*Question E6D01:
What is piezoelectricity?*  

*Correct Answer is  C: A characteristic of materials that generate a voltage when stressed and that flex when a voltage is applied*  

### AI Response: (may contain errors)  

**Explanation of Piezoelectricity:**

1. **Correct Answer: C**  
   Piezoelectricity is a property of certain materials that allows them to generate an electrical voltage when mechanical stress is applied. Conversely, these materials can also change shape (flex) when an electrical voltage is applied. This dual capability is what defines piezoelectric materials and is used in various applications like sensors, actuators, and frequency control devices.

2. **Why Other Choices Are Incorrect:**
   - **Choice A:** This describes a property more akin to electromagnetic wave generation, which is not related to piezoelectricity. Piezoelectric materials do not inherently generate electromagnetic waves when voltage is applied; they generate mechanical movement or stress.
   - **Choice B:** This describes birefringence, a property of materials where the index of refraction depends on the polarization of light passing through. This is unrelated to piezoelectricity.
   - **Choice D:** This suggests that materials generate voltage when exposed to electromagnetic waves, which is more related to the photoelectric effect or electromagnetic induction, not piezoelectricity.

3. **Useful Concepts:**
   - **Piezoelectric Effect:** When a piezoelectric material is mechanically stressed, it generates an electric charge. Conversely, applying an electric field causes the material to deform.
   - **Applications:** Common applications include quartz crystals in watches, microphones, and ultrasound equipment, where precise control of mechanical and electrical properties is needed.

Understanding piezoelectricity is crucial for recognizing how certain materials can be used in electronic and mechanical systems to convert between electrical and mechanical energy.