# Physics definitions

## Physics definitions

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# Physics definitions

Optics

State the laws of reflection of light. [2005 OL]

1. The incident ray, the normal at the point of incidence and the reflected ray all lie on the same plane.
2. The angle of incidence is equal to the angle of reflection (i = r).

A Real Image is an image formed by the actual intersection of light rays.

A Virtual Image is formed by the apparent intersection of light rays.

What is meant by refraction of light? [2008] [2006][2009 OL][2005 OL][2002 OL]
Refraction is the bending of light as it goes from one medium to another.

State Snell’s law of refraction. [2008]
The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

State the laws of refraction of light. [2002][2003 OL]

1. The incident ray, the refracted ray and the normal all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

What is meant by the refractive index of a material? [2004 OL]
The Refractive Index of a Medium is the ratio of the sine of the angle of incidence to the sine of the angle of refraction when light travels from a vacuum into that medium.

Define the term total internal reflection. [2003 OL]
Total internal reflection occurs when the angle of incidence in the denser of the two media is greater than the critical angle and light is reflected back into the denser medium.

Define the term critical angle. [2004 OL][2003 OL]
The critical angle corresponds to the angle of incidence in the denser of two media which causes the angle of refraction to be 900.

Mechanics

Velocity and acceleration
Displacement is distance in a given direction.

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

Define velocity [2009 OL][2008 OL][2004 OL][2002 OL]
Velocity is the rate of change of displacement with respect to time.

Define acceleration [2008 OL][2004 OL][2002 OL]
Acceleration is the rate of change of velocity with respect to time.

Explain the term acceleration due to gravity, g. [2006 OL][2003 OL]
It is the acceleration of an object which is in freefall due to the attraction of the earth.

Vectors
A Scalar Quantity is one which has magnitude only.

Explain what a vector quantity is. [2006 OL]
A vector is a quantity which has both magnitude and direction.

Force, Mass and Momentum
Define the Newton, the unit of force. [2008]
The Newton is the force that gives a mass of 1 kg an acceleration of 1 m s-2.

Define Force [2004] [2008 OL][2006 OL]
A force is something which causes an acceleration.

Define Momentum [2010 OL][2004][2004 OL]
Momentum = mass × velocity

Newton’s First Law of Motion states that every object will remain in a state of rest or travelling with a constant velocity unless an unbalanced external force acts on it.

State Newton’s Second Law of Motion [2009][2004] [2003][2007 OL]
The rate of change of an object’s momentum is directly proportional to the force which caused it, and takes place in the direction of the force.

State Newton’s Third law of Motion. [2006]
When body A exerts a force on body B, B exerts a force equal in magnitude (and) opposite in direction to A.

State The Principle of Conservation of Momentum.
[2010 OL][2009 OL][2008 OL][2007 OL][2005 OL][2004 OL][2002]
In any interaction between two objects, the total momentum before the interaction is equal to the total momentum after the collision provided no external forces act.

What is friction? [2007][2009 OL][2006 OL][2002 OL]
Friction is a force that opposes the relative motion between two surfaces in contact.

Pressure
The Density of a substance is a measure of its mass per unit volume.

Define pressure. [2006][2009 OL][2007 OL][2005 OL][2002 OL]
Pressure is Force per unit Area.

State Boyle’s law. [2010 OL][2009][2006][2007 OL][2003 OL]
For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.

State Archimedes Principle. [2010 OL][ [2007]
When an object is immersed in a fluid, the upthrust it experiences is equal to the weight of the displaced fluid.

State the law of flotation. [2008]
For a floating object, the weight of the object equals the weight of the fluid displaced.

Gravity
The weight of an object is the force of the Earth’s gravity acting on it.

State Newton’s Law of Gravitation. [2010][2008][2005] [2004][2008 OL][2003 OL]
Any two objects in the universe are attracted to each other with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Moments
Define the moment of a force. [2006 OL][2003 OL]
The moment of a force is equal to the force multiplied by the perpendicular distance between the force and the fulcrum.

Conditions for Equilibrium of an object under Coplanar Forces.
If an object is in equilibrium then:

1. The vector sum of the forces in any direction is zero.

(forces up = forces down)

1. The sum of the moments about any point is zero.

Work, Energy and Power
Define energy [2005 OL]
Energy is defined as the ability to do work

Define work. [2007 OL]
Work is defined as the product of displacement by force (in the direction of the displacement).

Define Power [2006] [2002] [2007 OL]
Power is the rate at which work is done.

What is the difference between potential energy and kinetic energy? [2010 OL][ [2007 OL]
Potential energy is energy a body has due to its position; kinetic energy is energy a body has due to its motion.

State the Principle of Conservation of Energy. [2008] [2005]
Energy cannot be created or destroyed; it can only be changed from one form to another.

Circular Motion
Define (i) velocity [2006] and (ii) angular velocity. [2006] [2005]

1. Velocity is the rate of change of displacement with respect to time.
2. Angular velocity is the rate of change of angle with respect to time.

Define ‘Centripetal Force’ [2005]
Centripetal Force is the force - acting in towards the centre - required to keep an object moving in a circle.

Simple Harmonic Motion
State Hooke’s Law. [2009][2007] [2003] [2002]
Hooke’s Law states that when an object is stretched the restoring force F is directly proportional to the displacement provided the elastic limit is not exceeded.

Temperature and Thermometers
What is meant by the temperature of a body? [2009 OL][2008 OL]
The temperature of an object is a measure of the hotness or coldness of that object.

What is heat? [2008 OL]
Heat is a form of energy

What is the difference between heat and temperature? [2010 OL][2003]
Heat is a form of energy.
Temperature is a measure of the hotness of an object.

Explain the term thermometric property. [2004] [2003][2005 OL][2004 OL][2002 OL]
A thermometric property is a property which changes measurably with temperature.

Why is it necessary to have a standard thermometer? [2003][2009][2009 OL]
Different thermometers have different thermometric properties at the same temperature.
Or
No two types of thermometers will give exactly similar readings at all temperatures. This is because different thermometric properties do not change proportionally with the same change in degree of hotness.

Heat
Define specific heat capacity. [2006] [2004][2008 OL][2002 OL]
Specific heat capacity is the heat energy required to raise the temperature of 1 kg of a substance by 1 K

Define specific latent heat. [2004]
Specific latent heat is the heat energy required to change the state of 1 kg of a substance without a change in temperature.

Define Specific Latent of Fusion
The Specific Latent of Fusion (lf) is the amount of heat energy need to change 1 kg of the substance from a solid to a liquid without a change in temperature.

Define Specific Latent of Vaporisation
The Specific Latent of Vaporisation(lv) is the amount of heat energy needed to change 1 kg of the substance from a liquid to a gas without a change in temperature.

What is convection? [2006 OL]
Convection is the transfer of heat through a fluid by means of circulating currents of fluid caused by the heat.

What is meant by conduction? [2004 OL]
Conduction is the movement of heat energy through a substance by the passing on of molecular vibration from molecule to molecule, without any overall movement of the substance.

Radiation is the transfer of heat energy from one place to another in the form of electromagnetic waves.

Waves and Wave Motion

Explain what is meant by the frequency of a wave? [2010 OL][2007 OL]
The frequency of a wave is the number of waves passing a fixed point per second.

What is meant by the amplitude of a wave? [2005 OL]
Amplitude corresponds to the height of the wave.

Explain the difference between Transverse and Longitudinal waves. [2005][2006 OL]
A Transverse wave is a wave where the direction of vibration is perpendicular to the direction in which the wave travels.
A Longitudinal Wave is a wave where the direction of vibration is parallel to the direction in which the wave travels.

Explain the term diffraction[2009][2005 OL][2004 OL][2002 OL]
Diffraction is the spreading out of a wave when it passes through a gap or passes by an obstacle.

Stationary waves are formed when two periodic travelling waves of the same frequency and amplitude travelling in opposite directions meet.

Explain the term interference. [2005 OL][2004 OL]
Interference occurs when waves from two sources meet to produce a wave of different amplitude.

Explain the term constructive interference. [2003]
Constructive interference occurs when two waves combine to produce a wave of greater amplitude.

Explain the term coherent sources. [2003]
Coherent sources are waves which have the same frequency and are at in phase.

What is the Doppler Effect? [2010][2008] [2007] [2006] [2003] [2002]
The Doppler Effect is the apparent change in frequency due to relative motion between source and observer.

Sound
Define Sound Intensity [2007] [2002]
Sound Intensity is defined as power per unit area.

Explain the term natural frequency. [2010 OL][2007 OL]
Natural frequency is the frequency at which an object will vibrate if free to do so.

Explain the term resonance. [2010][2007 OL]
Resonance is the transfer of energy so that a body vibrates at its natural frequency.

The threshold of hearing is the smallest sound intensity detectable by the average human ear at a frequency of 1 KHz.

The frequency limits of audibility are the highest and lowest frequencies that can be heard by a normal human ear.
The range is 20 Hz – 20,000 Hz.

Overtones are frequencies which are multiples of the fundamental frequency.

The wave nature of Light
Explain the term dispersion. [2010 OL][2009][2007 OL]
Dispersion is the splitting up of white light into its constituent colours.

Explain what is meant by a spectrum. [2007 OL]
A spectrum refers to the range of colours present in white light.

Explain the term monochromatic light. [2009 OL]
Monochromatic light is light of one wavelength only.

Explain the term diffraction grating. [2009 OL]
A diffraction grating consists of a piece of transparent material on which a very large number of opaque (black) parallel lines are engraved.

What are complementary colours? [2003 OL]
Complementary colours are pairs of colours consisting of a primary and a secondary colour, such that when combined they give white light.

A Polarised wave is a wave which vibrates in one plane only.

Electricity

Static Electricity
State Coulomb’s law of force between electric charges. [2010 OL][2005] [2003] [2007 OL]
Coulomb’s Law states that the force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.

Define electric field strength. [2010][2009][2007] [2005] [2003] [2002]
Electric field strength is defined as force per unit charge.

Electric Current
What is an electric current? [2010][2008 OL][2006] [2006 OL][2004 OL][2003 OL][2002 OL]
An electric current is a flow of charge.

Potential Difference and Capacitance
Define potential difference. [2009][2005] [2004][2002 OL]
Potential difference is the work done in moving a charge of one Coulomb from one point to another.

Define capacitance. [2009][2008] [2004][2002 OL]
The capacitance of a conductor is the ratio of the charge on the conductor to its potential.

The Volt
The potential difference between two points is one volt if one Joule of work is done when bringing a charge of one Coulomb from one point to another.

The potential at a point refers to the work done in bringing unit charge from that point to earth.

A voltage when applied to a circuit is an emf.

Resistance
Define resistance.  [2007] [2005]
The resistance of a conductor is the ratio of the potential difference across the conductor to the current flowing through it.

State Ohm’s Law [2010 OL][2007 OL][2006 OL][2005 OL]
Ohm’s Law states that the current flowing through a conductor is directly proportional to the potential difference across it, assuming constant temperature.

Define resistivity. [2008] [2007] [2002]
Resistivity is defined as the resistance of a cube of material of side one metre.

Effects of an Electric Current
What is the kilowatt-hour? [2004 OL]
The kilo-watt hour is the amount of energy used by a 1000 Watt appliance in one hour.

Joules Law states that the rate at which heat is produced in a conductor is proportional to the square of the current, provided its resistant is constant.

Semiconductors
What is a semiconductor? [2002][2009 OL][2006 OL][2003 OL]
A semiconductor is a material whose resistivity is between that of a conductor and that of an insulator.

What is doping? [2004][2009 OL][2003 OL]
Doping is the adding of a controlled quantity of an impurity to increase conduction.

Intrinsic Conduction is the movement of charges through a pure semiconductor.
Extrinsic Conduction is the movement of charges through a doped semiconductor.

An n-type semiconductor is a semiconductor in which electrons are the majority charge carriers.
A p-type semiconductor is a semiconductor in which holes are the majority charge carriers.

What is a p-n junction? [2006 OL][2003 OL]
A p-n junction is the region connecting the p-type semiconductor to the n-type semiconductor.

What is a diode? [2003 OL]
A diode is a device that allows current to flow in one direction only.

Magnets and magnetic fields
What is a magnetic field? [2009 OL][2006 OL][2005 OL][2003 OL]
A Magnetic Field is any region of space where magnetic forces can be felt.

Current in a Magnetic Field
Define the Ampere. [2006] [2003]
The Ampere is that current which, if flowing  in two infinitely long parallel wires of negligible cross-sectional area 1 m apart in vacuum experience a force of 2 x 10-7 N per metre.

State the principle on which the definition of the Ampere is based. [2007]
A current-carrying conductor in a magnetic field experiences a force.

Electromagnetic Induction
What is electromagnetic induction? [2009][2008][2004] [2002] [2008 OL][2007 OL][2004 OL][2002 OL]
Electromagnetic Induction occurs when an emf is induced in a coil due to a changing magnetic flux.

State Faraday’s law of electromagnetic induction. [2010][2007][2005]
Faraday’s Law states that the size of the induced emf is proportional to the rate of change of flux.

State Lenz’s law of electromagnetic induction. [2002]
Lenz’s Law states that the direction of the induced emf is always such as to oppose the change producing it.

State the laws of electromagnetic induction. [2008] [2003]

Define magnetic flux. [2006] [2005]
Magnetic flux is the product of magnetic flux density by area.

Modern Physics (which means Physics of the last 120 years)

The Electron
What are cathode rays? [2009 OL]
Cathode rays are streams of high speed electrons.

What is the photoelectric effect? [2008][2005][2003][2008 OL]
The photoelectric effect is the emission of electrons from the surface of a metal when light radiation of suitable frequency falls on it.

What is a photon? [2003 OL]
A photon is a discrete amount (or packet) of electromagnetic radiation.

What is thermionic emission? [2010][2009 OL][2002 OL]
Thermionic Emission is the giving off of electrons from the surface of a hot metal.

Distinguish between photoelectric emission and thermionic emission. [2004]
Thermionic emission is the emission of electrons from the surface of a hot metal.

What are X-rays? [2010][2009 OL] [2006]
X-rays are electromagnetic radiation of a high frequency.

Ionisation occurs when an atom loses electrons.

The electron-volt is the energy lost or gained by an electron when it moves through a potential difference of one volt.

The Atom, the Nucleus and Radioactivity
The Atomic Number (Z) of an atom tells us the number of protons present in the atom.

The Mass Number (A) of an atom tells us the number of protons plus neutrons present in the atom.

What is an isotope? [2003]
Isotopes are atoms which have the same atomic number but different mass number.

What are alpha particles [2008 OL]
An alpha particle is a helium nucleus (composed of 2 protons and 2 neutrons).

What is radioactivity? [2010 OL][ 2007 OL][2005][ 2005 OL][2004 OL][2003 OL]
Radioactivity / Radioactive decay is the breakup of unstable nuclei with the release of one or more types of radiation.

What is ionisation? [2005]
Ionisation occurs when an atom loses or gains an electron.

Explain the term half-life. [2007][2005 OL][2002 OL]
The half-life of a material is the time taken for half of the nuclei in the sample to decay.

The Law of Radioactive Decay states that the number of disintegrations per second is proportional to the number of nuclei present.

Fission, Fusion and Nuclear Energy
What is an isotope? [2007][2009 OL]
Isotopes are atoms which have the same atomic number but different mass number.

What is nuclear fission? [2007][2004] [2002][2006 OL][2004 OL][2003 OL]
Nuclear fission is the break-up of a large nucleus into two smaller nuclei with the release of energy (and neutrons).

What is a chain reaction? [2010][2003 OL]
This occurs when at least one neutron gets released during fission causing more fission to occur in another nucleus and this then becomes a self-sustaining reaction.

What is meant by nuclear fusion? [2003][2008 OL]
Nuclear Fusion is the combining of two small nuclei to form one large nucleus with the release of energy.

Distinguish between fission and fusion. [2006]

Distinguish between radioactivity and fission. [2005]

Particle Physics
What is a photon? [2006]
A photon is a bundle (discrete amount) of electromagnetic radiation.

What is anti-matter? [2010]
Antimatter is material/matter/particles that has same mass as another particle but opposite charge.

What is meant by pair production? [2010]
Pair production involves the production of a particle and its antiparticle from a gamma ray photon.

Source : http://www.thephysicsteacher.ie/LC%20Physics/Revision/3.%20Definitions.doc

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