Like every Friday, the "Taste of Physics" corner is back — number 9.
This time: quartz crystals, piezoelectricity, and the Mohs scale.
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You could write a whole book about quartz crystals — they are a fascinating material with an enormous range of industrial applications.
This time we'll focus on quartz's role in electronics as a timekeeping device, an application found in a wide variety of products such as wristwatches and motherboards.
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Piezoelectricity is a property whereby a material releases an electric current in response to physical pressure applied to it. In the reverse direction, an electric current applied to the material causes it to expand slightly. Quartz is an example of a material with this property.
At the heart of a wristwatch sits a tiny quartz crystal, and the physical pressure on it is delivered by an electric current.
When an electric current is applied to the crystal, its atoms vibrate at an exceptionally high rate. The current comes from the watch's battery and is converted from direct current to alternating current by a switch that repeatedly reverses the direction of flow back and forth.
The alternating current causes the quartz crystal to oscillate at a frequency of 32,768 Hz (= cycles per second).
A dedicated counter measures these oscillations and adds one digit to the seconds display upon the completion of each full cycle.
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The frequency of a quartz crystal is determined by its precise dimensions and shape.
To achieve the exact required frequency, lasers are used to cut the crystal to the correct size, and a gold coating is applied to the crystal's ends.
Even so, a slight deviation of about half a second per day remains. In addition, temperature changes can affect the crystal's dimensions and thereby alter its frequency.
Various methods are used to achieve maximum accuracy — for example, measuring several crystals in parallel, or using an electronic circuit.
Despite the remarkably low cost of modern wristwatches, it is worth remembering that the quartz crystal at their core is a genuine feat of engineering.
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The Mohs scale is used to measure the hardness of minerals, with diamond sitting at the highest hardness rating.
Quartz is rated at hardness 7, meaning it is an exceptionally hard material.
Since the precise shape and dimensions of quartz are critical to its ability to generate the required frequency, its high hardness rating carries great significance — it allows the crystal to maintain its shape over time and under varying temperature conditions.
In the video: a quartz crystal in nature.
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