pyroelectricity
/ PY-roh-ee-lek-TRISS-ih-tee /
Some crystals already carry a built-in polarization, with charge gently leaning one way inside them. Now warm or cool such a crystal a little. The built-in polarization changes strength as the temperature shifts, and that change makes a small voltage appear across the crystal. This sensitivity of polarization to temperature is pyroelectricity.
It happens because heating a crystal makes it expand and jiggle, which alters exactly how far off-center its charges sit, so the spontaneous polarization grows or shrinks with temperature. While the temperature is changing, the surface charge that had been quietly balancing the polarization no longer matches it, and a measurable voltage flickers out. Hold the temperature steady and the voltage fades as stray charges creep back to rebalance things — pyroelectricity responds only to change, not to a steady warmth.
This matters because it is the basis of the motion-sensing detectors that switch on porch lights and trigger alarms: they sense the changing infrared warmth of a body passing by. The key misconception is to expect a steady voltage from a steadily warm crystal; you only get a signal while the temperature is moving. Every pyroelectric crystal must also be polar and piezoelectric, but not every piezoelectric is pyroelectric.
The motion sensor that flicks on a porch light holds a pyroelectric crystal behind a lens. When a warm person walks into view, the crystal's temperature ticks up for an instant, its polarization shifts, and the brief voltage pulse tells the circuit 'something moved.'
A motion sensor reads the voltage pulse a pyroelectric crystal makes as a warm body passes.
Pyroelectricity gives a signal only while the temperature is changing; a crystal held at any constant temperature, hot or cold, produces no lasting voltage.