6.1 Waves Flashcards
Waves
Waves transfer energy from one place to another without transferring matter. Wave motion (the movement of waves) can be shown by the vibrations of a spring or by water waves.
Transverse waves
Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer.
For a transverse wave:
The energy transfer is perpendicular to wave motion.
They transfer energy, but not the particles of the medium.
They can move in solids and on the surfaces of liquids but not inside liquids or gases.
Some transverse waves (electromagnetic waves) can move in solids, liquids and gases and in a vacuum.
The point on the wave that is:
The highest above the rest position is called the peak, or crest.
The lowest below the rest position is called the trough.
Longitudinal waves
Waves where the points along its length vibrate parallel to the direction of energy transfer.
For a longitudinal wave:
The energy transfer is in the same direction as the wave motion.
They transfer energy, but not the particles of the medium.
They can move in solids, liquids and gases.
They can not move in a vacuum (since there are no particles).
The key features of a longitudinal wave are where the points are:
Close together, called compressions.
Spaced apart, called rarefactions.
Amplitude
The distance from the undisturbed position to the peak or trough of a wave.
It is given the symbol A and is measured in metres (m).
Amplitude is the maximum or minimum displacement from the undisturbed position.
Wavelength
The distance from one point on the wave to the same point on the next wave.
In a transverse wave:
The wavelength can be measured from one peak to the next peak
In a longitudinal wave.
The wavelength can be measured from the centre of one compression to the centre of the next.
The wavelength is given the symbol λ (lambda) and is measured in metres (m).
Frequency
The number of waves passing a point in a second.
Frequency is given the symbol f and is measured in Hertz (Hz).
Frequency = 1 / time period
Time period
The time taken for a single wave to pass a point.
This can also be stated as, The time taken for one full cycle of a wave.
The time period is given the symbol T and is measured in seconds (s).
Wave speed
The distance travelled by a wave each second.
Wave speed is given the symbol ν.
Wave speed is the speed at which energy is transferred through a medium.
v = fλ
v = wave speed in metres per second (m/s)
f = frequency in Hertz (Hz)
λ = wavelength in metres (m)
Practical 8 (water waves in a ripple tank)
Aim: To measure frequency, wavelength and wave speed by observing water waves in a ripple tank.
Procedure - Set up the apparatus as shown and fill the ripple tank with water to a depth of no more than 1 cm.
Turn on the power supply and the light source to produce a wave pattern on the screen.
The wavelength of the waves can be determined by using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts.
The frequency can be determined by timing how long it takes for a given number of waves to pass a particular point and dividing the number of wavefronts by the time taken.
Record the frequency and wavelength in a table and repeat the measurements.
Practical 8 (investigating waves in a solid)
Aim: To measure frequency, wavelength and wave speed by observing waves in a solid object, such as a stretched string or elastic cord.
Set up the apparatus as shown, then adjust the frequency of the signal generator until a “solid” wave is produced.
Once the “solid” wave is produced, record the frequency shown on the signal generator.
Use a ruler to measure the wavelength, the length to measure will depend on the number of “solid” waves produced. Or measure the length of multiple wavelengths, and divide by the number of wavelengths seen.
Repeat the procedure by adjusting the frequency until another “solid” wave is produced.
Reflection
Reflection occurs when A wave hits a boundary between two media and does not pass through, but instead stays in the original medium.
The law of reflection states:
The angle of incidence = The angle of reflection.
Transmission
Transmission occurs when A wave passes through a substance.
For light waves, the more transparent the material, the more light will pass through.
For the process to count as transmission, the wave must pass through the material and emerge from the other side.
When passing through a material, waves are usually partially absorbed.
The transmitted wave may have a lower amplitude because of some absorption.
Absorption
Absorption occurs when Energy is transferred from the wave into the particles of a substance.
Waves can be partially or completely absorbed.
Light will be absorbed if the frequency of light matches the energy levels of the electrons.
The light will be absorbed, and then reemitted over time as heat.
Transmission of sound waves
Sound waves are longitudinal waves.
They transfer energy by the molecules vibrating and knocking into neighbouring molecules.
The more molecules that are present the faster the wave can transfer energy, therefore:
Sound waves travel fastest in solids.
Sound waves travel slowest in gases.
When sound waves move from one medium to another, there will be changes to its:
Wave speed
Frequency
Wavelength
Sound waves in the ear
Sound waves can be heard by human beings because sound waves are transferred from the air to the solid components of the ear.
In the case of the human ear, the sound waves are transferred by two main solid components:
The eardrum which is made of tissue and skin and Three small bones.
The sound wave travels down the auditory canal towards the eardrum.
The pressure variations created by the sound wave exert a varying force on the eardrum causing it to vibrate.
The vibration pattern of the sound waves creates the same pattern of vibration in the eardrum.
The eardrum vibration is transferred to the three small bones
The vibration of these small bones then transfers the vibrations to the inner ear.
In the inner ear, nerve cells detect the sound and send a message to the brain giving the sensation of sound.
The transmission of sound to the human ear only works over a limited range of frequencies.
This limits the range of sound frequencies a human can hear.
The range of frequencies a human can hear is 20 Hz to 20 000 Hz.
Exploring structure using waves
Sound waves can be used to analyse structures that are hidden from direct observation.
Examples of the use of sound waves:
Echo sounding used by shipping to detect the ocean floor.
Ultrasound used to look inside the human body.
Ultrasound crack detection to find cracks in rail tracks.
Reflection seismology to detect oil and gas underground.
Seismic activity (Earthquakes) can be used to investigate the structure of the Earth.
Ultrasound
Sound waves with a frequency above the human hearing range of 20000 Hz.
When meeting a boundary between two media ultrasound may be:
Partially reflected
Partially transmitted
Partially absorbed
The percentage that is reflected or transmitted depends on the two media at the boundary.
The percentage of the wave reflected is greatest when the difference in speed of sound between the media is large.
Ultrasound is also used by doctors to perform scans of a developing foetus.
Echo sounding
Echo sounding uses ultrasound to detect objects underwater.
The sound wave is reflected off the ocean bottom.
The time it takes for the sound wave to return is used to calculate the depth of the water.
The distance the wave travels is twice the depth of the ocean.
Sesmic waves
Earthquakes produce two types of waves:
P-waves (primary waves)
S-waves (secondary waves)
These waves pass through the Earth’s centre and can be detected at various points around the Earth using seismometers.
By carefully timing the arrival of the waves at each point, the location of the earthquake, along with its magnitude, can be pinpointed.
P - waves
P-waves are longitudinal waves.
These waves can pass through solids and liquids.
P-waves are faster than S-waves.
They are very low frequency sound waves known as infrasound.
Infrasound is any sound below the frequency of human hearing (<20 Hz).
The waves refract as they pass through the different layers of the Earth.
This refraction affects the regions in which waves can be detected, yielding important information about the nature and size of the Earth’s various layers.
S - waves
S-waves are a type of transverse wave.
Unlike P-waves, S-waves are unable to travel through liquids.
They pass through solids only.
S-waves are slower than P-waves.
This means that they are unable to travel through the Earth’s molten (liquid) outer core – providing important evidence about its state and size.
Ultrasound used to produce an image
Sound waves bounce off structures inside your body and back to the probe, which converts the waves into electrical signals. A computer then converts the pattern of electrical signals into real-time images or videos, which are displayed on a computer screen nearby.
