List of Leaving Cert Physics Definitions

This page gives a comprehensive list of definitions you need to know if you are studying leaving cert Physics. It is important to know all of these definitions before your exam. 


Displacement is a distance in a given direction, it is a vector quantity.

Speed is the rate of change of distance with respect to time

Velocity is the rate of change of displacement with respect to time, it is a vector quantity.

Acceleration is the rate of change of velocity with respect to time, it is a vector quantity.

A scalar quantity is a quantity with magnitude only.

A vector quantity is a quantity with magnitude and direction.

Newton's 1st Law of Motion: every body will remain in a state of rest, or moving with a constant velocity unless an external force acts on it.

Newton's 2nd Law of Motion: when an external force acts on a body, the rate of change of the body's momentum is directly proportional to the force and is in the same direction of the force.

Newton's 3rd Law of Motion: if a body (A) exerts a force on body (B), then body (B) exerts an equal but opposite force on body (A). For every action there is an equal but opposite reaction.

Force and momentum: definitions

Vector nature of forces to be stressed

Friction: a force opposing motion

Principle of conservation of momentum.

Centripetal force required to maintain uniform motion in a circle.

Definition of angular velocity ω.

Newton’s law of universal gravitation. F = Gm1m2 d 2

Weight = mg

Density and pressure

Boyle’s law.

Archimedes’ principle. Law of flotation.

Moments Definition. Levers. Couple.

Vector sum of the forces in any direction is zero. The sum of the moments about any point is zero.

Hooke’s law: restoring force ∝ displacement.


Energy as the ability to do work.

Principle of conservation of energy

Power as the rate of doing work or rate of energy conversion.


Measure of hotness or coldness of a body.

The SI unit of temperature is the kelvin

Thermometric properties, A physical property that changes measurably with temperature.

Thermometers measure temperature


Heat as a form of energy that causes a rise in temperature when added or a fall in temperature when withdrawn.

Heat capacity, specific heat capacity

Latent heat, specific latent heat

1. Conduction 2. Convection 3. Radiation

U-values: use in domestic situations.


Longitudinal and transverse waves: frequency, amplitude, wavelength, velocity. Relationship c = f λ

Reflection. Refraction. Diffraction. Interference. Polarisation.

Stationary waves; relationship between inter-node distance and wavelength.

Doppler effect


Reflection, refraction, diffraction, interference.

Characteristics of notes, Amplitude and loudness, frequency and pitch, quality and overtones.

Natural frequency. Fundamental frequency. Definition of resonance

Sound intensity: definition

Sound intensity level is measured in decibels. Doubling the sound intensity increases the sound intensity level by 3 dB. 


Laws of reflection

Images formed by plane and spherical mirrors.

Laws of refraction

Refractive index.

Refractive index in terms of relative speeds.

Total internal reflection

Critical angle

The eye: optical structure; short sight, long sight,

Diffraction and interference

Light as a transverse wave motion

Dispersion by a prism and a diffraction grating. Recombination by a prism.

Primary, secondary, complementary colours.

Electromagnetic spectrum

The spectrometer and the function of its parts.


Electrification by contact Charging by rubbing together dissimilar materials.

Types of charge: positive, negative

Conductors and insulators. Unit of charge: coulomb.

Electrification by induction

Total charge resides on outside of a metal object.

Point discharge

Electroscope Structure.

Coulomb’s law

Electric fields Idea of lines of force.

Definition of electric field strength.

Definition of potential difference: work done per unit charge to transfer a charge from one point to another. Definition of volt. Concept of zero potential

Capacitors and capacitance, Definition: C = Q/V Unit of capacitance. Parallel plate capacitor.

Capacitors – conduct a.c. but not d.c.

Description of electric current as flow of charge; 1 A = 1 C s –1

Pd and voltage are the same thing; they are measured in volts. A voltage when applied to a circuit is called an emf.

Conduction in • metals • ionic solutions (active and inactive electrodes) • gases • vacuum • semiconductors. References in each case to charge carriers.

Conduction in semiconductors: the distinction between intrinsic and extrinsic conduction; p-type and n-type semiconductors.

The p-n junction: basic principles underlying current flow across a p-n junction.

Definition of resistance, unit. Ohm's law


Resistors in series and parallel.

Wheatstone bridge.

LDR – light-dependent resistor. Thermistor.

Potential divider.

Effects of electric current

Chemical effect – an electric current can cause a chemical reaction. Magnetic effect of an electric current.

Plugs, fuses, MCBs (miniature circuit breakers). Ring and radial circuits, bonding, earthing,

The kilowatt-hour.

RCDs (residual current devices).

Magnetic poles exist in pairs. Magnetic effect of an electric current.

Magnetic field due to • magnets • current in - a long straight wire - a loop - a solenoid.

Magnetic flux density

Definition of the ampere.

Magnetic flux Φ = BA Faraday’s law.

Lenz’s law.

Alternating current

Peak and rms values of alternating currents and voltages.

Mutual induction (two adjacent coils): when the magnetic field in one coil changes an emf is induced in the other, e.g. transformers.

Self-induction: a changing magnetic field in a coil induces an emf in the coil itself, e.g. inductor.


The electron as the indivisible quantity of charge.

Principle of thermionic emission and its application to the production of a beam of electrons.

Cathode ray tube consisting of heated filament, cathode, anode, and screen.

Photoelectric emission

Photoelectric effect. The photon as a packet of energy; E = hf

Threshold frequency. Einstein's photoelectric law.

X-rays produced when high-energy electrons collide with target.

Principles of the hot-cathode X-ray tube.

Principle of Rutherford’s experiment.

Bohr model, descriptive treatment only. Energy levels.

Atomic nucleus as protons plus neutrons. Mass number A, atomic number Z, A ZX, isotopes.


three kinds of radiation:

Nature and properties of alpha, beta and gamma emissions.

Definition of becquerel (Bq) as one disintegration per second.

Law of radioactive decay. Concept of half-life: T 1 /2 Concept of decay constant rate of decay = λ N

Principles of fission and fusion.

Nuclear reactor (fuel, moderator, control rods, shielding, and heat exchanger).


Radioactive decay resulting in two particles.

If momentum is not conserved, a third particle (neutrino) must be present.

Fundamental forces of nature

Strong nuclear force: force binding nucleus, short range. Weak nuclear force: force between particles that are not subject to the strong force, short range. Electromagnetic force: force between charged particles, inverse square law. Gravitational force: inverse square law

particle zoo

Leptons: indivisible point objects, not subject to strong force, e.g. electron, positron, and neutrino. Baryons: subject to all forces, e.g. protons, neutrons, and heavier particles. Mesons: subject to all forces, mass between electron and proton.


e+ positron, e– electron.

Pair production: two particles produced from energy.


Quark model

Six quarks – called up, down, strange, charmed, top, and bottom.

Anti-quark has opposite charge to quark and same mass

Baryons composed of three quarks: p = uud, n = udd, other baryons any three quarks. Mesons composed of any quark and an anti-quark.


Electromagnetic relay.

Simple d.c. motor. Principle of operation of moving-coil loudspeaker

Principle of moving-coil galvanometer.

Induction coil.

Structure and principle of operation of simple a.c. generator.

P-n diode used as half-wave rectifier.

The transistor as a voltage amplifier

AND, OR and NOT gates.