M

Magnet

A magnet is a material or object that produces a magnetic field. A simple permanent magnet like a bar magnet has two poles, called 'North' and 'South'. Like poles repel and unlike poles attract.

Magnetic domain

Tiny physical regions in permanent magnets, approximately 0.01 to 1 mm, that have magnetically aligned atoms, giving the domain an overall polarity.

Magnetic dipole

A dipole with opposing magnetic poles.

Magnetic field

A field of force, defined in terms of the torque exerted on a test dipole.

Magnetic poles

The ends, or sides, of a magnet about which the force of magnetic attraction seems to be concentrated.

Magnetic quantum number

The magnetic quantum number is the third of a set of quantum numbers (the principal quantum number, the azimuthal quantum number, the magnetic quantum number, and the spin quantum number) which describe the unique quantum state of an electron and is designated by the letter m.

Magnetic reversal

The flipping of polarity of the earth's magnetic field as the north magnetic pole and the south magnetic pole exchange positions.

Magnification

The factor by which an image's linear size is increased (or decreased). Cf. angular magnification.

Magnitude

The size of a measurement of a vector and scalar quantities that consist of a number and unit only, no direction.

Mass defect

The difference between the sum of the masses of the individual nucleons forming a nucleus and the actual mass of that nucleus.

Mass number

The sum of the number of protons and neutrons in a nucleus defines the mass number of an atom; used to identify isotopes; for example, Uranium 238.

Mass

The quantity of matter contained in a body is called its mass. The SI unit of mass is kg. The mass of a body remains the same everywhere. It is a measure of inertia, which means a resistance to a change of motion.

Matter

Objects that take up space and have mass are called matter.

Mechanical energy

The form of energy associated with machines, objects in motion, and objects having potential energy that results from gravity.

Mechanical wave

The waves, which need a material medium for their propagation, are called mechanical waves. Mechanical waves are also called elastic waves. Sound waves, water waves are examples of mechanical waves.

Melting point

The temperature at which a phase change of solid to liquid takes place; the same temperature as the freezing point for a given substance.

Mercury (metal)

Mercury is a metal which is liquid at room temperature. It is often used in liquid in glass thermometers. It is a toxic substance.

Metal

Matter having the physical properties of conductivity, malleability, ductility, and luster.

Meteorite

This is a lump of material which manages to reach Earth's surface, having only partially burned up on the way down.

Meteoroid

A solid object moving in interplanetary space, of a size considerably smaller than a asteroid and considerably larger than an atom or molecule.

Metre

The metre is the SI unit of length. The way it is defined means the speed of light is exactly 299,792,458 metres per second.

Microphone

A microphone is an input device in an electronic system. It converts sound energy into electrical energy. It is therefore a transducer.

Microwaves

Microwaves, along with light, UV, x-rays and gamma radiation form part of the electromagnetic spectrum. The wavelength of microwaves is longer than that of infra-red radiation, but shorter than that of radio waves. Microwaves are used commonly in mobile phones and in microwave ovens.

Millibar

A measure of atmospheric pressure equivalent to 1.000 dynes per cm ^2.

Milli rem

A unit of radiation exposure equal to one-thousandth of a REM.

Miniature Circuit Breaker (MCB)

Instead of using a fuse to protect an electrical flex, a miniature circuit breaker (MCB) can be used . MCBs are sometimes known as Earth Leakage Circuit Breakers (ELCBs) or Residual Current Devices (RCDs).

Miscible fluids

Fluids that can mix in any proportion.

Mixture

Matter made of unlike parts that have a variable composition and can be separated into their component parts by physical means.

Mks system

Mks system system of units of measurement based on the metric system and having the meter of length, the kilogram of mass, and the second of time as its fundamental units.

Model

A mental or physical representation of something that cannot be observed directly that is usually used as an aid to understanding.

Moderator

In a nuclear fission reactor, the moderator slows fast neutrons down. Slower moving neutrons are more effective at keeping the chain reaction going by causing more uranium nuclei to break apart. Each fission event releases fast neutrons - the moderator is therefore required to slow them down.

Modulation

Modulation means change. In physics, this is usually used to describe changes made to the properties of a radio-wave or microwave carrier wave in order to carry information. There are two types of modulation - amplitude modulation and frequency modulation.

Mole

An amount of a substance that contains Avogadro's number of atoms, ions, molecules, or any other chemical unit; a mole is thus 6.02 x 10^23 atoms, ions, or other chemical units.

Molecule

A group of atoms stuck together.

Momentum

Momentum is considered to be a measure of the quantity of motion in a body. The momentum of a body is defined as the product of its mass and velocity. Its SI units are kg m /s.

Morse Code

Morse Code is a versatile and simple method of transmitting information. It uses any method of telecommunication (such as flashing lights, tap keys and wires or radio transmissions) to send information coded as sequences of dots and dashes (e.g. short and long pulses of voltage or flashes of light).

Motor

Motors convert electrical energy into kinetic energy, along with some unwanted sound and heat energy.