Ultrasonic Wave Generation and Superconductor Properties

1. SONAR (Sound Navigation and Ranging)

SONAR is a system used to determine the presence of submerged submarines or enemy aircraft. It utilizes the properties of ultrasonic waves to identify the depth of the sea and detect obstacles.

• Mechanism: The SONAR transmitter emits ultrasonic rays in various directions. When these rays strike an obstacle, they reflect back to the receiver. By measuring the distance and time of the transmitted and reflected waves, the velocity of the ultrasonic wave is calculated.
• Mathematical Calculation: If the distance traveled by the transmitted and reflected wave is AC + BC, and velocity v = (AC + BC) / t, then v = 2CO / t. Therefore, the depth of the sea is CO = vt / 2.

2. Piezoelectric Method

Piezoelectric Effect

When pressure is applied to one pair of opposite faces of crystals like quartz, tourmaline, or Rochelle salt—cut with their faces perpendicular to the optic axis—equal and opposite charges appear across the other faces. This is known as the piezoelectric effect.

Inverse Piezoelectric Effect

If an alternating voltage is applied to one pair of opposite faces, the crystal undergoes mechanical contractions and expansions, causing it to vibrate. This is the inverse piezoelectric effect (or electrostriction). When the frequency of the applied voltage matches the natural frequency of the crystal, it produces ultrasonic waves.

Experimental Arrangement

A quartz crystal (Q) is placed between two metal plates (A and B) connected to coil L3. Coils L1, L2, and L3 are connected to a triode valve. Coil L1 is connected in parallel with a variable capacitor (C1) to form the tank circuit.

Working Principle

• When the high-tension battery is switched on, the oscillator produces a high-frequency alternating voltage: f = 1 / (2π√L1C1).
• The frequency can be controlled by the variable capacitor C1.
• Through transformer action, an EMF is induced in the secondary coil L3, exciting the quartz crystal into vibrations.
• By adjusting C1 to resonant conditions, the crystal produces ultrasonic waves at frequency: f = (1/2L) * √(E/ρ).

3. Magnetostriction Effect

When an alternating magnetic field is applied parallel to a ferromagnetic rod (e.g., iron or nickel), it experiences contraction and expansion at the same frequency as the applied field. This is the Magnetostriction Effect.

Construction and Working

• A nickel rod is clamped at the center and permanently magnetized by a DC current in a surrounding coil.
• Two ends are wound with coils L1 and L2. L1 is connected to the transistor output, and L2 to the base. A variable capacitor C1 forms the tank circuit.
• When the battery is switched on, the resonant circuit L1C1 sets up an alternating current. Changes in plate current alter the magnetization and length of the rod, inducing an EMF in L2 to maintain oscillations.
• When the tank circuit frequency matches the rod's natural frequency, resonance occurs, producing ultrasonic waves.

4. Properties of Superconductors

Superconductivity is characterized by several distinct physical properties:

• Electrical Resistance: Drops to zero below a critical temperature.
• Effect of Impurities: Can alter the superconducting transition.
• Effect of Pressure and Stress: Increasing pressure can induce superconductivity (e.g., Cesium at 110 Kbar).
• Isotope Effects: The transition temperature T is proportional to 1/√M, where M is the isotopic mass.
• Magnetic Field Effect: A critical magnetic field (H) destroys the superconducting state. This field decreases as temperature increases.
• Critical Current Density (Jc) and Current (Ic): Limits for maintaining the superconducting state.
• Meissner Effect (Diamagnetic Property): The complete expulsion of magnetic fields from a superconducting material. When placed in a magnetic field (H > Hc) at room temperature, the field penetrates the material.