Question

Ultrasonic waves are produced by

• A. Piezoelectric effect
• B. Coulomb's law
• C. Doppler's effect
• D. Peltier's effect

Answer 1

The correct option is A

Piezoelectric effect

Ultrasonic wave:

1. It is a longitudinal wave.
2. When an electrical signal generator provides a pulse of electrical energy to a piezoelectric crystal in a transducer, the crystal vibrates, converting the electrical pulses into mechanical vibrations then ultrasonic waves are produced.
3. The inaudible sound having frequency is more than $\text{20 kHz}$ , termed as Ultrasonic wave.

Diagram of ultrasonic wave is shown as follows:

Explanation of correct option.

Option (A) : Piezoelectric effect:

1. When the mechanical force or pressure is applied on a material then an electric charge is produced that material is known as piezoelectric material and the effect is termed as piezoelectric effect.
2. Two metal plates are sandwiched around a piezoelectric crystal. The material is perfectly balanced at this moment and does not conduct electricity. The metal plates apply mechanical pressure to the material, causing the electric charges within the crystal to become unbalanced..
3. Because of crystal deformation, the piezoelectric effect transfers mechanical or kinetic energy into electrical energy. The sound waves are received in this way by ultrasonic transducers.

Explanation of incorrect options.

1. According to Coulomb's law, the attraction or repulsion force depends on the product of the charged particle and inverse to the square of the distance. It affects the line that connects the two-point charges.
2. The apparent variation in frequency between the frequency at which sound or light waves leave a source and the frequency at which they reach an observer, produced by relative motion between the observer and the wave source, is known as the Doppler effect.
3. The Peltier effect causes one junction to cool and the other to heat when an electric current is maintained in a circuit of material consisting of two distinct conductors; the effect is substantially stronger in circuits including contrasting semiconductors.