UNIT 4 Vibrations &Ultrasonic waves
1.Define natural frequency and free vibrations
The frequency with which a body vibrates freely at its own is called its natural frequency.
Free vibrations can be defined as the vibrations in which the body vibrates with its own natural frequency when left free to itself.
2.Define forced vibrations and resonance
The forced vibrations can be defined as the vibrations in which the body vibrates with a frequency other than its natural frequency under the action of an external periodic force.
The phenomenon in which a body vibrates with its natural frequency under the influence of another vibrating body with the same frequency is called resonance.
3.what are ultrasonic waves?
The sound waves having frequencies above the audible range i.e. above 20 kHz are called ultrasonic waves.
4.Explain Magnetostriction effect
When a ferromagnetic rod is kept in an alternating magnetic field the rod is set into longitudinal vibrations
This happens due to change in the length of the rod.
This longitudinal vibrations, result in the formation of ultrasonic waves.
5.Give applications of Ultrasonic waves
Defects in a metal block can be detected using ultrasound
Defects in metal blocks do not allow ultrasound to pass through them and they are reflected back.
Ultrasound is passed through one end of a metal block and detectors are placed on the other end.
The defective part of the metal block does not allow ultrasound to pass through it. As a result, it will not be detected by the detector.
Hence, defects in metal blocks can be detected.
Ultrasonic drilling
Ultrasonics are used for making holes in very hard materials.
For this purpose, a suitable drilling tool bit is fixed at the end of a powerful ultrasonic generator.
Some slurry (paste of sediment) is made to flow between the bit and the plate in which the hole is to be made
Ultrasonic generator causes the tool bit to move up and down very quickly and a hole is drilled with the help of slurry.
Ultrasonic welding
A hammer is attached to a powerful ultrasonic generator as shown in Figure.
The metallic sheets to be welded are put together under the tip of the hammer .
The hammer is made to vibrate ultrasonically.
As a result, it presses the two metal sheets very rapidly and the molecules of one metal diffuse into the molecules of the other.
Thus, the two sheets get welded without heating.
aka. Cold welding
6.State Piezoelectric effect and converse of Piezoelectric effect.
Piezoelectric Effect is the ability of certain materials (eg pair of Quartz crystal) to generate an electric charge in response to applied mechanical stress.
The converse of Piezoelectric effect- If a potential difference is applied across two faces of a crystal,a change in the dimensions of the crystal(expansion or contraction)are observed in a direction perpendicular to these faces.
8.Explain application of ultrasonic waves in sonar system.
SONAR is a technique which stands for Sound Navigation and Ranging.
It uses ultrasonics for the detection and identification of under water objects.
The method consists of sending a powerful beam of ultrasonics in the suspected direction in water.
By noting the time interval between the emission and receipt of beam after reflection, the distance of the object can be easily calculated.
**[ For any numerical related to this topic use the formula d = vt/2
Where, d= distance (m), v= velocity (m/s), t= Time (s)]**
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