inertia

Inertia is a property of all bodies that resists any change in their state of motion. The relationship between mtion and inertia is given by Newton's first law of motion

See also Newton's laws of motion

infrared

A form of electromagnet radiation emitted from the surface of all bodies and invisible to the eye, but perceptible as warmth by the skin. Infrared is also known as thermal radiation since it will cause an object that absorbs it to warm up.

The Sun emits about half its radiant energy as infrared, and the other half as light.

See also electromagnetic spectrum

infrasound

Mechanical vibrations with a frequency below 20 Hz, i.e. below the threshold of human hearing. Infrasounds are thus not audible, though they can be felt when they are intense. Sources of infrasound: engine vibrations and earthquakes.

insulator

Materials that do not conduct an electrical current (except a very high voltages) are called electrical insulators. Any material without free electrons is an electrical insulator. Plastics, rubber and air are all good electrical insulators.

Materials that are poor conductors of thermal energy are known as thermal insulators. Gases and liquids are poor conductors of thermal energy and hence make good insulators as long as they are prevented from convecting. This may be done by trapping either gas or liquid in tiny pockets within a solid. Expanded polystyrene, which is an excellent thermal insulator, is an example of air trapped in this manner.

interference

Interference is a phenomenon common to all wave motion. It occurs whenever two or more waves combine.

The combination can be constructive or destructive. When waves arrive at a point in step they combine constructively, and destructive when waves arrive out of step.

If the interference is destructive, the wave appears to vanish, though its energy has actually been transferred elsewhere.

ion

An atom or molecule becomes an ion if it gains or looses one or more electrons.

See also ionisation & plasma

ionising radiation

Any radiation that causes ionisation when absorbed by an atom. Such radiation consists either of a stream of high energy particles (e.g. electrons, protons, alpha particle, beta particle) or high frequency electromagnetic radiation (e.g. ultraviolet, X-rays, gamma rays).

Ionising radiation can be detected with a Geiger counter or photographic film, which it will fog (i.e. when the film is developed the exposed film comes out dark, whereas an unexposed film will come out clear)

Ionising radiation is harmful to living organisms since it will cause cells to die or affect genetic material.

ionisation

All electrical phenomena are due to the equal and opposite charges carried by the protons and electrons which make up all atoms. In the normal state an atom is electrically neutral since it has as many protons (which have a positive charge) as it has electrons (which are negatively charged). If it loses or gains electrons it is said to become ionised. Losing electrons gives the atom an overall positive charge while gaining them makes it negative. Under normal circumstances it is not possible for the atom to lose or gain protons because they are bound up within the nucleus and therefore cannot be removed without destroying the atom. If all electrons are lost, a substance becomes a plasma.

isotope

All atoms of the same element must have the same number of protons in the nucleus. However, they may have different number of neutrons. This makes them isotopes of the element. All elements have isotopes. e.g. there are 3 isotopes of hydrogen. Information about the number of nucleons and protons in an isotope is given by

where A is the number of nucleons (or atomic mass), Z is the number of protons (or atomic number) and X is the element.

All elements have unstable isotopes, i.e. isotopes that emit alpha particles or beta particles to make isotopes of other elements.