Transcript Longitudinal waves

13/04/2015
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
13/04/2015
Some definitions…
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)
13/04/2015
“Wave behaviour”
Anything that travels as a
wave demonstrates wave
behaviour – in other words, it
can be reflected, refracted
and diffracted:
Reflection
Refraction
Diffraction
13/04/2015
Transverse vs. longitudinal waves
Transverse waves are
when the displacement
is at right angles to
the direction of the
wave (e.g. light and
other electromagnetic
waves)…
Displacement
13/04/2015
Displacement
Direction
Direction
Longitudinal waves
are when the
displacement is
parallel to the
direction of the wave
(e.g. sound waves)…
Where are the compressions and rarefactions?
The Electromagnetic Spectrum
13/04/2015
Each type of radiation shown in the electromagnetic spectrum has a
different wavelength and a different frequency:
High frequency,
_____ wavelength
Gamma
rays
X-rays
Low frequency, _____
(high) wavelength
Ultra violet
Visible
light
Infra red
Microwaves
Radio/TV
γ
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
The Electromagnetic Spectrum
13/04/2015
Type of radiation
Uses
Dangers
Gamma rays
Treating cancer,
sterilisation
Cell mutation
X rays
Medical
Cell mutation
Ultra violet
Sun beds
Skin cancer
Visible light
Seeing things
None (unless you
look at the sun)
Infra red
Remote controls,
heat transfer
Sunburn
Microwaves
Satellites, phones
Very few
TV/radio
Communications
Very few
The Wave Equation
13/04/2015
All E-M waves obey the Wave Equation:
Wave speed (v) = frequency (f) x wavelength ()
in m/s
in Hz
in m
V
f

Some example wave equation questions
13/04/2015
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 6x10-7m and a
frequency of 5x1014Hz. What is the speed of purple
light?
3x108ms-1
How sound travels…
13/04/2015
As we know, sound waves are formed when something vibrates.
But how does the sound reach our ears?
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
13/04/2015
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
13/04/2015
Angle of incidence = Angle of reflection
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
13/04/2015
Evidence about the origins of
the universe…
13/04/2015
Source of
light
“Spectra”
13/04/2015
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…
13/04/2015
Before
helium
helium
After
The absorption lines have all been “shifted”
towards the longer wavelength end (red end)…
This is called red
shift. The faster
the light source
moves the further
its light will be
“shifted”
Before
After
A similar effect happens with sound –
this is called “The Doppler Effect”
13/04/2015
Red Shift simplified
13/04/2015
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…
13/04/2015
13/04/2015
13/04/2015
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
13/04/2015
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
13/04/2015
This slideshow has been made freely available on the TES
Resources website.
More Science PowerPoints like this can be found at the
website www.educationusingpowerpoint.com. This site
contains slideshows that cover the 2011 AQA, EdExcel,
OCR Gateway and OCR 21st Century courses (with more
material being added every year) and A Level Physics and
KS3 material.
Some slideshows are free, others require a small
subscription fee to be taken out (currently only £50 for a
year). Further details can be found at Education Using
PowerPoint.