How many base units are there in the International System of Units (SI)?
- A.Five
- B.Six
- C.Seven
- D.Nine
Correct answer
Explanation
The correct answer is C, seven. The SI system, adopted in 1960 by the General Conference on Weights and Measures, rests on seven base units: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela. Every other unit in science is derived from these by multiplication and division.
Options A and B fall short of the list and usually come from forgetting the mole or the candela, which are the two least familiar. Option D, nine, comes from adding the radian and the steradian, the units of plane angle and solid angle; these were long treated as a separate class called supplementary units and are not counted among the base units. A safe way to remember the seven is by their quantities: length, mass, time, current, temperature, amount of substance and luminous intensity.
Read the full article: Units and Measurements: SI Units, Dimensions and Instruments
Practice Questions
View allWhat is the SI unit of force?
- A.Joule
- B.Newton
- C.Watt
- D.Pascal
Show answer
One newton is the force that gives a mass of one kilogram an acceleration of one metre per second squared, which follows directly from the equation force equals mass times acceleration.
Read the full explanation →Which one of the following is NOT a base unit of the SI system?
- A.Kelvin
- B.Mole
- C.Newton
- D.Candela
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The newton is a derived unit: it is built from the kilogram, the metre and the second, since force equals mass times acceleration, and it can be written as one kilogram metre per second squared.
Read the full explanation →The candela is the SI unit of which physical quantity?
- A.Luminous intensity
- B.Electric current
- C.Amount of substance
- D.Pressure
Show answer
The candela takes its name from the candle, which was once used as a rough standard of light, and it is one of the seven base units of the SI system.
Read the full explanation →A light year is a unit of which of the following?
- A.Time
- B.Distance
- C.Speed
- D.Brightness of a star
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A light year is the distance that light, moving at about three lakh kilometres per second, covers in one year, which works out to roughly 9.46 trillion kilometres.
Read the full explanation →The SI unit of pressure is
- A.Newton
- B.Pascal
- C.Hertz
- D.Tesla
Show answer
Pressure is force divided by area, so its unit is one newton per square metre, which is given the name pascal after Blaise Pascal.
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