4.2 travelling waves. what is a (travelling) wave?
Post on 19-Jan-2016
264 Views
Preview:
TRANSCRIPT
4.2 Travelling waves
What is a (travelling) wave?
Waves
Waves can transfer energy and information without a net motion of the medium through which they travel.
They involve vibrations (oscillations) of some sort.
Wave fronts
Wave fronts highlight the part of a wave that is moving together (in phase).
= wavefront
Ripples formed by a stone falling in water
Rays
Rays highlight the direction of energy transfer.
Transverse waves
The oscillations are perpendicular to the direction of energy transfer.
Direction of energy transfer
oscillation
Transverse waves
Transverse waves
Transverse waves
Transverse waves
peak
trough
Transverse waves
• Water ripples
• Light
• On a rope/slinky
• Earthquake (s)
Longitudinal waves
The oscillations are parallel to the direction of energy transfer.
Direction of energy transfer
oscillation
Longitudinal waves
compression
rarefraction
Longitudinal waves
• Sound
• Slinky
• Earthquake (p)
Other waves - water
A reminder – wave measurements
Displacement - x
This measures the change that has taken place as a result of a wave passing a particular point.
Zero displacement refers to the average position.
= displacement
Amplitude - A
The maximum displacement from the mean position.
amplitude
Period - T
The time taken (in seconds) for one complete oscillation. It is also the time taken for a complete wave to pass a given point.
One complete wave
Frequency - f
The number of oscillations in one second. Measured in Hertz.
50 Hz = 50 vibrations/waves/oscillations in one second.
Wavelength - λ
The shortest distance between points that are in phase (points moving together or “in step”).
wavelength
Wave speed - v
The speed at which the wave fronts pass a stationary observer.
330 m.s-1
Period and frequency
Period and frequency are reciprocals of each other
f = 1/T T = 1/f
The Wave Equation
The time taken for one complete oscillation is the period T. In this time, the wave will have moved one wavelength λ.
The speed of the wave therefore is distance/time
v = λ/T = fλYou need to be able to derive this!
1) A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?
2) A water wave travels through a pond with a speed of 1m/s and a frequency of 5Hz. What is the wavelength of the waves?
3) The speed of sound is 330m/s (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
4) Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light?
Some example wave equation questions
0.2m
0.5m
0.6m/s
3x108m/s
Let’s try some questions!
4.2 Wave equation questions
Representing waves
There are two ways we can represent a wave in a graph;
Displacement/time graph
This looks at the movement of one point of the wave over a period of time
1
Time s
-1
-2
0.1 0.2 0.3 0.4
displacement
cm
Displacement/time graph
This looks at the movement of one point of the wave over a period of time
1
Time s
-1
-2
0.1 0.2 0.3 0.4
displacement
cm
PERIOD
Displacement/time graph
This looks at the movement of one point of the wave over a period of time
1
Time s
-1
-2
0.1 0.2 0.3 0.4
displacement
cm
PERIOD
Displacement/time graph
This looks at the movement of one point of the wave over a period of time
1
Time s
-1
-2
0.1 0.2 0.3 0.4
displacement
cm
PERIOD
IMPORTANT NOTE: This wave could be either transverse or longitudnal
Displacement/distance graph
This is a “snapshot” of the wave at a particular moment
1
Distance cm
-1
-2
0.4 0.8 1.2 1.6
displacement
cm
Displacement/distance graph
This is a “snapshot” of the wave at a particular moment
1
Distance cm
-1
-2
0.4 0.8 1.2 1.6
displacement
cm
WAVELENGTH
Displacement/distance graph
This is a “snapshot” of the wave at a particular moment
1
Distance cm
-1
-2
0.4 0.8 1.2 1.6
displacement
cm
WAVELENGTH
Displacement/distance graph
This is a “snapshot” of the wave at a particular moment
1
Distance cm
-1
-2
0.4 0.8 1.2 1.6
displacement
cm
WAVELENGTH
IMPORTANT NOTE: This wave could also be either transverse or longitudnal
Electromagnetic spectrum
James Clerk Maxwell
Visible light
Visible light
λ ≈ 700 nm λ ≈ 420 nm
Ultraviolet waves
λ ≈ 700 - 420 nm
Ultraviolet waves
λ ≈ 700 - 420 nm λ ≈ 10-7 - 10-8 m
X-rays
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
X-rays
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
Gamma rays
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
Gamma rays
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 mλ ≈ 10-12 - 10-15 m
Infrared waves
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
Infrared waves
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
Microwaves
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
Microwaves
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
λ ≈ 10-2 - 10-3 m
Radio waves
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
λ ≈ 10-2 - 10-3 m
Radio waves
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
λ ≈ 10-2 - 10-3 m
λ ≈ 10-1 - 103 m
Electromagnetic spectrum
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
λ ≈ 10-2 - 10-3 m
λ ≈ 10-1 - 103 m
What do they all have in common?
λ ≈ 700 - 420 nm
λ ≈ 10-7 - 10-8 m
λ ≈ 10-9 - 10-11 m
λ ≈ 10-12 - 10-15 m
λ ≈ 10-4 - 10-6 m
λ ≈ 10-2 - 10-3 m
λ ≈ 10-1 - 103 m
What do they all have in common?
• They can travel in a vacuum• They travel at 3 x 108m.s-1 in a vacuum
(the speed of light)• They are transverse• They are electromagnetic waves (electric
and magnetic fields at right angles to each oscillating perpendicularly to the direction of energy transfer)
What do you need to know?
• Order of the waves
• Approximate wavelength
• Properties (all have the same speed in a vacuum, transverse, electromagnetic waves)
• The Electromagnetic Spectrum
Sound
Sound travels as Longitudinal waves
The oscillations are parallel to the direction of energy transfer.
Direction of energy transfer
oscillation
Longitudinal waves
compression
rarefaction
Amplitude = volume
Pitch = frequency
Range of hearing
Range of hearing
Humans can hear up to a frequency of around 20 000 Hz (20 kHz)
Measuring the speed of sound
Can you quietly and sensibly follow Mr
Porter?
Measuring the speed of sound
• Distance = 140 m
• Three Times =
• Average time =
• Speed = Distance/Average time = m/s
4.2 Measuring the speed of sound
• Measuring the speed of sound using Audacity
String telephones
Sound in solids
• Speed ≈ 6000 m/s
Sound in liquids
• Speed ≈ 1500 m/s
Sound in gases (air)
• Speed ≈ 330 m/s
Sound in a vacuum?
echo
• An echo is simply the reflection of a sound
top related