Pearson Edexcel GCSE (9-1) Physics revision notes
Waves Revision Notes
Review revision notes for Waves in Edexcel Physics.
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Waves
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Measuring Wave Speeds: Sound in Air and Ripples on Water Revision Notes
Measuring the Speed of Sound in Air
One method uses two microphones connected to an oscilloscope. A sound is generated, and its arrival at each microphone is displayed on the oscilloscope screen. The time difference between the two signals is measured, and given the known distance between the microphones, the speed of sound can be calculated using the formula: speed = distance / time. For more precision, repeat measurements for different distances and average the speed. Another method involves a single microphone at a known distance from a wall, timing the sound and its echo.
Measuring the Speed of Ripples on Water
This is typically done using a ripple tank. A motor with an eccentric weight (vibrator) generates ripples on the water surface. A light source above the tank projects the ripple pattern onto a screen below. A stroboscope, flashing at a controllable frequency, is used to make the ripples appear stationary. When the ripples appear frozen, the stroboscope's frequency matches the ripple frequency (or a multiple). The wavelength can then be measured directly from the projection on the screen using a ruler. The wave speed is then calculated using the wave equation: wave speed (v) = frequency (f) × wavelength (λ).
General Principle for Wave Speed Measurement
All wave speeds can fundamentally be determined using the wave equation, v = fλ, where 'v' is wave speed, 'f' is frequency, and 'λ' is wavelength. The challenge lies in accurately measuring these quantities for different types of waves. For sound, direct timing over a distance is also feasible. For ripples, the stroboscope helps in visualising and measuring wavelength.
Sound Wave and Solid Vibration Conversion Revision Notes
Conversion Between Sound Waves and Solid Vibrations
Sound waves, which are pressure oscillations in a medium, can cause solids to vibrate by transferring energy to the solid's particles. Conversely, a vibrating solid can transfer energy to the surrounding medium, generating new sound waves. This energy transfer occurs through particle-to-particle collisions. The efficiency of this conversion depends on the physical properties of both the medium and the solid, such as their density and elasticity.
Limited Frequency Range and Resonant Frequencies
The conversion of wave disturbances between sound and solid vibrations is frequency-dependent. Solids have natural resonant frequencies; when an incoming sound wave's frequency matches one of these, the solid will vibrate with maximum amplitude due to resonance. Outside these specific frequencies, the energy transfer is much less efficient, leading to a limited functional frequency range for such conversions. This is crucial for transducers like microphones and speakers.
The Human Ear: A Natural Transducer System
The human ear exemplifies this conversion. Sound waves enter the ear canal and cause the eardrum (a solid membrane) to vibrate. These vibrations are then amplified by small bones (ossicles) and transferred to the fluid in the cochlea, which stimulates hair cells. The cochlea's structure means that different parts are sensitive to different frequencies, acting as a natural frequency analyser. This complex system ensures that only a specific range of sound frequencies can be effectively perceived, which defines the human audible range.
Sound Wave Transmission Through Different Media Revision Notes
Frequency Remains Constant
When sound moves from one medium to another, its frequency does not change. The frequency is determined by the source that produced the sound wave. Imagine a speaker vibrating at a set rate; it will continue to produce sound waves at that same rate, regardless of whether those waves then travel through air, water, or a solid.
Velocity Changes
The speed (velocity) of sound depends on the properties of the medium. Sound generally travels faster in liquids and solids than in gases because particles are closer together and can transmit vibrations more efficiently. For example, sound travels much faster in water than in air. This change in velocity is a primary characteristic of a sound wave crossing a boundary.
Wavelength Changes with Velocity
Since the frequency (f) remains constant and the velocity (v) changes, the wavelength (λ) must also change. This relationship is described by the wave equation: v = fλ. If the velocity increases when entering a new medium, the wavelength must also increase to keep the frequency constant. Conversely, if the velocity decreases, the wavelength will decrease.
Calculating Depth or Distance with Wave Speed and Time Revision Notes
The Fundamental Relationship
The basic physics relationship is wave speed (v) equals distance (d) divided by time (t), so v = d / t. You can rearrange this to d = v × t for use in problems involving any type of wave, such as sound or seismic waves.
Calculating Depth or Distance from Echoes
For reflected waves (like sonar echoes), the measured total time is for a round trip. Use the formula: depth = (wave speed × round-trip time) / 2. Divide by two to get the one-way depth. This is critical for accurate results in sonar and radar applications.
Factors Affecting Wave Speed
Wave speed depends on both the stiffness (elastic modulus) and the density of the medium, with v ≈ sqrt(stiffness/density). Sound travels faster in stiffer media (and lower-density if stiffness is constant). For example, sound travels much faster in water (about 1500 m/s) than in air (about 340 m/s), and is usually faster still in solids. In gases, higher temperatures also increase the speed.
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