Ultrasound waves are produced when alternating current causes a crystal to do what?

Ultrasound waves are produced when alternating current causes a crystal to do what?

• A. Compress and expand the crystal (piezo-electric action)
• B. Heat the crystal
• C. Move laterally
• D. Emit light

Answer: (A)

The essential idea is the piezoelectric effect: applying an alternating electric field makes a crystal physically vibrate. When the current reverses direction, the crystal repeatedly compresses and expands. This rapid back-and-forth motion pushes against the surrounding medium and creates pressure waves that travel as ultrasound. The frequency of these waves matches the applied electrical frequency, and the wave amplitude grows with the applied voltage. Heating the crystal would be just heat, not a directed mechanical wave. Lateral movement by itself doesn’t produce the coherent longitudinal waves needed for ultrasound, and emitting light involves a completely different process. The piezoelectric crystal’s thickness-wise compression–expansion motion is what generates the ultrasound.

The essential idea is the piezoelectric effect: applying an alternating electric field makes a crystal physically vibrate. When the current reverses direction, the crystal repeatedly compresses and expands. This rapid back-and-forth motion pushes against the surrounding medium and creates pressure waves that travel as ultrasound. The frequency of these waves matches the applied electrical frequency, and the wave amplitude grows with the applied voltage.

Heating the crystal would be just heat, not a directed mechanical wave. Lateral movement by itself doesn’t produce the coherent longitudinal waves needed for ultrasound, and emitting light involves a completely different process. The piezoelectric crystal’s thickness-wise compression–expansion motion is what generates the ultrasound.