Longitudinal wave Class 11

Hello dear students we know that, a wave is a small disturbance travelling in medium and wave motion is transfer of energy from one place to another without actual change in position of particles.

The waves which do not require material medium for propagation is called electromagnetic waves. Also the waves which requires material medium for propagation is called as mechanical waves.

For transfer of energy, medium plays very important role. For propagation of waves, medium should possess some characteristics as,

  1. Medium should possess ability to store energy
  2. Medium should have modulus of elasticity related to change in volume

When waves are travelling through a medium, we-observe the following facts:

1) At any point of the medium, the form of the wave repeats after equal internal of time. This shows that wave motion is periodic in time.

2) At any instant, the form of waves repeats at equal distance. This shows that wave motion is periodic in space.

Thus wave motion has double periodicity.

The waves are categorised as 1. Transverse wave, 2. Longitudinal waves.

Let’s discuss the concept of longitudinal wave……………!

Longitudinal wave is defined as ‘the wave in which the particles of medium vibrate in a direction parallel to direction of propagation’. Eg. Sound wave produced with the help of tuning fork.

Since the direction of propagation of wave and direction of vibration of particles is parallel, medium divides itself in to alternate compressions and rarefactions as shown in fig. below.

Compression: Compressions are the reasons where particles are crowded. During formation of compression the volume of medium decreases so as to increase the density of medium. Pressure at compression also rises due to rise in density.

Rarefaction: Rarefactions are the reasons where the particles are widely spaced. During formation of compression, volume of medium decreases so as to increase the density. This decreases the pressure of particles of medium.

One compression and one rarefaction form a set of longitudinal wave. The distance between two successive compressions / rarefaction or distance between centres of two consecutive compression / rarefaction is known as wavelength of longitudinal wave.

Following are some important properties of longitudinal wave…….!

  1. The particles of the medium vibrate in the direction parallel to the direction on propagation of waves.
  2. Performs SHM’s of same amplitude and period.
  3. The distance between two consecutive compression (and rarefactions) is equal to the wavelength o the waves.
  4. The Compressions & rarefactions are alternately produced.
  5. The density &Pressure at any point of the medium varies between maximum and minimum values.
  6. In Longitudinal waves the medium must possess the elasticity of volume.
  7. Velocity of longitudinal wave travelling in medium of density , and modulus of elasticity E, is given as

v = √E/ρ


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