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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twelve (Why do digital watches lose time in the winter)

Lesson twelve
ProfVietanhNgày: Thứ năm, 2010-04-29, 3:27 PM | Tin nhắn # 1
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Why do digital watches lose time in the winter

A digital watch keeps time by applying a tiny potential (voltage) across a crystal of quartz, causing it to vibrate at a precise frequency v cycles per second. The watch keeps time by counting off 1 s each time the quartz vibrates v times, explaining why the majority of the components within the watch comprise a counting mechanism.

Unfortunately, the number of vibrations of the crystal per second is dictated by the potential applied to the quartz, so a larger voltage makes the frequency v increase, and a smaller voltage causes v to slow. For this reason, the potential of the watch battery must be constant.

As we have known the value of deltaG is never independent of temperature, except in those rare cases when deltaS(cell) = 0. Accordingly, the value of deltaG(cell) for a battery depends on whether someone is wearing the watch while playing outside in the cold snow or is sunbathing in the blistering heat of a tropical summer.

And the electromotive force, emf, of the watch battery is itself a function of the change in Gibbs function, deltaG(cell) according to

deltaG(cell) = -nF × emf

Where F is the Faraday constant and n is the number of moles of electrons transferred per mole of reaction. The value of deltaG(cell) is negative if the reaction proceeds reversibly, so the emf is defined as positive to ensure that deltaG(cell) is always negative. In other words, the value of deltaG(cell) relates to the spontaneous cell reaction.

So we understand that as the emf changes with temperature, so the quartz crystal vibrates at a different frequency – all because deltaG(cell) is a function of temperature. Ultimately, then, a digital watch loses time in the winter as a simple result of the cold.

Vocabulary
quartz: thạch anh
vibrate: dao động, rung
precise: chính xác, đúng
frequency: tần số, gồm có
dictate: điều khiển, bức chế
blistering: khắc nghiệt, nghiêm khắc
electromotive force: sức điện động


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
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