3.1 General properties of waves (3)
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1.
Consider sound waves. Explain how sound waves demonstrate that waves transfer energy without transferring matter. Include a discussion of the medium through which sound travels.
Sound waves are longitudinal waves, meaning the displacement of the particles in the medium is parallel to the direction the wave travels. Sound travels through a medium – typically air, water, or solids – by causing vibrations in the particles of that medium. These vibrations create areas of compression (where particles are close together) and rarefaction (where particles are spread apart). The energy from the source of the sound is transferred through these compressions and rarefactions.
The individual particles of the medium do not travel a long distance with the sound wave; they simply vibrate back and forth. The energy is carried by the propagation of these compressions and rarefactions. For example, when you speak, your vocal cords vibrate, causing the air molecules around them to compress and expand. These compressions and expansions propagate outwards as a sound wave, carrying the energy of your voice to your listener. The air molecules themselves do not travel to the listener; only the disturbance (the wave) does.
| Medium | Description |
| Solid | Particles are closely packed, allowing for efficient energy transfer. |
| Liquid | Particles are less closely packed than in solids, but still allow for energy transfer. |
| Gas | Particles are widely spaced, so energy transfer is less efficient than in solids or liquids. |
2.
A student is demonstrating the properties of longitudinal waves using a slinky. They push and pull one end of the slinky to create a wave.
- Describe how the displacement of the coils in the slinky relates to the direction in which the wave travels.
- Explain why sound waves and seismic P-waves can be modelled as longitudinal waves.
1. Description of displacement and wave direction:
In a longitudinal wave, the displacement of the particles (in this case, the coils of the slinky) is parallel to the direction in which the wave propagates. This means that as the wave travels, the coils of the slinky move back and forth in the same direction as the wave is moving. The compressions and rarefactions (regions of high and low density) travel along the slinky in the same direction as the pushes and pulls.
2. Explanation for sound and P-waves being longitudinal:
Sound waves and P-waves are longitudinal because they involve compressions and rarefactions of the material through which they travel. The particles of the medium vibrate parallel to the direction the wave is moving. For sound waves, the vibrations are typically in the same direction as the sound is traveling (e.g., from a speaker to your ear). For P-waves, the compressions and rarefactions propagate through the Earth's interior, and the particles of the rock vibrate back and forth in the same direction as the wave's propagation. This characteristic vibration pattern is a defining feature of longitudinal waves.
3.
A sound wave has a frequency of 440 Hz in air. The speed of sound in air is approximately 343 m/s. Calculate the wavelength of the sound wave.
Given:
- Frequency (f) = 440 Hz
- Speed of sound (v) = 343 m/s
Equation: v = fλ
Rearranging for wavelength (λ): λ = v / f
Calculation: λ = 343 m/s / 440 Hz = 0.784 m
Answer: The wavelength of the sound wave is 0.784 m.