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Leaving Cert Physics glossary

105 key terms in plain English, grouped by strand and labelled Higher or Ordinary. The same definitions the H1 Owl app teaches, written from the published syllabus.

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Nature of Science 13 terms

AccuracyHigher: How close a measured value lies to the true or accepted value of the quantity being measured. It is limited by systematic error, so it is improved by calibrating the instrument and removing bias from the method, not by simply repeating the reading.
AccuracyOrdinary: How close a measured value is to the true or correct value of what you are measuring.
Directly proportionalHigher: A relationship in which one quantity increases by the same factor as another, so that the ratio between them stays constant. Plotted against each other the readings give a straight line through the origin, and the slope of that line is the constant of proportionality.
Percentage errorHigher: The difference between a measured value and the accepted value, divided by the accepted value and multiplied by 100. Expressing the error this way lets results of very different sizes be compared fairly and shows which measurement limits the experiment.
Percentage errorOrdinary: How far a measured value is from the correct value, given as a percentage of the correct value. A smaller figure means a better result.
PrecisionHigher: How closely repeated readings of the same quantity agree with one another, and how finely an instrument can resolve a measurement. Readings can be tightly grouped and still be wrong, because a systematic error shifts every one of them the same way.
PrecisionOrdinary: How close repeated readings of the same thing are to each other. Readings can be close together and still be wrong.
Random errorHigher: An unpredictable variation that makes repeated readings scatter above and below the true value, caused by things such as reaction time or changing room conditions. Its effect is reduced by taking several readings and working out the mean.
SI base unitHigher: One of the seven agreed units from which every other unit is built, including the metre, the kilogram, the second, the ampere and the kelvin. Units such as the newton and the joule are derived by combining these, which lets an equation be checked for consistency.
Significant figuresHigher: The digits in a recorded measurement that carry real information about its size, counted from the first non-zero digit. A calculated answer should be quoted to no more of them than the least precise measurement used in the calculation.
Significant figuresOrdinary: The digits in a measurement that really tell you its size. Give an answer to no more of them than your least precise reading.
Systematic errorHigher: A fault in the apparatus or the method that shifts every reading in the same direction by about the same amount, such as a zero error on a balance. Repeating the measurement will not remove it; the instrument must be checked or the readings corrected.
Systematic errorOrdinary: A fault that shifts every reading the same way, such as a scale that does not start at zero. Repeating the reading will not fix it.

Forces and Motion 37 terms

AccelerationHigher: The rate of change of velocity with respect to time, measured in m s⁻². Because velocity is a vector, a body has a non-zero value whenever its speed changes, its direction changes, or both; on a velocity-time graph it is given by the slope.
AccelerationOrdinary: The rate of change of velocity with respect to time, measured in m s⁻². A car pulling away from traffic lights has a positive value.
Acceleration due to gravityHigher: The rate at which the velocity of a freely falling body increases near the surface of the Earth, taken as 9.8 m s⁻² and directed downwards. Where air resistance can be ignored it has the same value for every body, whatever its mass.
Centripetal forceHigher: The net inward force needed to keep a body moving along a circular path, always directed towards the centre of the circle. It changes the direction of the velocity without changing its size, and it is supplied by tension, gravity or friction.
DisplacementHigher: The change in position of a body, measured in a straight line from its starting point to its finishing point and quoted with the direction of that line. It differs from distance travelled, which follows the actual path and ignores direction.
ForceOrdinary: A push or a pull that can change the speed, the direction or the shape of an object. It is measured in newtons (N).
FrictionHigher: A force that acts where two surfaces touch and opposes their relative sliding motion. It depends on how hard the surfaces are pressed together and on the materials involved, and it converts kinetic energy into internal energy.
FrictionOrdinary: A force that acts between two surfaces rubbing together and slows the movement down. It also makes the surfaces warm up.
Gravitational potential energyHigher: The store of energy a body has because of its height above a chosen reference level, found by multiplying mass, gravitational field strength and height. Lifting a body increases it, and it converts into kinetic energy as the body falls.
ImpulseHigher: The product of a force and the time for which it acts, equal to the change in momentum that it produces. Spreading the same change in momentum over a longer time reduces the force, which is how crumple zones and air bags protect passengers.
Kinetic energyHigher: The store of energy a body has because it is moving, calculated as half the mass multiplied by the square of the speed. Because the speed is squared, doubling the speed of a car quadruples the energy that its brakes have to remove.
Kinetic energyOrdinary: The store of energy an object has because it is moving. The faster and the heavier it is, the more of it there is.
MassOrdinary: The amount of matter in an object, measured in kilograms. It is the same on the Earth as it is on the Moon.
MomentumHigher: The product of the mass of a body and its velocity, measured in kg m s⁻¹ and pointing in the same direction as the velocity. A large value makes a body harder to stop, which is why it is central to the study of collisions.
MomentumOrdinary: The mass of an object multiplied by its velocity. A lorry has far more of it than a bicycle moving at the same speed.
Newton's first law of motionHigher: A body stays at rest, or keeps moving in a straight line at constant speed, unless an external unbalanced force acts on it. This is why a passenger continues forward when a car brakes sharply, and why a seat belt is needed.
Newton's law of universal gravitationHigher: Any two point masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centres. Doubling that separation cuts the force to a quarter of its size.
Newton's second law of motionHigher: The rate of change of momentum of a body is directly proportional to the unbalanced force acting on it and takes place in the direction of that force. For a body of constant mass this reduces to F = ma, which defines the newton.
Newton's third law of motionHigher: When one body exerts a force on a second body, the second exerts a force on the first that is equal in magnitude and opposite in direction. The two forces act on different bodies, so they never cancel each other out.
Potential energyOrdinary: The store of energy an object has because of its height above the ground. Lifting a bag onto a shelf gives it more.
PowerHigher: The rate at which energy is transferred or converted from one form into another, measured in watts, where one watt is one joule per second. Two machines can transfer the same energy, but the more powerful one does it in less time.
PowerOrdinary: The energy transferred each second, measured in watts. A 2000 W kettle uses energy far faster than a 60 W lamp.
Principle of conservation of energyHigher: In any closed system the total energy stays constant; it can be converted from one form into another but it can be neither created nor destroyed. Solving a problem then becomes a matter of tracking where all of the energy has gone.
Principle of conservation of energyOrdinary: In any change the total energy stays the same. It is never made or destroyed, only turned from one form into another.
Principle of conservation of momentumHigher: In any collision or explosion the total momentum of a closed system stays the same, provided no external force acts on it. The vector total before the interaction equals the vector total afterwards, which lets an unknown velocity be calculated.
Resultant forceHigher: The single push or pull that would have the same effect as all the forces acting on a body put together, found by adding those forces as vectors. When it comes to zero the body stays at rest or keeps moving with constant velocity.
Scalar quantityHigher: A physical property that has size only, with no direction attached to it, so a single number and a unit describe it in full. Mass, time, distance, speed and energy are examples, and they combine by ordinary arithmetic rather than by drawing a diagram.
SpeedOrdinary: The distance travelled in each second, measured in metres per second (m s⁻¹).
Terminal velocityHigher: The steady speed a falling body settles at once the upward drag force has grown large enough to balance its weight. With no unbalanced force left, the acceleration drops to zero and the speed stops increasing, as with a parachutist.
Vector quantityHigher: A physical property that needs a size and a direction before it is fully described, such as force, displacement, velocity, acceleration and momentum. Two of them are added head to tail on a diagram or by resolving them into perpendicular components.
Vector quantityOrdinary: A measurement that needs a size and a direction, such as force or velocity. Mass and time need only a size.
VelocityHigher: The rate of change of displacement with respect to time, measured in m s⁻¹ and always quoted with a direction. On a displacement-time graph it is given by the slope, so a curved graph shows that it is not staying constant.
VelocityOrdinary: Speed given together with the direction of travel, such as 12 m s⁻¹ due north.
WeightHigher: The force with which gravity pulls on a body, found by multiplying its mass by the gravitational field strength and measured in newtons. It changes from place to place, while the mass of the body stays the same everywhere.
WeightOrdinary: The pull of gravity on an object, measured in newtons. On the Earth it is the mass multiplied by 9.8 m s⁻².
WorkHigher: The energy transferred when a force moves its point of application, found by multiplying the force by the distance moved in the direction of that force. It is measured in joules, and nothing is transferred when the movement is at right angles to the force.
WorkOrdinary: The energy transferred when a force moves an object, found by multiplying the force by the distance moved. It is measured in joules.

Waves and Energy Transfer 17 terms

AmplitudeHigher: The greatest distance a particle of the medium moves away from its rest position as a wave passes through it. It sets how much energy the wave carries, showing up as loudness in a sound wave and as brightness in a light wave.
DiffractionHigher: The spreading of a wave after it passes through a narrow gap or around the edge of an obstacle. The effect is greatest when the gap is about the same size as the wavelength, which is why sound bends around a doorway but light appears not to.
Doppler effectHigher: The change in the observed frequency of a wave when the source and the observer are moving relative to one another. The observed frequency rises as they approach and falls as they separate, heard in a passing siren and seen as red shift.
FrequencyHigher: The number of complete vibrations or cycles that pass a point each second, measured in hertz. It is fixed by the source of the wave and does not change when the wave crosses into a new medium, unlike speed and wavelength.
FrequencyOrdinary: The number of complete waves that pass a point each second, measured in hertz (Hz).
InterferenceHigher: The result of two waves of the same type meeting at a point, where their displacements add together. Waves arriving in step reinforce to give a loud or bright maximum, while waves arriving out of step cancel to give a minimum.
Longitudinal waveHigher: A disturbance in which the particles of the medium vibrate along the same line as the direction of travel, producing regions of compression and rarefaction. Sound is the standard example, which is why it cannot travel through a vacuum.
ReflectionHigher: The return of a wave into the same medium after it strikes a boundary, with the angle of incidence equal to the angle it leaves at, both measured from the normal. This is what allows a plane mirror to form an image behind it.
ReflectionOrdinary: The bouncing back of light or sound when it hits a surface. A mirror image and an echo are both caused by it.
RefractionHigher: The change in direction of a wave as it crosses a boundary between two media, caused by the change in its speed. It bends towards the normal on entering a denser medium and away from the normal on leaving one.
RefractionOrdinary: The bending of light as it passes from one material into another, such as air into water. It makes a straw look bent in a glass.
Refractive indexHigher: A number comparing the speed of light in a vacuum with its speed in a given material, equal to the sine of the angle of incidence divided by the sine of the angle of refraction. Light in a vacuum travels at 3.0 x 10⁸ m s⁻¹.
Specific heat capacityHigher: The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in J kg⁻¹ K⁻¹. Water has an unusually high value, which is why it is used in heating systems and why coastal climates are mild.
Total internal reflectionHigher: The complete return of light into a denser medium when it meets the boundary with a less dense medium at an angle of incidence greater than the critical angle. Optical fibres and prism periscopes rely on it, since no light escapes.
Transverse waveHigher: A disturbance in which the particles of the medium vibrate at right angles to the direction in which the wave travels, producing crests and troughs. Light, every other kind of electromagnetic radiation and ripples on water all behave this way.
WavelengthHigher: The distance between two neighbouring points on a wave that are vibrating in step, for example from one crest to the next. It is tied to the other wave quantities by speed = frequency x wavelength, so it shortens in a slower medium.
WavelengthOrdinary: The distance from one crest of a wave to the very next crest, measured in metres.

Electricity and Magnetism 23 terms

Alternating currentHigher: A flow of charge that reverses its direction regularly, many times each second, because the potential difference driving it keeps changing sign. The mains supply in Ireland behaves this way at a frequency of 50 Hz.
CapacitanceHigher: The charge stored on a conductor divided by the potential difference needed to store it, measured in farads. A capacitor built from two parallel plates holds energy in the electric field between them and can release it very quickly.
Coulomb's lawHigher: 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. Like charges are pushed apart and unlike charges pulled together along the line joining them.
Electric chargeHigher: A basic property of matter that comes in two kinds, positive and negative, and makes bodies carrying it attract or repel one another. It is measured in coulombs and is carried by protons and electrons in equal and opposite amounts.
Electric currentHigher: The rate of flow of charge past a point in a circuit, measured in amperes, where one ampere is one coulomb per second. In a metal it is carried by free electrons, which drift the opposite way to the conventional arrow on a diagram.
Electric currentOrdinary: The flow of charge around a circuit, measured in amperes (A) using an ammeter.
Electric field strengthHigher: The force per unit charge acting on a small positive test charge placed at a point, measured in newtons per coulomb. Its direction is the direction of the force on that positive charge, so it points away from a positive source charge.
Electrical powerHigher: The rate at which electrical energy is converted into other forms in a component, found by multiplying the current through it by the potential difference across it. This is why a kettle drawing a large current is rated in kilowatts.
Electrical powerOrdinary: The electrical energy changed into other forms each second, found by multiplying current by potential difference.
Electromagnetic inductionHigher: The production of an emf in a conductor whenever the magnetic flux through it changes, either by moving the conductor in a field or by changing the field itself. It is the principle behind generators, dynamos and transformers.
Faraday's law of electromagnetic inductionHigher: The size of the emf induced in a circuit is proportional to the rate of change of magnetic flux through that circuit. Moving a magnet into a coil more quickly, or using a coil with more turns, therefore gives a larger induced emf.
FuseOrdinary: A thin wire in a plug that melts and breaks the circuit if the current gets too high, protecting the appliance.
Lenz's lawHigher: The direction of an induced current is always such that its magnetic effect opposes the change that is producing it. Work must therefore be done against that opposition, which is how the principle of conservation of energy is respected.
Magnetic fieldHigher: The region around a magnet, a moving charge or a conductor carrying current in which a magnetic force can be detected. It is mapped with lines running from north to south outside a magnet, and their spacing shows how strong it is.
Magnetic fieldOrdinary: The space around a magnet or a wire carrying current where a magnetic force can be felt.
Ohm's lawHigher: The current flowing through a metallic conductor kept at constant temperature is directly proportional to the potential difference across it. A conductor that obeys it gives a straight line through the origin on a graph of current against potential difference.
Ohm's lawOrdinary: The current in a metal conductor at constant temperature is directly proportional to the potential difference across it.
Potential differenceHigher: The work done in moving one coulomb of charge from one point in a circuit to another, measured in volts. A cell supplies it, and each component in a series loop takes a share that adds up to the total supplied by the source.
Potential differenceOrdinary: The energy given to each unit of charge between two points in a circuit, measured in volts (V).
ResistanceHigher: The opposition a component offers to the flow of charge, defined as the potential difference across it divided by the current through it and measured in ohms. Electrical energy is converted to internal energy as charge passes through.
ResistanceOrdinary: How strongly a component opposes the flow of charge, measured in ohms. A long thin wire has more of it than a short thick one.
ResistivityHigher: A property of a material rather than of one particular sample, measuring how strongly it opposes the flow of charge. It equals the resistance of a piece of that material of unit length and unit cross-sectional area, quoted in ohm metres.
TransformerHigher: A device made of two coils wound on a common soft iron core that changes the size of an alternating potential difference. The ratio of the potential differences equals the ratio of the numbers of turns, and it will not work on direct current.

Modern Physics 15 terms

Half-lifeHigher: The time taken for half the unstable nuclei in a sample to decay, or equally for the activity of that sample to fall to half its value. It is a fixed characteristic of a nuclide and ranges from fractions of a second to billions of years.
Half-lifeOrdinary: The time it takes for half the unstable nuclei in a sample to decay away.
Ionising radiationHigher: Any emission with enough energy to knock electrons out of the atoms it passes through, leaving charged ions behind. Alpha particles, beta particles, gamma rays and X-rays all do this, which is why exposure has to be kept low.
IsotopeHigher: Atoms of the same element that contain the same number of protons but different numbers of neutrons in the nucleus. They behave identically in chemical reactions but differ in mass and in whether the nucleus is stable.
IsotopeOrdinary: Atoms of the same element with the same number of protons but different numbers of neutrons.
Mass-energy equivalenceHigher: The principle that a quantity of matter can be converted into a quantity of energy and back again, the two being linked by E = mc². Because the speed of light is 3.0 x 10⁸ m s⁻¹, a tiny loss of mass releases an enormous amount of energy.
Nuclear fissionHigher: The splitting of a large unstable nucleus into two smaller nuclei, releasing neutrons and a large amount of energy. The neutrons released can trigger further splits, giving the chain reaction used in a nuclear power station.
Nuclear fissionOrdinary: The splitting of a large nucleus into two smaller ones, releasing a lot of energy. It is used in nuclear power stations.
Nuclear fusionHigher: The joining of two light nuclei to form a single heavier nucleus, releasing far more energy per kilogram than splitting one does. It needs enormous temperatures to overcome the repulsion between the nuclei, and it is what powers the Sun.
Nuclear fusionOrdinary: The joining of two small nuclei to make a bigger one, releasing energy. It is what makes the Sun shine.
Photoelectric effectHigher: The emission of electrons from the surface of a metal when electromagnetic radiation above a certain threshold frequency falls on it. Raising the intensity releases more electrons, but only a higher frequency gives each one more energy.
PhotonHigher: A single packet, or quantum, of electromagnetic radiation whose energy equals Planck's constant multiplied by the frequency of that radiation. Treating light as a stream of these explains effects that a pure wave picture cannot.
QuarkHigher: A fundamental particle that combines in twos or threes to build protons, neutrons and other heavy particles, and that is never found on its own. Everyday matter is made from the up and down kinds, which carry fractional electric charge.
RadioactivityHigher: The spontaneous breakdown of an unstable nucleus, which emits alpha particles, beta particles or gamma rays and becomes more stable. The process is random, unaffected by temperature or chemical state, and cannot be switched off.
RadioactivityOrdinary: The breaking up of unstable nuclei, giving out alpha particles, beta particles or gamma rays.

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