Physics - Units

The international system of units (SI units) has 3 primary units and a number of secondary units. Other quantities such as the joule, newton... can be defined by different combinations of these units as described on the standards page.

Primary Units

unit   of definition
Second s Time The time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom.
Metre m Length The length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.
kilogram kg Mass

The mass of the international prototype kept in Sèvres, France.

It is proposed that this is changed such that the Plank constant is exactly 6.6260693×10-34 joule seconds.

Is it nesessary to have 3 units? We could relate distance and time by the speed of light which is a physical constant. So for example distance could be measured in seconds. One second would be the distance it
takes light to go in a second (similar to a light-year). We would then only
need two units: seconds and kg.

So instead of the hard to remember equation:
E = m c^2
we would have the much simpler:
E = m

This idea for a system of units where c=1 is mentioned in The Feynman lectures on Physics, book 1, 17-3.

Another question is whether Newtons (force) is more fundamental than kg (mass)?

Secondary Units

unit   of current definition proposed definition
ampere A Current The current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vacuum would produce between these conductors a force equal to 2 ×10-7newtons per metre of length. The ampere is such that the elementary charge is exactly 1.60217653×10-19 coulombs (1 coulomb = 1 ampere second)
kelvin K Temperature The fraction 1/273.16 of the thermodynamic temerature of the triple point of water. The kelvin is such that the Bolzmann constant is exactly 1.3806505×10-23 joules per kelvin.
mole mol Amount of substance The amount of substance that contains as many elementary units as there are atoms in 0.012 kilograms of carbon-12. The mole is such that the Avogadro constant is exactly 6.0221415 ×1023 per mole.
candela cd Luminous intensity The intensity, in a given direction, of a light souce that emits monochromatic radiation of frequency 540 ×1012 hertz with a radiant intensity in that direction of 1/683 watts per steradian. unchanged




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