Leaving Cert. Physics Definitions for 2016

Displacement

The distance from a fixed point along a particular direction.

Velocity

The rate of change of displacement (m s-1).

Acceleration

The rate of change of velocity (m s-2).

Scalar quantity

A quantity that has no direction associated with it, e.g., mass.

Vector quantity

A quantity that has direction and magnitude associated with it, e.g., velocity.

Force

Something that can cause acceleration (N) and is a vector quantity.

Friction

A force that opposes the motion of one surface over another.

Momentum

Mass multiplied by velocity (momentum = mv) (kg m s-1) and is a vector quantity.

Conservation of momentum

In any interaction within a closed system, total momentum before is equal to total momentum afterwards.

Newton’s First Law of Motion

An object remains in a state of rest or of uniform motion in a straight line unless an external force acts on it.

Newton’s Second Law of Motion

The resultant force on an object is proportional to its rate of change of momentum.

Newton’s Third Law of Motion

If A exerts a force on B, then B exerts an equal and opposite force on A.

Weight

The force of gravity acting on a body.

Newton’s Law of Universal Gravitation

Every two objects attract each other with a force given by F = Gm1m2/d².

Density

The mass per unit volume of a substance (g cm-3 or kg m-3).

Pressure

The force on unit area (P = F/A) measured in Pascals (Pa = N m-2).

Archimedes Principle

The upthrust is equal in magnitude to the weight of the fluid displaced when a body is partially or wholly immersed in a fluid.

Law of flotation

The weight of a floating body is equal to the weight of the fluid displaced.

Moment

The product of the force and the perpendicular distance between the axis and the line of action of the force (Moment = force x perpendicular distance).

Equilibrium

A body is in equilibrium when its acceleration is zero.

Work

The product of the force and the distance moved (W = Fs).

Power

The work done per second or the energy changed per second (P = W/t).

Efficiency

The ratio of power output to power input expressed as a percentage (Efficiency = (Power out / Power in) x 100).

Centripetal acceleration

The acceleration of a body moving in a circular path towards the centre of the circle (a = ω²r = v²/r).

Simple Harmonic motion

The motion of a particle is simple harmonic if its acceleration is proportional to its displacement from a point (a = -ω²s).

Specific Heat Capacity

The amount of energy needed to change the temperature of 1kg of substance by 1 Kelvin (J kg-1 K-1).

Conduction

The transfer of energy through a substance without the movement of the particles of the substance.

Convection

The transfer of energy through a substance by the movement of the particles.

Radiation

The movement of energy in straight lines from the surface of a hot body.

Boyle’s Law

When a fixed mass of gas is kept at constant temperature, its volume is inversely proportional to its pressure.

Stationary Wave

A wave with no net transfer of energy formed by interference of two waves of equal frequencies moving in opposite directions.

Resonance

The transfer of energy between two bodies with the same or similar natural frequency.

Doppler effect

The apparent change in frequency of a wave due to relative motion between the source of a wave and the observer.

Snell’s Law

sin i / sin r = n, where n is the refractive index.

Power of a lens

P = 1/focal length.

Critical angle

The angle of incidence in the denser medium for which the angle of refraction is 90 degrees.

Total Internal Reflection

Occurs when the angle of incidence in the denser medium is greater than the critical angle.

Transverse Wave

A wave in which the vibrations are perpendicular to the direction of wave travel.

Longitudinal Wave

A wave in which the vibrations are parallel to the direction of wave travel.

Frequency

The number of complete cycles of a wave passing a point per second, measured in Hertz (Hz).

Period

The time taken for one complete cycle of a wave.

Electric field strength

The force per unit charge (E = F/q).

Potential difference

Work done per unit charge (V = W/q).

Coulomb’s Law

F = q1q2/(4πεd²), where F is the force between two charges.

Electric Current

A flow of charge, defined as I = q/t.

Resistance

The ratio of voltage to current (R = V/I).

Ohm’s Law

The current flowing through a wire is proportional to the potential difference across it (V = IR).

Thermionic emission

The emission of electrons from the surface of a hot metal.

Electron-Volt

The amount of energy gained by an electron when accelerated through a potential difference of 1 Volt.

Photon

A packet of electromagnetic energy, with energy given by E = hf.

Radioactivity

The decay of a nucleus of an atom with the emission of one or more types of radiation.

Half-life

The time taken for half of the nuclei present in a radioactive substance to decay.