bang, billion, wavelength, further
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