The core difference between the thermoelectric and pyroelectric effects lies in the thermal condition that triggers an electrical response:
Pyroelectric Effect: This effect generates a temporary voltage or current in certain materials when their temperature is changing uniformly over time (dT/dt ≠ 0). The material possesses a spontaneous electric polarization that is temperature-dependent. A change in temperature causes a change in this polarization, resulting in surface charges. When the temperature becomes stable (dT/dt = 0), even if it’s elevated, the effect ceases, and the charge dissipates. Pyroelectric sensors are therefore sensitive to temporal changes in temperature.
Thermoelectric Effect (specifically the Seebeck effect): This effect generates a continuous voltage across a material (or more commonly, at the junction of two dissimilar materials) when there is a spatial temperature difference (gradient) across it (ΔT ≠ 0). One end or junction is hotter than the other. This voltage is maintained as long as the temperature difference exists. Thermoelectric devices like thermocouples and thermopiles rely on this principle to measure static temperature differences or absolute temperatures (when one junction is held at a known reference temperature).
In summary:
- Pyroelectric: Responds to a change in temperature over time (dynamic). Generates charge due to changing polarization. Needs dT/dt.
- Thermoelectric (Seebeck): Responds to a temperature difference across space (static or dynamic gradient). Generates voltage due to charge carrier diffusion driven by ΔT. Needs ΔT.
A PIR motion sensor uses the pyroelectric effect because it needs to detect the transient temperature change caused by a moving warm object entering or leaving its field of view. A thermocouple uses the thermoelectric effect to measure a steady temperature by sensing the stable temperature difference between its measurement junction and reference junction.