OCR A Level Physics Definitions

Boyle's Law

The pressure of an ideal gas is inversely proportional to its volume, provided that the mass of gas and the temperature do not vary.

Absorption line spectrum

A set of specific frequencies of electromagnetic radiation, visible as dark lines in an otherwise continuous spectrum on spectroscopy. They are absorbed by atoms as their electrons are excited between energy states by absorbing the corresponding amount of energy in the form of photons - every element has a characteristic line spectrum

Activity

The rate at which nuclei decay or disintegrate in a radioactive source, measured in becquerels (Bq) or decays per second

Alpha radiation

Ionising radiation consisting of particles comprising two protons and two neutrons (a helium nucleus), with a charge of +2e

Angular velocity

The rate of change of angle for an object moving in a circular path - symbol ω

Annihilation

The complete destruction of a particle and its antiparticle in an interaction that releases energy in the form of identical photons

Aphelion

The furthest point from the Sun in an orbit

Arcminute

A minute of arc; 1° = 60 arcminutes

Arcsecond

A second of arc; 1 arcminute = 60 arcseconds

Astronomical unit

The mean distance from the Earth to the Sun, i.e. 150 million km or 1.50 x 10¹¹ m

Atomic mass unit

One atomic mass unit (1 u) is one-twelfth the mass of a neutral carbon-12 atom

Attenuation

The decrease in the intensity of electromagnetic radiation as it passes through matter and/or space

Attenuation coefficient

A measure of the absorption of X-ray photons by a substance, also known as absorption coefficient - SI unit m⁻¹

Avogadro constant

6.02 x 10²³, the number of atoms in 0.012 kg (12 g) of carbon-12; symbol Nₐ

Background radiation

The radiation emitted by the surroundings, which must be measured before radiation produced in an experiment can usefully be measured

Baryon

Any hadron made with a combination of three quarks

Becquerel

A unit of activity - one becquerel is an activity of one decay per second

Beta decay

A neutron in an unstable nucleus decays into a proton, an electron, and an electron antineutrino (β⁻ decay), or a proton into a neutron, a positron, and an electron neutrino (β⁺ decay)

Beta radiation

Ionising radiation consisting of fast-moving electrons (β⁻) or (β⁺) emitted from unstable nuclei, with a charge of -e or +e, respectively

Big Bang

The theory that at a moment in the past all the matter in the Universe was contained in a singularity (a single point), the beginning of space and time, that expanded rapidly outwards

Binding energy

The minimum energy required to completely separate a nucleus into its constituent protons and neutrons

Binding energy per nucleon

The binding energy divided by the number of protons and neutrons in the nucleus; the greater the binding energy per nucleon, the more tightly bound are the nucleons within the nucleus

Black body

An idealised object that absorbs all the electromagnetic radiation incident on it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature

Black hole

The remnant core of a massive star after it has gone supernova and the core has collapsed so far that in order to escape it an object would need an escape velocity greater than the speed of light, and therefore nothing, not even photons, can escape

Blue shift

The shortening of observed wavelength that occurs when a wave source is moving towards the observer - in astronomy, if a galaxy is moving towards the Earth, the absorption lines in its spectrum will be blue-shifted, that is, moved towards the blue end of the spectrum

Boltzmann constant

The molar gas constant R divided by the Avogadro constant (N_A), a constant that relates the mean kinetic energy of the atoms or molecules in a gas to the gas temperature - symbol k.

Brownian motion

The continuous random motion of small particles suspended in a fluid, visible under a microscope.

capacitance

The charge stored per unit potential difference across a capacitor.

carbon dating

A method for determining the age of organic material, by comparing the activities, or the ratios, of carbon-14 to carbon-12 nuclei of the dead material of interest and similar living material.

centripetal acceleration

The acceleration of any object travelling in a circular path at constant speed, which always acts towards the centre of the circle.

centripetal force

A force that keeps a body moving with a constant speed in a circular path.

chain reaction

A reaction in which the neutrons from an earlier fission stage are responsible for further fission reactions leading to an exponential growth in the rate of the reactions.

Chandrasekhar limit

The mass of a star's core beneath which the electron degeneracy pressure is sufficient to prevent gravitational collapse, 1.44 solar masses.

charge carrier

A particle with charge that moves through a material to form an electric current - for example, an electron in a metal wire.

cloud chamber

A detector of ionising radiation consisting of a chamber filled with air saturated with vapour at a very low temperature so that droplets of liquid condense around ionised particles left along the path of radiation.

collimator

Part of a gamma camera, a honeycomb of long, thin tubes made from lead that absorbs any photons arriving at an angle to the axis of the tubes so that a clear picture is obtained.

comet

A small, irregular body made of ice, dust, and small pieces of rock in an (often highly eccentric elliptical) orbit around the Sun - as they approach the Sun, some comets develop spectacular tails.

conical pendulum

A simple pendulum that, instead of swinging back and forth, rotates in a horizontal circle at constant speed.

continuous spectrum

A spectrum in which all visible frequencies or wavelengths are present (a heated solid metal such as a lamp filament will produce this type of spectrum).

control rods

Rods made of a material whose nuclei readily absorb neutrons (commonly boron or cadmium), which can be moved into or out of a reactor core to ensure that exactly one slow neutron survives per fission reaction or to completely stop the fission reaction.

coolant

A substance that removes the thermal energy produced from reactions within a fission reactor.

corrected count rate

The radiation count rate measured in an experiment minus the background count rate.

cosmological principle

The assumption that, when viewed on a large enough scale, the Universe is homogeneous and isotropic, and the laws of physics are universal.

Coulomb's law

Any two point charges exert an electrostatic (electrical) force on each other that is directly proportional to the product of their charges and inversely proportional to the square of their separation.

coupling gel

A gel with acoustic impedance similar to that of skin smeared onto the transducer and the patient's skin before an ultrasound scan in order to fill air gaps and ensure that almost all the ultrasound enters the patient's body.

damping

An oscillation is damped when an external force that acts on the oscillator has the effect of reducing the amplitude of its oscillations.

dark energy

A hypothetical form of energy that fills all of space and would explain the accelerating expansion of the Universe.

dark matter

A hypothetical form of matter spread throughout the galaxy that neither emits nor absorbs light - it could explain the differences between the predicted and observed velocities of stars in galaxies.

daughter nucleus

A new nucleus formed following a radioactive decay.

decay constant

The probability of decay of an individual nucleus per unit time.

diffraction grating

A glass or plastic slide on which as many as 1000 lines in a millimetre are ruled, at a spacing that diffracts visible wavelengths of light.

Doppler effect

The change in the frequency and wavelength of waves received from an object moving relative to an observer compared with what would be observed without relative motion.

driving frequency

The frequency with which the periodic driver force is applied to a system in forced oscillation.

electric field strength

The force experienced per unit positive charge at that point.

electric potential

The work done by an external force per unit positive charge to bring a charge from infinity to a point in an electric field - unit volt or J C(^{-1}).

electric potential difference

The work done by an external force per unit positive charge to move a charge between two points in an electric field.

electron degeneracy pressure

A quantum mechanical pressure created by the electrons in the core of a collapsing star due to the Pauli exclusion principle.

ellipse

An elongated 'circle' with two foci.

emission line spectrum

A set of specific frequencies of electromagnetic radiation, visible as bright lines in spectroscopy, emitted by excited atoms as their electrons make transitions between higher and lower energy states, losing the corresponding amount of energy in the form of photons as they do so - every element has a characteristic line spectrum.

energy level

A discrete (quantised) amount of energy that an electron within an atom is permitted to possess.

equation of state of an ideal gas

pV = nRT, where n is the number of moles.

escape velocity

The minimum velocity at which an object has just enough energy to leave a specified gravitational field

excited (an atom)

Containing an electron or electrons that have absorbed energy and been boosted into a higher energy level

expanding Universe

The idea that the fabric of space and time is expanding in all directions and that as a result any point, in any part of the Universe, is moving away from every other point in the Universe, and the further the points are apart the faster their relative motion away from each other

exponential decay

A constant ratio process in which a quantity decreases by the same factor in equal time intervals

Faraday's law

The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage

fiducial marker

A marker for a point used as a fixed basis for measurement or comparison

fission

A process in which a large nucleus splits into two smaller nuclei after absorbing a neutron

Fleming's left-hand rule

A mnemonic for the direction of the force experienced by a current-carrying wire placed perpendicular to the external magnetic field: on the left hand, the first finger gives the direction of the external magnetic field, the second finger gives the direction of the conventional current, and the thumb gives the direction of motion (force) of the wire

forced oscillation

An oscillation in which a periodic driver force is applied to an oscillator

free oscillation

The motion of a mechanical system displaced from its equilibrium position and then allowed to oscillate without any external forces

frequency (oscillation)

The number of complete oscillations per unit time - unit Hertz (Hz)

fundamental particle

A particle that has no internal structure and hence cannot be split into smaller particles

fusion

A process in which two smaller nuclei join together to form one larger nucleus

galaxy

A collection of stars and interstellar dust and gas bound together by their mutual gravitational force

gamma radiation

Ionising radiation consisting of high-energy photons, with wavelengths less than about (10^{-11}) m, which travel at the speed of light

gas pressure in stars

The pressure of the matter in the star's core pushing outwards and counteracting the gravitational force pulling the matter in the star inwards

geostationary satellite

A satellite that remains in the same position relative to a spot on the Earth's surface, by orbiting in the direction of the Earth's rotation over the equator with a period of 24 hours

grating equation

An equation that can be used to determine accurately the wavelength of monochromatic light sent through a diffraction grating, (d \sin \theta = n \lambda)

grating spacing

The separation between adjacent lines or slits in a diffraction grating

gravitational field

A field created around any object with mass, extending all the way to infinity

gravitational field strength, g

The gravitational force exerted per unit mass at a point within a gravitational field.

gravitational potential

The work done per unit mass to bring an object from infinity to a point in the gravitational field - unit J kg⁻¹.

ground state

The energy level with the most negative value possible for an electron within an atom - the most stable energy state of an electron.

hadron

A particle or antiparticle that is affected by the strong nuclear force, and, if charged, by the electromagnetic force - for example, a proton.

half-life

The average time it takes for half the number of active nuclei in a sample of an isotope to decay.

heavy damping

Damping that occurs when the damping forces are large and the period of the oscillations increases slightly with the rapid decrease in amplitude.

Hertzsprung-Russell diagram

A graph showing the relationship between the luminosity of stars in our galaxy (on the y-axis) and their average surface temperature (on the x-axis, with temperature increasing from right to left).

homogeneous

Uniform in terms of the distribution of matter across the Universe when viewed on a sufficiently large scale.

Hubble constant

The gradient of a best-fit line for a plot of recessional speed against distance from Earth of other galaxies.

Hubble's law

The recessional speed v of a galaxy is almost directly proportional to its distance d from the Earth.

impedance matching (or acoustic matching)

The use of two substances with similar acoustic impedance to minimise reflection of ultrasound at the boundary between them.

induced fission

Nuclear fission occurring when a nucleus becomes unstable on absorbing another particle (such as a neutron).

intensity reflection coefficient

The ratio of reflected intensity over incident intensity for ultrasound incident at a boundary.

internal energy

The sum of the randomly distributed kinetic and potential energies of the atoms, ions, or molecules within the substance.

ionising radiation

Any form of radiation that can ionise atoms by removing an electron to leave a positive ion.

isochronous oscillator

An oscillator that has the same period regardless of amplitude.

isotherm

A line on a pressure-volume graph that connects points at the same temperature.

isotopes

Nuclei of the same element that have the same atomic number (number of protons) but different nucleon numbers (numbers of neutrons).

isotropic

The same in all directions (for example the Universe, appearing the same to any observer regardless of position).