Physics Study Guide/Basic Units

The SI System of Measurement

Simple Units

Time

It is defined as the duration between two events. In the international system of measurement (S.I.) the second (s) is the basic unit of time and it is defined as the time it takes a cesium (Cs) atom to perform 9 192 631 770 complete oscillations. The Earth revolves around its own axis in 86400 seconds; this time is known as 1 day, and the 86400th part of one day is known as a second.

Distance

In the international system of measurement (S.I.) the meter (m) is the basic unit of distance and is defined as the distance traveled by light in 1/(299 792 458)th of a second.

Mass

In the international system of measurement (S.I.) the kilogram (kg) is the basic unit of mass and is defined as the mass of a platinum-iridium alloy cylinder kept at the Bureau International des Poids et Mesures in Paris France. See Wikipedia article.

Current

In the international system of measurement (S.I.) the ampere (A) is the basic measure of electrical current. It is defined as the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre (m) apart in vacuum, would produce between these conductors a force equal to 2×10^-7 newton (N) per metre of length.

Unit of Thermodynamic Temperature

The kelvin (K), unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Unit of Amount of Substance

1. The mole (mol) is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.

2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Luminous Intensity

The candela (cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Derived Units

Charge

The SI unit of charge is the coulomb (C). It is equal to 1 A*s (ampere times second).

Velocity

The SI unit for velocity is in m/s or meters per second.

Energy

The SI unit of energy is the joule (J). The joule has base units of kg·m²/s² = N·m. A joule is defined as the work done or energy required to exert a force of one newton for a distance of one metre. See Wikipedia article.

Force

The SI unit of force is the newton (N), named after Sir Isaac Newton. It is equal to $1\mathrm{k}\mathrm{g}\cdot \mathrm{m}/{\mathrm{s}}^2$.

Prefixes

Prefix	yotta	zetta	exa	peta	tera	giga	mega	kilo	hecto	deca		deci	centi	milli	micro	nano	pico	femto	atto	zepto	yocto
Symbol	Y	Z	E	P	T	G	M	k	h	da		d	c	m	µ	n	p	f	a	z	y
10n	1024	1021	1018	1015	1012	109	106	103	102	101	100	10-1	10-2	10-3	10-6	10-9	10-12	10-15	10-18	10-21	10-24
1000n	10008	10007	10006	10005	10004	10003	10002	10001						1000-1	1000-2	1000-3	1000-4	1000-5	1000-6	1000-7	1000-8