Question E6D03:
Which of the following is an aspect of the piezoelectric effect?  

Choices:  
A: Mechanical deformation of material due to the application of a voltage  
B: Mechanical deformation of material due to the application of a magnetic field  
C: Generation of electrical energy in the presence of light  
D: Increased conductivity in the presence of light    

Correct Answer is  A: Mechanical deformation of material due to the application of a voltage  

To understand the piezoelectric effect, remember it’s a two-way coupling between electrical and mechanical energy in certain crystals (e.g., quartz, ceramics).

1) Why the correct answer is correct
- Mechanical deformation of material due to the application of a voltage is the converse piezoelectric effect. Apply a voltage → the crystal strains (changes shape). This is used in buzzers and crystal oscillators, where an AC voltage makes the crystal mechanically vibrate.

Useful relation (converse effect):
$$S = d\,E$$
where:
- $S$ = mechanical strain (unitless)
- $E$ = electric field (V/m)
- $d$ = piezoelectric coefficient (m/V)

2) Why the other choices are incorrect
- Mechanical deformation due to a magnetic field: That’s magnetostriction, not piezoelectricity.
- Generation of electrical energy in the presence of light: That’s the photovoltaic effect (e.g., solar cells).
- Increased conductivity in the presence of light: That’s photoconductivity (photoresistors, photoconductive cells).

3) Other useful formulas/rules of thumb
- Direct piezoelectric effect (sensor mode): Stress generates charge/voltage.
$$D = d\,T$$
where:
- $D$ = electric displacement (C/m²)
- $T$ = mechanical stress (N/m²)
- $d$ = piezoelectric coefficient (C/N)

Rule of thumb:
- Direct effect: push it → get voltage (microphones, contact pickups).
- Converse effect: apply voltage → it moves (buzzers, actuators, crystal filters/oscillators).