Wave Behaviour
Waves transfer energy and information without transferring matter. There is proof of this in water and music. Dropping a twig into a calm pool of water causes ripples to form and move across the water, while the twig and the water themselves are not carried. Furthermore, strumming a guitar string causes a person to hear a sound, however the air does not move to carry this sound, or it would cause a vacuum
Amplitude: displacement from rest position to the peak/trough. Synonymous with volume
Wavelength: length of a full cycle of the wave
Frequency: the number of complete cycles of the wave passing a certain point per second, measured in Hz
Period: length of time taken for one wave to pass a given point
Velocity: speed of the wave in direction of travel in a certain medium
In longitudinal waves, oscillations are backwards and forwards along the same direction as the direction of travel and energy transfer. They travel in compressions and rarefactions e.g. Sound, P-Waves
In transverse waves, oscillations are at right angles to the direction of travel. All EM Waves, S-Waves and Ripples are transverse
where v = Velocity (m/s), f = Frequency (Hz) and λ = Wavelength (m)
where v = Velocity (m/s), x = Distance (m) and t = Time (s) – Used to determine depth using SONAR, where x/2 = depth
where P = Period (s) and f = Frequency (Hz)
- To measure the speed of sound in air, use an oscilloscope and set it up to show detected waves of two microphones separately. Move one microphone way until the two waves are aligned on the display but move exactly one wavelength apart. The distance between the microphones is the wavelength. Then use v = fλ where the frequency is that used by the microphone
- Or, measure the distance between a person and a wall and measure the time taken for an echo of a loud sound to reach the person. Calculate the speed of the sound using s=d/t
- To measure the speed waves on water, measure the time it takes for a wave to travel for a wave to travel between two fixed points. The speed is then calculated from s=d/t
CORE PRACTICAL: Investigating Waves
Part 1: Waves on Water
A – Set up a ripple tank with a dipper near one side of the tank. Fasten a ruler to one of the sides so the marking can be seen above water level
B – Vary the voltage of the motor until waves with two waves are always seen
C – Count how many waves are formed in a given time. Find the number of waves per second
D – Estimate the wavelength using a ruler the use v = fλ to find velocity
Part 2: Waves in Solids
A – Suspend a metal rod using clamp stands and rubber bands
B – Hit one end with a hammer. Note down peak frequency using a frequency app on a smartphone
C – Measure the length of the rod. Wavelength is twice this length. Put into v=fλ
Wave Behaviour
| Substances may absorb, transmit, reflect or refract ways in ways that vary with wavelength | |
| Absorption | A wave transfers its energy into the stores of a second material, often a thermal store |
| Transmission | Carries on travelling at a boundary |
| Reflection | Wave is sent away from a boundary; how echoes are created |
| Diffraction | Waves spread out as they pass through a gap in a boundary |
| Refraction | Waves travel in different speeds in mediums of different densities, so at a boundary there is an immediate change in speed. When a wave hits the boundary at an angle, this causes a change in direction |
Sound
Sound waves are caused by vibrating objects that pass through a medium as compressions and rarefactions. The processes which allow us to hear only works over a frequency range of 20 – 20000 Hz
- Sound waves are recepted by the pinna and are reflected into the ear canal
- The eardrum, a thin membrane, is made to vibrate
- Vibrations pass through the oscicles which amplify the sound
- These vibrations enter the cochlea, a coiled tube containing a liquid and a membrane
- The wave passes through the liquid and causes a part of the membrane which correspond to certain frequencies to vibrate, moving transmitters underneath which causes an electrical impulse which is sent to the brain
Ultrasound is sound with frequencies greater than 20000Hz, and is used in scans and SONAR
- Foetal Scanning: a gel is placed above the womb to stop the waves from reflecting. A probe emits ultrasonic waves. Some sound is reflected when the ultrasound waves pass into a different medium. These reflected waves are detected by the probe. The exact timing and distribution of these echoes are processed by a computer to produce images of the foetus
- SONAR: used to detect the depth of sea under a ship or by animals for communication or predation. An ultrasonic pulse is emitted, and the time taken for it to be recepted is multiplied by the speed of the pulse, then divided by two to get the distance
Infrasound is sound with frequencies less than 20Hz and is used in the exploration of the Earth’s core
- Vibrations caused by earthquakes are called seismic waves. Energy travels as longitudinal waves and as transverse S waves, and can be detected by seismometers
- Longitudinal waves can pass through solids, liquids and gases, whilst transverse waves tha need a medium can only be transmitted by solid
- Places where waves are detected depend on where the focus of the earthquake is, but there always a large area on the opposite side of the earth where no S waves are detected, called a S wave shadow zone and occurs because the outer core of the earth is liquid
- There is a band around the Earth called the P wave shadow zone. However, it has bee discovered that there are a few weak P waves arriving here, which can only occur if waves i the liquid core had been reflected by something solid, meaning the inner core must be solid