Mumbai university > FE > SEM 1 > Applied Physics 1

Marks: 5M

Year: May 2015

Principle

It works under the principle of 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 at resonance.

Resonance is the condition at which the frequency of the oscillator match with the natural frequency of rod.

Construction

Working:

1) The rod is initially magnetized by the DC power supply.

2) When the battery is initially switched on the current is induced in the transistor.

3) The transistor is properly biased.

4) Emitter- forward biased, Collector- reverse biased.

5) An emf is induced in coil L1, this induces an emf to coil L2 & a part of it is feed as input to the base. Hence, this feedback system makes transistor operates continuously.

6) The varying frequency can be obtained by altering the capacitance value of condenser.

7) At resonance Ultrasonic waves will be produced.

Natural frequency of rod= Frequency of oscillator circuit

$\frac{K}{2l}\sqrt{\frac{γ}{ρ}} = \frac{1}{2π\sqrt{L_1C_1}}$

Where,

K - Fundamental overtone, here K=1

l - length of rod

γ -Youngs’modulus

ρ - density of rod

$L_1$ - Inductance

$C_1$ - Capacitance