Question

Which of the following SI unit has been expressed in terms of certain interval of time?
1. Mass - kilogram
2. Electric current - ampere
3. Temperature - kelvin
4. Length - meter

Answer 1

Correct Answer - Option 4 : Length - meter

CONCEPT:

• At the present time, We use an atomic standard of time, based on the cesium atom.
• Atomic clocks are very accurate clocks.
• Due to their accuracy, the SI unit of length has been expressed in terms of the time interval of a light-year.

EXPLANATION:

• At the present time, We use an atomic standard of time, based on the cesium atom.
• These are very accurate clocks.
• Due to their accuracy, the SI unit of length has been expressed in terms of the path length light travels in a certain interval of time (1/299, 792, 458 of a second).
• Definition of meter: A meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. (1983)
• So the correct answer is option 4.

Some definitions of SI units:

• Kilogram: The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at the International Bureau of Weights and Measures, at Sevres, near Paris, France.
• Ampere: The ampere is that constant current which, if maintained in current two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 210ñ7 newton per meter of length.
• Kelvin: The kelvin, is the fraction 1/273.16 of the thermodynamic dynamic temperature of the triple point of water.