Direct Energy, 2018a

2 CAPACITORS AND PIEZOELECTRIC DEVICES 49
2.10. A piezoelectric device isplaced in an electric eld of strength | −→ E | =
500 m V . The device is tested twice. In the rst test, a stress of
| −→ ς | = 1000 m N wasput on the device, and the material polarization
2
wasmeasured to be | −→ P | =2.75·10 −8
m C . In the second test also with
2
| −→ E | = 500 m V , a stress of |−→ ς | = 100 m N wasput on the device, and the
2
material polarization wasmeasured to be | −→ P | =6.50 · 10 −9
m C . Find
2
the piezoelectric strain constant d, and nd the relative permittivity
of the material ɛ r .
2.11. According to the data sheet, a piezoelectric device is 3% ecient. A
coworker says that energy is not conserved in the device because 97%
of the energy is lost when it is used. Explain what is wrong with your
coworker'sexplanation.
2.12. Match the material property with itsdenition. (Not all denitions
will be used.)
1. A mechanical stress will cause a
(material) polarization in thistype of
material.
2. This type of material is glassy and
noncrystalline.
3. Chargesdo not easily ow through
thistype of material
4. In the presence of a weak external
voltage, chargesdo not ow in thistype
of material. In the presence of a strong
external voltage, chargesow easily.
5. A material polarization in one atom
inducesmaterial polarization in nearby
atomsin thistype of material.
A. Amorphous
B. Dielectric
C. Ferroelectric
D. Piezoelectric