The working of the quartz crystal in a wristwatch or electronic clock is based on which physical effect?

Introduction / Context:
Modern wristwatches and many electronic clocks use a quartz crystal to keep accurate time. This tiny crystal vibrates at a very stable frequency when an electric field is applied. Understanding which physical effect allows an electric signal to create mechanical vibrations in the crystal (and vice versa) is essential in basic electronics and general physics. This question checks whether you know the name of that effect.

Given Data / Assumptions:

• The device is a quartz crystal used inside a watch or clock for timekeeping.
• The crystal is made of quartz, a piezoelectric material.
• When an alternating voltage is applied, the crystal vibrates mechanically at a characteristic frequency.
• The options mention piezoelectric, Edison, photoelectric and Johnson effects.

Concept / Approach:
The piezoelectric effect is the phenomenon in which certain crystals, like quartz, develop an electric potential when mechanically stressed and conversely undergo mechanical deformation when an electric field is applied. In a quartz watch, an electronic circuit applies an alternating voltage at or near the crystal's natural resonant frequency, causing it to vibrate at a very stable rate, often 32,768 Hz. The electronic circuitry then divides down this frequency to generate one pulse per second, which drives the display. The other effects listed relate to different phenomena: the Edison effect to thermionic emission, the photoelectric effect to emission of electrons by light, and the Johnson effect to thermal noise.

Step-by-Step Solution:
Step 1: Recall that quartz is a piezoelectric material.Step 2: The piezoelectric effect states that mechanical stress produces electric charges and electric fields can produce mechanical strain in the crystal.Step 3: In a quartz watch, an electronic oscillator applies an alternating voltage across the crystal.Step 4: The crystal responds by vibrating mechanically at a very precise natural frequency.Step 5: This stable vibration is used as a time reference; digital circuits count these oscillations to measure seconds and minutes.Step 6: Conclude that the operation of the quartz crystal in a watch is based on the piezoelectric effect.

Verification / Alternative check:
Electronics references describe quartz oscillators as piezoelectric resonators. The term quartz crystal oscillator is tightly linked with piezoelectric behaviour. None of the other effects mentioned is ever cited in the context of crystal timekeeping. For instance, the photoelectric effect is used in light sensors, not in quartz time bases, and the Edison effect is connected with vacuum tubes rather than wristwatches.

Why Other Options Are Wrong:
The Edison effect refers to thermionic emission of electrons from a heated filament within a vacuum, forming the basis of early vacuum diodes. The photoelectric effect involves electrons emitted from a metal surface when light shines on it, foundational for photovoltaic cells and quantum physics. The Johnson effect, or Johnson–Nyquist noise, is thermal noise due to random motion of electrons in resistors. None of these effects describe the mechanical–electrical coupling in crystals used for timing.

Common Pitfalls:
Some students confuse photoelectric or other electric effects because they see the word electric and think any such phenomenon might be involved. Others may not recall the specific term piezoelectric even if they know the basic idea. To avoid such confusion, link the word piezo (meaning pressure) with the idea of stress and mechanical deformation, and remember that quartz watches are classic examples of piezoelectric applications.

Final Answer:
The quartz crystal in a watch works on the principle of the piezoelectric effect.