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| The balls of a pendulum are in perfect rhyme, measuring the passage of time. |
Alpha particle A particle released by radioactive decay that consists of two protons and two neutrons – equivalent to the nucleus of a helium atom.
Beta particle A particle released by radioactive beta decay – usually an electron, but rarely a positron. Beta particles are released from unstable atomic nuclei when a neutron transforms into a proton or, more rarely, vice versa.
Gamma radiation A form of high-energy electromagnetic radiation released by various processes such as radioactive decay.
Electron A low-mass elementary particle carrying negative electrical charge. Electrons are found in the orbital shells surrounding an atomic nucleus, and play a key role in chemical bonding. They are also the principal means by which electric current flows through materials.
Proton A subatomic particle with substantial mass and positive electric charge, normally found in the atomic nucleus.
Neutron An electrically neutral subatomic particle with substantial mass, found in the nuclei of atoms.
Photon A discrete burst or ‘packet’ of electromagnetic energy that can display wave-like, as well as particle-like, behaviour.
Nucleon A catch-all term for the principal particles found in atomic nuclei – protons and neutrons. The number of nucleons in an atom determines its atomic mass.
Fermion Any particle with a half-integer ‘spin’ property, including all the elementary matter particles (known as quarks and leptons). Fermions are governed by the Pauli exclusion principle, which limits their behaviour and explains much of the structure of matter.
Boson A particle with a zero or whole-number ‘spin’. Bosons display unique physics because they are unaffected by the Pauli exclusion principle.
Lepton Any member of a family of elementary particles that are not susceptible to the strong nuclear force, including electrons and neutrinos.
Isotope A term used to distinguish between atoms of the same elements with different masses, due to differing numbers of neutrons in their atomic nuclei.
Radioisotope An isotope of an atom that is unstable (typically due to an excess of neutrons over protons in its nucleus) and prone to undergoing radioactive decay in order to reach a more stable configuration.
Exponential decay A common pattern found in physical processes in which the rate of a process decreases in proportion to its value. For example, radioactive decay, in which the rate of decay is proportional to the number of radioactive nuclei present in a material, and therefore decreases over time.
Atomic number A property that indicates the number of protons in an atomic nucleus, and therefore the number of electrons in the corresponding neutral atom. An atom’s atomic number defines which element it forms.
Atomic mass A measure of the mass of any atom in ‘atomic mass units’ – in practice, equivalent to the total number of protons plus neutrons it contains.
Endothermic process Any chemical or physical process that absorbs energy from the surrounding environment.
Exothermic process Any chemical or physical process that generates and releases an excess of energy.
Cosmic rays High-energy particles generated by various processes in deep space, which give rise to showers of lower-energy particles as they interact with Earth’s upper atmosphere.
Electromagnetic radiation A natural wave phenomenon consisting of electrical and magnetic waves interfering with and reinforcing each other. It can exhibit very different properties depending on its wavelength, frequency and energy.
Conventional current The notional direction in which positive charge flows around a circuit – opposite to the physical flow of electrons.
Potential difference A measure of the ‘electric potential energy’ between two points. Measured in volts, it indicates the work that must be done in moving a unit of electric charge through the electric field between the points.
Vacuum tube An electronic device that controls the flow of electric current from a beam fired through a vacuum between two electrodes.
Magnetic moment A property determining the strength of the magnetic field created by an object, and its susceptibility to the influence of other magnetic fields.
Momentum A property found by multiplying an object’s mass by its velocity in a particular direction. Momentum determines the force required to stop an object, or accelerate it to a particular velocity.
Angular momentum A property of rotating objects analogous to momentum, and linked to their inertia and rate of rotation around an axis of rotation.
Spin A property of subatomic particles, analogous to angular momentum in larger objects, which affects many aspects of their behaviour.
Orbital shell A region surrounding an atomic nucleus, in which electrons are found.
Frame of reference Any consistent system of coordinates that can be used to measure the properties of objects. Frames of reference in motion to one another are particularly relevant to the study of relativity, since observations made in different reference frames can differ greatly.
Inverse square law A pattern found in physical processes in which the strength of a property such as a force decreases as the square of the distance from its source increases, as the force becomes more ‘spread out’ across a spherical region of space.
Pauli exclusion principle A law that prevents fermion particles from occupying identical ‘states’ in a system, and is therefore responsible for much of the structure of matter.
Planck’s constant A physical constant that helps define quantum-scale relations such as that between the frequency of a photon and the energy it contains.
Quantum The minimum possible amount of a particular physical property that may be involved in a physical interaction. Certain phenomena, such as the energy of light waves and of electrons in an atom, are inherently ‘quantized’ on the smallest scale. By extension, quantum physics is a term used to describe the strange and sometimes counterintuitive physics that occurs on very small, subatomic scales.
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