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Glossary of Physics Definitions

Acceleration
The change in velocity per unit time.
Accommodation
The ability of the eye to produce sharp images of objects at different distances, by altering the focal length of the eye lens.
Activity (of a radioisotope)
The number of radioactive disintegrations per second.
Alternating current
Current which reverses its direction at a fixed frequency.
Ampere
The current which when flowing in two straight parallel conductors of infinite length, placed on metre apart in vacuum, produces a force of 2 x 10–7 newtons per metre of length on each conductor.
Amplitude
The maximum extent of vibration or oscillation from the position of equilibrium.
Angle of incidence
The angle between the incident ray of light striking a surface, and the normal to the surface at the point of incidence.
Angle of reflection
The angle between the reflected ray of light and the normal to the surface at the point of reflection.
Angle of refraction
The angle between the refracted ray of light and the normal to the surface at the point of refraction.
Angular speed
The rate of motion of a body through an angle about an axis.
Anode
The positive electrode in an electrolytic cell or an electron tube.
Antinode
A point of maximum displacement in a standing wave.
Atomic number
The number of protons in the nucleus of an atom.

Becquerel
One radioactive disintegration per second.
Bel
The ratio of intensities of two sounds in which the power of one sound is ten times that of another.

Capacitance
The ability of an electrical device to store electric charge.
Cathode
The negative electrode in an electrolytic cell or an electron tube.
Centre of curvature (of a spherical mirror)
The centre of the sphere of which the mirror is part.
Centripetal acceleration
The acceleration caused by centripetal force.
Centripetal force
The inwards-acting force that keeps a body rotating around a central point.
Chain reaction
A nuclear reaction in which energy is continuously released as a result of neutrons, emitted by the fission of atomic nuclei, each splitting other nuclei and causing the emission of more neutrons.
Complementary colours
Two colours, which when mixed, produce white light.
Conductor
A material which is able to conduct heat and/or electricity.
Constructive
See Interference
Converging (or convex) lens

A lens that converges (brings together) parallel rays of light.
Couple
A force, or system of forces, that produces a turning effect.
Critical angle
The least angle of incidence of a ray of light passing from a dense to a less-dense medium, for which the angle of refraction is 90°.

Decay constant (of a radioisotope)
The proportion of the number of atoms that decay each second.
Density
The mass per unit volume of a substance.
Destructive
See Interference
Diffraction
The bending of waves, when they pass the edge of an obstacle, or through a small gap.
Diode
An electrical device through which current can only flow in one direction.
Dispersion
The separation of light into its component colours.
Displacement
The straight-line distance between two points.
Diverging lens
A lens that diverges (spreads out) parallel rays of light.
Doping
The addition to a semiconductor material of very small, but controlled amount, of another element.
Doppler effect
The apparent change in the frequency of a note due to the relative motion between the source of the note and the observer.

Eddy currents
Induced currents circulating throughout the cores of transformers, etc., as a result of changing magnetic flux.
Electric field intensity (strength)
The force acting on unit point charge when placed in that field.
Electromagnetic induction
The inducing of an emf in a conductor when it cuts, or is cut by, lines of magnetic force.
Electroplating
The placing of a thin layer of one metal on top of another by the process of electrolysis.
Electrolysis
The decomposition of a compound by passing an electric current through it.
Electrolyte
A substance, which when either molten or dissolved in water, conducts an electric current.

Farad
The capacitance of a capacitor such that a potential difference of one volt across its plates causes one coulomb of charge to be stored.
Focal length
The distance from the pole of a spherical mirror, or the centre of a lens, to its principle focus.
Focus
See Principle focus
Force

That which changes, or tends to change, the uniform motion of a body.
Forward bias
Descriptive of a diode connected to a battery so that it conducts.
Frequency
The number of complete cycles or oscillations of a vibrating system per unit time.
Friction
A force that opposes the relative sliding motion between two surfaces in contact with each other.
Fundamental frequency
The frequency of the note sounded when a string or column of air is vibrating as a whole unit.

Half life
The time taken for one half of a radioisotope to decay or change into another element.
Harmonic
A frequency that is a whole number multiple of the fundamental frequency of a vibrating system.
Heat
Energy possessed by a system due to the kinetic energy of its atoms or molecules.
Heat capacity
The amount of heat required to heat an object by 1 K.
Hertz
A frequency of one cycle per second.

Incident ray (of light)
A ray of light that strikes a mirror or other optical device.
Intensity (of sound)
The energy passing per second through each square metre of area, at right angles to its path.
Interference
The addition or combination of waves; it can be of two types: constructive, in which waves reinforce one another, and destructive, in which waves cancel each other out.
Isotopes
Atoms of the same element which have different numbers of neutrons.

Joule
The amount of work done when a force of one newton moves a distance of one metre.
Joule’s law
The heat produced electrically is proportional to the square of the current, the resistance of the heating wire, and the time of heating.

Kinetic energy
The energy possessed by a body by virtue of its motion.


Lever
A rigid body that can rotate about a fixed point.
Line of electric force
A line along which a positive electric charge would travel if placed in an electric field.
Longitudinal wave
A wave in which the direction of vibration of the particles is in line with the direction in which the wave is travelling.
Long sight
A defect of the eyeball in which parallel rays of light come to a focus beyond the retina.
Latent heat of vaporisation
The heat involved when a liquid changes to a vapour (or vice-versa).
Latent heat of fusion
The heat involved when when a solid changes to a liquid (or vice-versa).

Magnetic declination
The angle between magnetic north and geographical north at a given place.
Magnetic dip
The angle made with the horizontal by a freely-suspended magnetic needle at a given place on the Earth’s surface.
Mass number
The number of nucleons (protons + neutrons) in the nucleus of an atom.
Moment
The product of the magnitude of a force multiplied by the perpendicular distance between the force and the fulcrum.
Momentum
The product of a body’s mass and velocity.
Mutual induction
The inducing of an emf in one circuit as the result of changing magnetic flux in an adjacent circuit.
Myopia
See Short sight

Node
A point of zero displacement in a standing wave.
Normal, the
A line at right angles angles to the surface of a mirror or other optical device.
N-type material
A semiconductor material in which the presence of an impurity has increased the number of free electrons.
Nuclear fission
The splitting of large atomic nuclei into into smaller ones, with the release of energy.
Nuclear fusion
The combining together of small atomic nuclei into larger ones, with the release of energy.

Ohm
The SI unit of resistance, which is the resistance between two points such that a potential difference on one volt between the points produces a current of one ampere.
Ohm’s law
The current flowing through a device is proportional to the potential difference across it.

Pascal
The SI unit of pressure, of one newton per square metre.
Peak value (of a.c.)
The maximum positive or negative voltage reached by an alternating current or voltage.
Period
The time taken for any part of a wave to move forward one wavelength.
Permittivity
A measure of the degree to which a medium can resist the flow of electric charge.
Phase (of a wave)
The position of any particle of the medium in its cycle.
Photoelectric effect (or emission)
The emission of electrons from metals by the action of light.
Photon
A single quantum of radiant energy.
Plancks’s constant
The constant h in E = hf, which relates the energy E of a photon of electromagnetic radiation to its frequency f.
Potential difference (voltage)

The potential difference between two points is equal to the work done in moving one coulomb of charge from one point to the other.
Potential energy
The energy possessed by a body by virtue of its position or conformation.
Power (mechanical)
The rate at which work is done.
Power (of a lens)
The reciprocal of the lens’ focal length (expressed in metres).
Primary cell
A cell that produces electricity as a result of an irreversible chemical reaction in the cell.
Pressure
Force acting per unit area.
Primary colour
A colour that cannot be produced by mixing any combination of other colours. The three primary colours of light are red, green and blue.
Principle focus
The point relative to a spherical mirror or lens through which all parallel rays pass (or appear to come from) after being reflected or refracted.
P -type material
A semiconductor material in which the presence of an impurity has increased the number of “holes”.

Radioactivity
The spontaneous disintegration of unstable atomic nuclei with the resulting emission of radiation.
Radioisotope
An isotope possessing radioactivity.
Radioactive decay
The spontaneous disintegration of radioactive nuclei into nuclei of other elements.
Radius of curvature (of a spherical mirror)
The distance between the mirror and its centre of curvature.
Real image
The image produced by the actual intersection of light rays.
Rectifier
An electrical component or circuit that converts alternating current to direct current.
Refracted ray
A ray of light that has been refracted at an optical interface.
Refractive index
The constant n in Snell’s Law, i.e., n = sin i/sin r.
Reflected ray
A ray of light that has been reflected from a mirror.
Relative permittivity
The permittivity of a medium relative to the permittivity of a vacuum.
Resistance
A measure of the ability of a material to oppose the passage of electricity.
Resistivity
The resistance of a conductor of length 1 m and of cross-sectional area 1 m2.
Reverse bias
Descriptive of a diode connected to a battery so that it does not conduct.
Rms value (of a.c.)
The square root of the average of the squares of the current (I2) or the voltage (V2), taken over a complete cycle.
Root mean square
The square root of the average of the squares of a group of values.

Scalar quantity
A quantity that has magnitude but no direction.
Secondary cell
A cell that must be “charged” by passing an electric current through it in the reverse direction to its discharge.
Secondary colour
The colour produced when two primary colours are mixed.
Self induction
The inducing of an emf in a circuit due to it being cut by its own changing magnetic flux.
Semiconductor
A material with electrical resistance between those of conductors and insulators.
Simple harmonic motion
The motion of a body that is being acted on by a force (i) whose magnitude is proportional to the displacement of the body from a fixed point, and (ii) whose direction is always towards that point.
Short sight
A defect of the eyeball in which parallel rays of light come to a focus short of (in front of) the retina.
Sound intensity
See intensity of sound.
Specific heat capacity
The amount of heat needed to increase the temperature of 1 kg of a substance by 1 K.
Specific latent heat of fusion
The quantity of heat needed to change 1 kg of a substance from solid to liquid, at its melting point (or vice-versa).
Specific latent heat of vaporisation
The quantity of heat needed to change 1 kg of a substance from liquid to vapour, at its boiling point (or vice-versa).
Spectrum
The band of colours or lines produced when light or other electromagnetic radiations are separated into their component wavelengths.
Standing wave
The interference pattern produced when two waves of the same frequency and amplitude travel at the same time in opposite directions.


Temperature
A measure of the “hotness” of a substance.
Thermionic emission
The emission of electrons from the surface of a hot metal.
Threshold frequency
The minimum frequency of incident radiation that can cause photoelectric emission.
Total internal reflection
The total reflection of light travelling from one medium to another of lower refractive index.
Transverse wave
A wave in which the direction of vibration of the particles is at right angles to the direction in which the wave is travelling.

Universal constant of gravitation
The force of attraction between two bodies each of mass 1 kg and separated by a distance of 1 m in vacuum.

Vector quantity
A quantity that has both magnitude and direction.
Velocity
Displacement per unit time.
Virtual image
The image produced by the apparent intersection of light rays.
Volt
The potential difference between two points when one joule of work is needed to move one coulomb from one point to the other.

Watt
A rate of working of one joule per second.
Wavelengthf)
The minimum energy needed to release an electron from the metal surface.

In a wave motion, the distance between two successive crests (in a transverse wave) or compressions (in a longitudinal wave).
Wave motion
A method of transferring energy through a medium by means of the periodic motion of its particles.
Weber
The magnetic flux passing through an area of 1 m2 placed in a field of 1 T.
Weight
The force of attraction between an object and the Earth (or other astronomical body).
Work
This is done when a force moves an object; the amount of work is the product of the force and the distance moved.
Work function of a metal (

 

Glossary of Physics Definitions

Acceleration
The change in velocity per unit time.
Accommodation
The ability of the eye to produce sharp images of objects at different distances, by altering the focal length of the eye lens.
Activity (of a radioisotope)
The number of radioactive disintegrations per second.
Alternating current
Current which reverses its direction at a fixed frequency.
Ampere
The current which when flowing in two straight parallel conductors of infinite length, placed on metre apart in vacuum, produces a force of 2 x 10–7 newtons per metre of length on each conductor.
Amplitude
The maximum extent of vibration or oscillation from the position of equilibrium.
Angle of incidence
The angle between the incident ray of light striking a surface, and the normal to the surface at the point of incidence.
Angle of reflection
The angle between the reflected ray of light and the normal to the surface at the point of reflection.
Angle of refraction
The angle between the refracted ray of light and the normal to the surface at the point of refraction.
Angular speed
The rate of motion of a body through an angle about an axis.
Anode
The positive electrode in an electrolytic cell or an electron tube.
Antinode
A point of maximum displacement in a standing wave.
Atomic number
The number of protons in the nucleus of an atom.

Becquerel
One radioactive disintegration per second.
Bel
The ratio of intensities of two sounds in which the power of one sound is ten times that of another.

Capacitance
The ability of an electrical device to store electric charge.
Cathode
The negative electrode in an electrolytic cell or an electron tube.
Centre of curvature (of a spherical mirror)
The centre of the sphere of which the mirror is part.
Centripetal acceleration
The acceleration caused by centripetal force.
Centripetal force
The inwards-acting force that keeps a body rotating around a central point.
Chain reaction
A nuclear reaction in which energy is continuously released as a result of neutrons, emitted by the fission of atomic nuclei, each splitting other nuclei and causing the emission of more neutrons.
Complementary colours
Two colours, which when mixed, produce white light.
Conductor
A material which is able to conduct heat and/or electricity.
Constructive
See Interference
Converging (or convex) lens

A lens that converges (brings together) parallel rays of light.
Couple
A force, or system of forces, that produces a turning effect.
Critical angle
The least angle of incidence of a ray of light passing from a dense to a less-dense medium, for which the angle of refraction is 90°.

Decay constant (of a radioisotope)
The proportion of the number of atoms that decay each second.
Density
The mass per unit volume of a substance.
Destructive
See Interference
Diffraction
The bending of waves, when they pass the edge of an obstacle, or through a small gap.
Diode
An electrical device through which current can only flow in one direction.
Dispersion
The separation of light into its component colours.
Displacement
The straight-line distance between two points.
Diverging lens
A lens that diverges (spreads out) parallel rays of light.
Doping
The addition to a semiconductor material of very small, but controlled amount, of another element.
Doppler effect
The apparent change in the frequency of a note due to the relative motion between the source of the note and the observer.

Eddy currents
Induced currents circulating throughout the cores of transformers, etc., as a result of changing magnetic flux.
Electric field intensity (strength)
The force acting on unit point charge when placed in that field.
Electromagnetic induction
The inducing of an emf in a conductor when it cuts, or is cut by, lines of magnetic force.
Electroplating
The placing of a thin layer of one metal on top of another by the process of electrolysis.
Electrolysis
The decomposition of a compound by passing an electric current through it.
Electrolyte
A substance, which when either molten or dissolved in water, conducts an electric current.

Farad
The capacitance of a capacitor such that a potential difference of one volt across its plates causes one coulomb of charge to be stored.
Focal length
The distance from the pole of a spherical mirror, or the centre of a lens, to its principle focus.
Focus
See Principle focus
Force

That which changes, or tends to change, the uniform motion of a body.
Forward bias
Descriptive of a diode connected to a battery so that it conducts.
Frequency
The number of complete cycles or oscillations of a vibrating system per unit time.
Friction
A force that opposes the relative sliding motion between two surfaces in contact with each other.
Fundamental frequency
The frequency of the note sounded when a string or column of air is vibrating as a whole unit.

Half life
The time taken for one half of a radioisotope to decay or change into another element.
Harmonic
A frequency that is a whole number multiple of the fundamental frequency of a vibrating system.
Heat
Energy possessed by a system due to the kinetic energy of its atoms or molecules.
Heat capacity
The amount of heat required to heat an object by 1 K.
Hertz
A frequency of one cycle per second.

Incident ray (of light)
A ray of light that strikes a mirror or other optical device.
Intensity (of sound)
The energy passing per second through each square metre of area, at right angles to its path.
Interference
The addition or combination of waves; it can be of two types: constructive, in which waves reinforce one another, and destructive, in which waves cancel each other out.
Isotopes
Atoms of the same element which have different numbers of neutrons.

Joule
The amount of work done when a force of one newton moves a distance of one metre.
Joule’s law
The heat produced electrically is proportional to the square of the current, the resistance of the heating wire, and the time of heating.

Kinetic energy
The energy possessed by a body by virtue of its motion.


Lever
A rigid body that can rotate about a fixed point.
Line of electric force
A line along which a positive electric charge would travel if placed in an electric field.
Longitudinal wave
A wave in which the direction of vibration of the particles is in line with the direction in which the wave is travelling.
Long sight
A defect of the eyeball in which parallel rays of light come to a focus beyond the retina.
Latent heat of vaporisation
The heat involved when a liquid changes to a vapour (or vice-versa).
Latent heat of fusion
The heat involved when when a solid changes to a liquid (or vice-versa).

Magnetic declination
The angle between magnetic north and geographical north at a given place.
Magnetic dip
The angle made with the horizontal by a freely-suspended magnetic needle at a given place on the Earth’s surface.
Mass number
The number of nucleons (protons + neutrons) in the nucleus of an atom.
Moment
The product of the magnitude of a force multiplied by the perpendicular distance between the force and the fulcrum.
Momentum
The product of a body’s mass and velocity.
Mutual induction
The inducing of an emf in one circuit as the result of changing magnetic flux in an adjacent circuit.
Myopia
See Short sight

Node
A point of zero displacement in a standing wave.
Normal, the
A line at right angles angles to the surface of a mirror or other optical device.
N-type material
A semiconductor material in which the presence of an impurity has increased the number of free electrons.
Nuclear fission
The splitting of large atomic nuclei into into smaller ones, with the release of energy.
Nuclear fusion
The combining together of small atomic nuclei into larger ones, with the release of energy.

Ohm
The SI unit of resistance, which is the resistance between two points such that a potential difference on one volt between the points produces a current of one ampere.
Ohm’s law
The current flowing through a device is proportional to the potential difference across it.

Pascal
The SI unit of pressure, of one newton per square metre.
Peak value (of a.c.)
The maximum positive or negative voltage reached by an alternating current or voltage.
Period
The time taken for any part of a wave to move forward one wavelength.
Permittivity
A measure of the degree to which a medium can resist the flow of electric charge.
Phase (of a wave)
The position of any particle of the medium in its cycle.
Photoelectric effect (or emission)
The emission of electrons from metals by the action of light.
Photon
A single quantum of radiant energy.
Plancks’s constant
The constant h in E = hf, which relates the energy E of a photon of electromagnetic radiation to its frequency f.
Potential difference (voltage)

The potential difference between two points is equal to the work done in moving one coulomb of charge from one point to the other.
Potential energy
The energy possessed by a body by virtue of its position or conformation.
Power (mechanical)
The rate at which work is done.
Power (of a lens)
The reciprocal of the lens’ focal length (expressed in metres).
Primary cell
A cell that produces electricity as a result of an irreversible chemical reaction in the cell.
Pressure
Force acting per unit area.
Primary colour
A colour that cannot be produced by mixing any combination of other colours. The three primary colours of light are red, green and blue.
Principle focus
The point relative to a spherical mirror or lens through which all parallel rays pass (or appear to come from) after being reflected or refracted.
P -type material
A semiconductor material in which the presence of an impurity has increased the number of “holes”.

Radioactivity
The spontaneous disintegration of unstable atomic nuclei with the resulting emission of radiation.
Radioisotope
An isotope possessing radioactivity.
Radioactive decay
The spontaneous disintegration of radioactive nuclei into nuclei of other elements.
Radius of curvature (of a spherical mirror)
The distance between the mirror and its centre of curvature.
Real image
The image produced by the actual intersection of light rays.
Rectifier
An electrical component or circuit that converts alternating current to direct current.
Refracted ray
A ray of light that has been refracted at an optical interface.
Refractive index
The constant n in Snell’s Law, i.e., n = sin i/sin r.
Reflected ray
A ray of light that has been reflected from a mirror.
Relative permittivity
The permittivity of a medium relative to the permittivity of a vacuum.
Resistance
A measure of the ability of a material to oppose the passage of electricity.
Resistivity
The resistance of a conductor of length 1 m and of cross-sectional area 1 m2.
Reverse bias
Descriptive of a diode connected to a battery so that it does not conduct.
Rms value (of a.c.)
The square root of the average of the squares of the current (I2) or the voltage (V2), taken over a complete cycle.
Root mean square
The square root of the average of the squares of a group of values.

Scalar quantity
A quantity that has magnitude but no direction.
Secondary cell
A cell that must be “charged” by passing an electric current through it in the reverse direction to its discharge.
Secondary colour
The colour produced when two primary colours are mixed.
Self induction
The inducing of an emf in a circuit due to it being cut by its own changing magnetic flux.
Semiconductor
A material with electrical resistance between those of conductors and insulators.
Simple harmonic motion
The motion of a body that is being acted on by a force (i) whose magnitude is proportional to the displacement of the body from a fixed point, and (ii) whose direction is always towards that point.
Short sight
A defect of the eyeball in which parallel rays of light come to a focus short of (in front of) the retina.
Sound intensity
See intensity of sound.
Specific heat capacity
The amount of heat needed to increase the temperature of 1 kg of a substance by 1 K.
Specific latent heat of fusion
The quantity of heat needed to change 1 kg of a substance from solid to liquid, at its melting point (or vice-versa).
Specific latent heat of vaporisation
The quantity of heat needed to change 1 kg of a substance from liquid to vapour, at its boiling point (or vice-versa).
Spectrum
The band of colours or lines produced when light or other electromagnetic radiations are separated into their component wavelengths.
Standing wave
The interference pattern produced when two waves of the same frequency and amplitude travel at the same time in opposite directions.


Temperature
A measure of the “hotness” of a substance.
Thermionic emission
The emission of electrons from the surface of a hot metal.
Threshold frequency
The minimum frequency of incident radiation that can cause photoelectric emission.
Total internal reflection
The total reflection of light travelling from one medium to another of lower refractive index.
Transverse wave
A wave in which the direction of vibration of the particles is at right angles to the direction in which the wave is travelling.

Universal constant of gravitation
The force of attraction between two bodies each of mass 1 kg and separated by a distance of 1 m in vacuum.

Vector quantity
A quantity that has both magnitude and direction.
Velocity
Displacement per unit time.
Virtual image
The image produced by the apparent intersection of light rays.
Volt
The potential difference between two points when one joule of work is needed to move one coulomb from one point to the other.

Watt
A rate of working of one joule per second.
Wavelengthf)
The minimum energy needed to release an electron from the metal surface.

In a wave motion, the distance between two successive crests (in a transverse wave) or compressions (in a longitudinal wave).
Wave motion
A method of transferring energy through a medium by means of the periodic motion of its particles.
Weber
The magnetic flux passing through an area of 1 m2 placed in a field of 1 T.
Weight
The force of attraction between an object and the Earth (or other astronomical body).
Work
This is done when a force moves an object; the amount of work is the product of the force and the distance moved.
Work function of a metal (