The present SI unit of time is defined as:
Correct answer: C. Duration of 9,192,631,770 vibrations of a cesium-133 atom
- A. 1/24 x 1/60 x 1/60 of a mean solar day
- B. The second as defined by the electronic digital watch
- C. Duration of 9,192,631,770 vibrations of a cesium-133 atom
- D. Minute
Explanation
The SI unit of time is the second, symbolized as 's'. The modern definition is based on the cesium-133 atom's vibrations: specifically, 9,192,631,770 vibrations. This provides a highly precise standard. Option A refers to an outdated definition based on the Earth's rotation, which is variable. Option B is incorrect because digital watches use the standard definition rather than define it. Option D is incorrect because a minute is a derivative unit, not the base SI unit.
This question appeared on the ETEA MDCAT 2008 paper, which you can sit online with every answer explained.
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