International System of Units

Question 1

Length

Answer 1

meter (m)

The distance travelled by light in vacuum in
1/299792458 second

Question 2

Mass

Answer 2

kilogram (kg)

The mass of the international prototype kilogram

Question 3

Time

Answer 3

second (s)

The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

Question 4

Electric Current

Answer 4

ampere (a)

The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10^−7 newtons per meter of length

Question 5

thermodynamic temperature

Answer 5

kelvin (k)

1/273.16 of the thermodynamic temperature of the triple point of water

Question 6

Amount of Substance

Answer 6

mole (mol)

The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.

Question 7

Luminous Intensity

Answer 7

candela (cd)

The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.