Transcript of Topic 4.4 Wave characteristics. Learning outcomesTeacher’s notes 4.4.1Describe a wave pulse and a...
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- Topic 4.4 Wave characteristics
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- Learning outcomesTeachers notes 4.4.1Describe a wave pulse and
a continuous progressive (travelling) wave. Students should be able
to distinguish between oscillations and wave motion, and appreciate
that, in many examples, the oscillations of the particles are
simple harmonic. 4.4.2State that progressive (travelling) waves
transfer energy. Students should understand that there is no net
motion of the medium through which the wave travels. 4.4.3Describe
and give examples of transverse and of longitudinal waves. Students
should describe the waves in terms of the direction of oscillation
of particles in the wave relative to the direction of transfer of
energy by the wave. Students should know that sound waves are
longitudinal, that light waves are transverse and that transverse
waves cannot be propagated in gases. 4.4.4Describe waves in two
dimensions, including the concepts of wavefronts and of rays.
4.4.5Describe the terms crest, trough, compression and rarefaction.
4.4.6Define the terms displacement, amplitude, frequency, period,
wavelength, wave speed and intensity. Students should know that
intensity amplitude2. 4.4.7Draw and explain displacementtime graphs
and displacementposition graphs for transverse and for longitudinal
waves. 4.4.8Derive and apply the relationship between wave speed,
wavelength and frequency. 4.4.9State that all electromagnetic waves
travel with the same speed in free space, and recall the orders of
magnitude of the wavelengths of the principal radiations in the
electromagnetic spectrum.
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- Waves and Oscillations Oscillation: the vibration of an object
Wave: can be a pulse wave or a continuous progressive (travelling)
wave. o Pulse: single oscillation or disturbance o Continuous
traveling wave: succession of oscillations (series of periodic
pulses) Both pulses and traveling waves: transfer energy though
there is no net motion of the medium through which the wave passes.
o A wave transfers energy without a transfer of matter
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- TRAVELLING WAVES Definition: A travelling wave (or progressive
wave) is one which travels out from the source that made it and
transfers energy from one point to another. Students should
understand that there is no net motion of the medium through which
the wave travels.
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- Transverse vs longitudinal Types of Traveling Waves
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- Transverse vs. longitudinal waves Transverse waves are when the
particles in the medium vibrate at right angles to the direction of
energy transfer Longitudinal waves are when the particles (of the
medium) vibrate in the same direction as the direction of energy
transfer; Displacement of particles Direction of wave/energy
transfer Displacement of particles Direction of wave/energy
transfer
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- Transverse wave
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- Transverse waves Examples of transverse waves: light, violin
and guitar strings, ropes, earthquake S waves Transverse waves
cannot propagate in a gas or a liquid because there is no mechanism
for driving motion perpendicular to the propagation of the wave
http://www.acoustics.salford.ac.uk/f eschools/waves/bungyvideo.htm
http://www.acoustics.salford.ac.uk/f
eschools/waves/wavetypes.htm
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-
http://www.acoustics.salford.ac.uk/feschools/waves/guitarvideo.htm
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- Longitudinal waves Sound waves and earthquake P-waves are
longitudinal
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- Longitudinal slinky http://www.acoustics.salford.ac.uk/f
eschools/waves/slinkyvideo.htm
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- Loudspeaker
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- Be a Thinker!
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- Waves in Two Dimensions
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- Wavefronts and rays Ray: direction in which wave (energy) is
travelling; Wavefront: line joining neighbouring points that have
the same phase /displacement. The distance between two successive
wavefronts is one wavelength The ray is always normal to a
wavefront;
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- Describe waves in two dimensions, including the concepts of
wavefronts and of rays. Energy is transferred in 2 dimensions Watch
the wavefront(s) propagate http://www.acoustics.salford.ac.uk/f
eschools/waves/dripvideo.htm
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- Wavefronts and rays Rays show the direction of travel of the
energy. The wavefronts are where the crests of the waves are. The
rays are always at 90 deg to the wavefronts. rays Wavefronts
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- Characteristics of a Wave.
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- Longitudinal waves v Compression is a term used in connection
with longitudinal wave and refers to the region where the particles
of the medium are "bunched up". High density High pressure v
Rarefaction is a term used in connection with longitudinal waves
referring to the regions where the particles are "stretched out".
Low density Low pressure
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- Longitudinal waves v The wavelength will be equal to the
distance between successive points of maximum compression and
successive points of maximum rarefaction. v The compression is the
region in which the molecules of the air are pushed together v The
rarefaction is the region where the molecules move apart.
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- Transverse waves Crests Troughs
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- Displacement graphs This graph represents a snapshot of the
vibrating medium (ex: string) at a specific instant of time. It
shows the displacement of each particle of the medium away from the
equilibrium position at that specific instant of time The
information that can be deduced from this graph: The amplitude of
the wave The wavelength The relative position of each particle in
the medium at this specific time
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- Displacement graphs This graph represents the of the variation
of the displacement of one specific particle of the medium away
from the equilibrium position with time The information that can be
deduced from this graph: The amplitude of the wave The period of
the wave
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- Define the terms displacement, amplitude, frequency, period,
wavelength, wave speed and intensity WAVELENGTH - the distance from
one crest to another or one trough to another. (In fact generally
from any point on the wave to the next exactly similar point i.e. 2
consecutive points in phase) FREQUENCY - the number of vibrations
of any part of the wave per second. The bigger the frequency the
higher the pitch of the note or the bluer the light AMPLITUDE the
maximum displacement that any point on the wave moves from its mean
position. The bigger the amplitude the louder the sound, the
rougher the sea, or the brighter the light
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- More terms Period (T) The time it takes for one complete cycle
of the wave. Displacement (x) How far the particle has travelled
from its mean/equilibrium position. Wave speed (v) The speed at
which the wavefronts pass a stationary observer Intensity (I) The
power per unit area that is received by an observer. Students
should know that intensity ~ amplitude 2
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- Derive and apply the relationship between wave speed,
wavelength and frequency. Wave speed (v) = frequency (f) x
wavelength ( ) in m/s in Hz in m The wave equation relates the
speed of the wave to its frequency and wavelength:
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- More about the speed of the Wave The Speed of waves only
depends on the nature and the properties of the medium w Water
waves do travel faster in deeper water w Light travels slower in
more optically dense material The frequency of a wave depends only
on the source producing the wave w It will therefore not change if
the wave enters a different medium or the properties of the medium
change The wavelength changes according to the wave equation V=f
Example: when water waves approach the shore (shallow water),
velocity decreases, frequency remains constant, so, wavelength
decreases. (Tsunami only appears in shallow water)
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- Apply your Knowledge!
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- Be a Thinker! Which of the following best describes the wave
speed of a progressive wave travelling through a medium? A.The
maximum speed of the vibrating particles of the medium B.The
average speed of the vibrating particles of the medium C.The speed
of the medium through which the wave travels D.The speed of
transfer of energy through the medium
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- Be a Thinker!
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- Intensity Source:
http://www.physicsclassroom.com/Class/sound/u11l2b.cfm
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- Intensity The diagram at the right shows that the sound wave in
a 2-dimensional medium is spreading out in space over a circular
pattern. Since energy is conserved and the area through which this
energy is transported is increasing, the power (being a quantity
that is measured on a per area basis) must decrease. The
mathematical relationship between intensity and distance is
sometimes referred to as an inverse square relationship. The
intensity varies inversely with the square of the distance from the
source. So if the distance from the source is: doubled (increased
by a factor of 2), then the intensity is quartered (decreased by a
factor of 4). Similarly, if the distance from the source is
quadrupled, then the intensity is decreased by a factor of 16.
Source: http://www.physicsclassroom.com/Class/sound/u11l2b.cfm
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- Electromagnetic waves
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- Frequencies of Regions (Hz) Gamma Rays >10 21 X-rays 10 17 -
10 21 Ultraviolet 10 14 - 10 17 Violet 7.5 x 10 14 > Visible
> Red 4.3 x 10 14 Infrared 10 11 -10 14 Microwaves 10 9 -10 11
Radio and TV < 10 9
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- The Different Regions v In the context of wave motion, common
properties of all parts of the electromagnetic spectrum are w all
transverse waves w all travel at the speed of light in vacuum (3.0
x 10 8 ms -1 ) w all can travel in a vacuum
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- Sources of Regions w Gamma certain radioactive materials nuclei
w X-rays by firing an electron stream at a tungsten metal target in
a highly evacuated tube. w Ultraviolet the Sun, ultraviolet lamp w
Visible hot bodies w Infrared the Sun (heat), hot bodies w
Microwaves Ovens, communication systems w Radio and TV transmitter
stations, Azteca TV