## 25.1 PIEZOELECTRIC EFFECT AND ULTRASONIC WAVES Certain materials exhibit a remarkable property: when subjected to mechanical stress, they generate an electric charge, and conversely, when an electric field is applied, they change shape. This two-way relationship is called the **piezoelectric effect**, and it forms the foundation for producing and detecting ultrasonic waves. This section examines this unique property and its applications in medical imaging and other fields. ### Piezoelectric Effect The piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. This occurs due to a shift of positive and negative charges within the material, creating an external electric field. ### Inverse Piezoelectric Effect The inverse piezoelectric effect is the reversible phenomenon where a piezoelectric material generates mechanical stress (or changes shape) when an electric field is applied. ### Applications of Piezoelectric Effect The piezoelectric effect is utilized in: - Production and detection of sound, especially ultrasound - Generation of high voltages - Electronic frequency generation and microbalances - Scientific instrumental techniques such as scanning probe microscopes ### Ultrasound **Audible frequency range**: The normal human ear can detect sound frequencies from 20 Hz to 20,000 Hz. **Ultrasound definition**: Sound waves with frequencies above 20,000 Hz are called ultrasound and are inaudible to the normal human ear. ### Piezoelectric Materials Common piezoelectric materials include: - Lithium niobate ($\mathrm{LiNbO_3}$) - Lead zirconate titanate (PZT) - Quartz ### Generation of Ultrasound Ultrasound is generated using the **inverse piezoelectric effect**. A piezoelectric transducer converts electrical energy into extremely rapid mechanical vibrations, producing high-frequency sound waves. To produce these waves, we must apply an alternating voltage to the crystal. ### Detection of Ultrasound Ultrasound detection utilizes the **piezoelectric effect**. When reflected or scattered ultrasonic waves return to a detector, they cause piezoelectric materials to vibrate. These vibrations generate an electric signal that can be detected and processed to form an image in medical ultrasound. ### Applications of Ultrasound in Medical Technology - **Sonography or Echography**: Imaging internal tissues and organs. This method is preferred over X-ray imaging because it is harmless and safe for examining unborn babies. - **Neurosurgery**: Focusing intense ultrasonic waves in small regions without disturbing surrounding tissues. - **Therapeutic uses**: Treatment of arthritis, muscular rheumatism, and sciatica. - **Blood flow monitoring**: Detecting blood clots, blocked arteries, and assessing cardiac function in adults and fetuses using the Doppler shift. ### Industrial Applications of Ultrasound **Ultrasonic flow meters** measure the flow rate of liquids. They transmit high-frequency sound waves into the liquid. These waves reflect off particles or bubbles, experiencing a frequency shift (Doppler shift) due to the liquid's velocity. This shift is measured to calculate the flow rate based on the pipe's cross-sectional area. Ultrasound is also used in SONAR to calculate depth $d$ using the formula $d = \frac{v \times t}{2}$. ### Animal Use of Ultrasound Some animals, such as bats and frogs, use ultrasound for communication and navigation. **Echolocation** is the process by which bats emit high-pitched, short-wavelength sounds and interpret the reflected waves to navigate and locate prey, creating an accurate map of their surroundings.