Mumbai university > FE > SEM 1 > Applied Physics 1

Marks: 5M

Year: May 2013,Dec 2014

Piezoelectric Effect:-

• When a mechanical compression or tension is applied to some crystals like quartz, a potential difference is developed across the crystal which is proportional to the applied pressure. This phenomenon is known as Piezo-electric Effect.

• Also if the potential difference is applied across the crystal, a mechanical compression or tension is developed. This is called Inverse Piezo-electric Effect.

Working:-

• A variable capacitor (C) and an inductor $(L_2)$ form a tank circuit.

• The frequency of the oscillations can be changed by changing the value of capacitance.

• When the circuit is closed, the current flows through the circuit and charges the capacitor.

• Then the capacitor starts discharging through the inductor, thus the electric energy is stored in the form of electric and magnetic field of capacitor and inductor respectively.

• Thus electric oscillations are produced in the tank circuit.

• The frequency of this oscillating electricity is given by

  $f =\frac{1}{2π\sqrt{L_2 C}}$

• With the help of the other electronic components including a transistor, electrical oscillations are produced continuously.

• This is fed to the secondary circuit connected to Quartz crystal (Q).

• The oscillating electric field is converted to mechanical vibration of crystal owing to the piezoelectric effect.

• This vibration produces sound wave of the frequency equal to the frequency of vibration, which is the frequency of electric oscillations.

• In this way ultrasonic sound waves can be produced.

• Natural frequency of crystal is given by

  $f = \frac{k}{2t}\sqrt{\frac{Y}{ρ}}$

  • t = Thickness of crystal slab
  • Y = Young's Modulus
  • ρ = Density
  • k = 1, 2, 3, ... (Integer Multiple)

• When the frequency of electric oscillations is equal to that of natural frequency of the crystal, resonance is achieved and the sound waves of maximum amplitude are produced.