Physics.ie - Focus on Physics

Definitions

Acceleration
rate of change of velocity with respect to time
Ampere
The current which when flowing in two straight parallel conductors
of infinite length, placed one meter apart in a vacuum, would
produce a force of 2×10-7 newtons per metre of
length.
Amplitude
The distance top (or bottom) of a wave to the middle of
the wave.
Angular Speed/Velocity
The rate of change of angle with respect to time
Anode
The positive electrode in a cell or solution.
Atomic Number
The number of protons in the nucleus.

Becquerel
SI unit for radioactive decay/activity, Becquerel(Bq) = 1
disintegration per second.

Capacitance
The ability of a material to store electric charge.
Cathode
The negative electrode in a cell or solution.
Centre of curvature
Centripetal acceleration
The acceleration towards the centre of a body moving in a
circle
Centripetal force
The force towards the centre needed to need a body moving
in a circle.
Chain reaction
A self-sustaining nuclear reaction, the products of which
need the reaction going.
Coherent
Light is coherent if the waves are in phase or have a
constant phase relationship.
Conductor

A material capable of carrying an electric charge.
Constructive interference
Crests from one source always meet the crest from another
source.
Converging/convex lens
Lens capable of bring to a point a beam of light passing through
it.
Coulomb
SI unit of electric charge, 1 coulomb(C) = 1 ampere per second.
Couple
Two equal and opposite parallel, but not linear forces acting
on a body.
Critical angle
The minimum angle of incidence at which total internal reflection
occurs.

Decay constant
The probability of the decay of an atomic nucleus per second.
Density
Mass per unit volume
Destruction interference
Crests from one source always meet the trough from another
source.
Diffraction
When the sideways spreading as a wave goes round obstacle
or through a small gap.
Diode
A device that allows current to flow only in one direction.
Dispersion
The splitting of light into different wavelengths, creating
a spectrum.
Displacement
A specified distance in a specified direction. (Vector)
Diverging lens
A lens that causes a parallel beam of light to diverge as
it passes through.
Doping
Adding small amounts of impurity to a semiconductor to make
it N or P type.
Doppler effect
The apparent change in frequency of sound or radiation
due to the relative motion of the source and observer.

Eddy currents
Induced electric currents in conductors due to changing magnetic
flux.
Electric field strength
The force per unit charge that a positive charge would experience
if placed at that point in the field.
Electromagnetic induction
Whenever the magnetic field passing through a coil changes
this causes an electromagnetic force in the coil.
Electromotive Force
The energy given to each coulomb of charge as it goes through
a battery or generator.

Farad
SI unit of capacitance, 1F = 1 coulomb across potential difference
of 1 volt.
Focal Length
The distance for the focal point to the mirror.
Force
Anything that causes the velocity of an object to change.
Forward bias
When a diode is connected to a battery so that current flows.

Frequency
The number of oscillation or cycles per second.
Friction
The force that opposes the sliding of surfaces in contact.
Fundamental frequency
The lowest possible vibration of a stationary wave.

Half life
The time taken for half of the undecayed atoms undergo a decay
in a radioactive isotope.
Harmonic
A wave whose vibration is a whole multiple of the fundamental
frequency.
Heat
Energy transfer from one place to another due to a temperature
difference.
Hertz
Measure of frequency, 1 hertz is one cycle per second.

Interference
The addition or combination of waves, it can be either destructive
or constructive.
Isotope
Atoms of that same element that have different mass numbers.
They have different amounts of neutrons.

Joule
SI unit of Work or Energy, 1 joule = 1 newton metre
Joule’s Law
The heat power loss in a circuit is directly proportional
to the square of the current flowing.

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

Lever
A rigid body which is free to rotate about a fixed point.
Longitudinal wave
A wave whose direction of vibration is parallel to the direction
the wave is travelling.

Magnetic declination
The angle between the magnetic North and true North at a point
on the Earth’s surface.
Magnetic dip
The angle between the Earth’s magnetic field and the horizontal
at a point on Earth.
Magnetic Flux Density
is the magnetic flux per unit area at right angles to the
magnetic field.
Mass number
Number of protons and neutrons in the nucleus.
Moment
The force x perpendicular distance from the point to the line
of action of the force.
Momentum
Mass x Velocity
Mutual induction
An emf is generated in a secondary coil due to a change in
the magnetic field produced by the primary coil.

Node
A point of zero displacement in a standing wave.
N-type semiconductor
A type of semiconductor whose conductivity is caused by a
flow of electrons.
Nuclear Fission
The Splitting up of a large nucleus into two smaller but similar
sized nucleii.
Nuclear Fusion
The joining of two small nucleii to form a larger nucleus.

Ohm
The resistance of a conductor when a current of 1 amp flows
in the conductor which has a potential difference of 1 volt
across it.
Ohm’s Law
For a metal at constant temperature, voltage is equal to the
current by the resistance.

Pascal
SI unit of pressure, 1 pascal = 1 newton per metre squared.
Period
The time a wave takes to travel one wavelength or for a object
to make one orbit.
Permittivity
The measure of how a material can resist the flow of electric
charge
Phase
Photoelectric effect
The emission of electrons from the surface of a metal when
electromagnetic radiation of suitable frequency falls on it.
Photon
A quantum of electromagetic radiation
Pitch
A measure of the frequency of a note.
High pitch means high frequency.
Plancks’s constant

A constant relating the frequency of radiation to its quantum
of energy.
Potential difference/Voltage
Between two points in an electric field is the work done in
bringing a charge of +1 Coulomb from one point to the other.
Potential energy
The energy a body has due to its position.
Power
Rate at which work is done or the rate at which energy is
converted from one form to another.
Pressure
The force per unit area acting on a surface.
P-type semiconductor
A type of semiconductor whose conductivity is caused by a
flow of holes.

Radioactivity
The disintegration or decay of unstable nucleus with the emission
of alpha, beta or gamma radiation.
Real image
An image formed by the actual intersection of rays and can
be projected onto a screen.
Refractive index
The amount that light is bent when it goes from one medium
to another.
Resistance

The potential difference between the ends of a conductor divided
by current flowing through it.
Resistivity
The inverse of conductivity = RA/L where R is resistance,
A is cross sectional area and L is the length.
Reverse bias
When a diode is connected to a battery so that no current
flows.
Root Mean Square (RMS)
The square root of the average of the squares of current or
voltage.

Scalar quantity
A quantity that depends only on magnitude.
Self induction
When an electromagnetic force is generated in a coil by a
change in the magnetic field of the same coil.
Semiconductor
A material whose resistance is between an insulator and a
conductor.
Simple harmonic motion

Specific heat capacity
Of a substance is the amount of heat energy needed to change
the temperature of one kilogram by one kelvin.
Specific latent heat of fusion
Of a substance is the amount of heat energy needed to change
1 Kg of that substance from a solid to a liquid.
Specific latent heat of vapourisation
Of a substance is the amount of heat energy needed to change
1 Kg of that substance from a liquid to a gas/vapour.
Spectrum
A band of colour produced when electromagnetic radiation are
separated into different frequencies.
Standing wave
A wave formed when two identical (same frequency and amplitude)
waves travelling in opposite directions meet.

Temperature
The measure of the hotness or coldness of a body.
Tesla
SI unit of magnetic flux density, 1 tesla = 1 weber per square
metre.
Thermionic emission
The emission of electrons from a heated metal.
Thermometric property
A property of a body that changes with temperature.
Thermistor
Is a semiconductor device whose resistance decreases rapidly
with increase in temperature.
Threshold frequency
The minimum frequency of the incident radiation for the photoelectric
effect to take place.
Total internal reflection
The total reflection of light travelling cross for a medium
to a medium of lower refractive index
Triple point of water

The unique temperature at which ice, water and vapour exist
together (273.16K)
Transverse wave
A wave whose direction of vibration is perpendicular to the
direction the wave is travelling.

Vector quantity
A quantity that depends on magnitude and direction
Velocity
The rate of displacement (speed) of a object in a given direction.
Virtual image
An image produced by the apparent intersection of rays.
Volt
SI unit of potential difference, 1 volt = 1 joule per
coulomb.

Watt
SI unit for Power, 1 watt = 1 Joule per second.
Wavelength
The distance between two successive crests.
Weber (Wb)
SI unit for magnetic flux, 1 weber = 1 Tesla metre squared.
Weight
The force of attraction of the earth on a given mass.
Work
Force x Distance
Work function
The minimum energy needed to release an electron from a metals
surface.

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