half life

Half life is the time taken for half the unstable nuclei in a radioactive sample to decay. Half life is also the time in which the activity of a sample of material to fall to half its initial value.

The isotope with the longest half life is Uranium-238, which has a half life of approximately 4.5 billion years (equal to the age of the Earth).

WWW: JAVA applet: half life

heliocentric

Heliocentric means centred on the Sun. The heliocentric theory was put forward by Nicholas Copernicus (1473-1543) a Polish astronomer and mathematician, in 1543 as a solution to the problem that astronomers had in predicting the motion of planets accurately based on the geocentric theory that was then current.

hertz

Frequency is measured in herz (Hz). 1 herz is one cycle per second.

Hooke's Law

The rule, discovered by Robert Hooke (1635-1703) an English scientist, architect and inventor, that links the amount by which an object is deformed to the force causing it to deform. Hooke's law states that extension is proportional to force, and applies only to elastic deformations.

Force = stiffness x extension

See also elastic deformation

hydro electricity

Electrical energy that is produced by making use of the transfer of g.p.e. of water into k.e. as it flows downhill.

WWW: HYDROELECTRICITY