When an electric signal is applied to a piezoelectric element, what happens to the element?

• A. it undergoes temperature fluctuations
• B. it expands and contracts
• C. it produces an electric signal of the opposite polarity
• D. it loses its piezoelectric properties

Answer: (B)

When a piezoelectric element is driven by an electric signal, it undergoes mechanical deformation—the direct piezoelectric effect. The applied electric field causes the crystal lattice to change shape along certain axes, so the element expands and contracts at the frequency of the signal. This is what lets ultrasound transducers convert electrical energy into vibrating, sound-producing motion.

In practice, the direction and amount of expansion or contraction depend on the material orientation and the polarity of the drive, but the essential idea is that electrical energy becomes mechanical motion. The other options aren’t the primary behavior: heating can occur but isn’t the main response to the applied signal, piezoelectric materials respond to mechanical stimulation with an electric signal (the reciprocal effect), and the properties don’t disappear unless the material becomes depolarized or damaged.