Measurements

Outcomes:SI Units

Measuring Skills

SI Units

• All scientific formula use metric units, for example speed is measured in metres per second and is calculated by dividing the distance in metres by the time in seconds.

• The metric system is a French system.• Britain and the USA still use miles and pounds,

but all their children are taught SI units.

• The speed of your car is measured in kilometres per hour. This looks like it is in SI units but as we have already discussed speed is measured in metres per second.

SI Units

Unit of length is the metreThe metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.Unit of time is the secondThe second is 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 cesium 133 atom.

Unit of mass is the kilogram The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.Unit of electric current is the ampereThe ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length.

Unit of thermodynamic temperature is the kelvinThe kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

• What are the SI units for volume?• What are the SI units for density?• What are the SI units for potential difference?• What are the SI units for frequency?