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School-college Physics Notes: Waves 4. LONGITUDINAL WAVES

Introduction to waves: 4. The technical description, properties and examples of a LONGITUDINAL WAVE

Doc Brown's Physics exam study revision notes

4. The technical description and properties of a LONGITUDINAL WAVE

The oscillations/vibrations of a longitudinal wave are in the same direction as the wave is moving.

The disturbance of the medium is parallel to the direction the wave is moving.

Know and understand why longitudinal waves show areas of compression and rarefaction.

The above diagram shows the compression and decompression (rarefaction) of a longitudinal sound wave, illustrated 'visually' by the pushing pulling of a slinky spring (see below 'picture' below).

The 'to and fro' effect is due to the particles of the medium being compressed to give a point of maximum particle density (maximum pressure), squashed up to give a compression.

At the same time, further along the wave, the arrangement of particles is stretched out to give a point of minimum density (or minimum pressure) called a rarefaction.

You can appreciate this by the way the vertical lines and spaced out or compressed together - the vertical lines represent the relative density of particles in the medium (gas, liquid or solid).

Also, the diagram above illustrates longitudinal sound waves travelling at the same speed where wave B has twice the frequency and half the wavelength of wave A. You can deduce this because in wave B the distance between two compression or two rarefactions is halved, so twice as many waves will pass a given point in the same time.

The diagram above illustrates a slinky spring 'pulsed' with longitudinal waves. It also illustrates in a way what happens to the air when a sound wave passes through it and the ground with one of the types of earthquake wave (the compressional P waves), which go right through the Earth to the other side of the world!)

Examples of longitudinal waves:

Sound waves - e.g. from your vocal chords or musical instrument

Earthquake P-waves - that can go right through the Earth

Slinky spring - 'pushed and pulled' to send pulses of energy along it.

Keywords, phrases and learning objectives for waves

Be able to describe and explain with a technical description, the properties and examples of a longitudinal wave.

Be able to do longitudinal wave calculations using the wave equation formula.

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