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IB Glossary

The following terms are taken from the IB Physics specification. The list is separated into temrs from the Core part of the course (relevant to all students) and terms taken from the Additional Higher Level part (relevant to Higher level students and Standard level student staking associated options). IB students are expected to be familiar with the meanings of each of these terms, and are expected to be able to give the definitions of many of the terms (see the specification for further information on this). The definitions given below are based on those given in a variety of sources, including IB past paper mark schemes. I have tried my best to provide definitions that are consistent with the expectations of examiners but I cannot guarentee that each of the definitions I have given would be acceptable.






Topic 1: Physics and physical measurement


Fundamental units:

A set of seven independently defined units of measurement that forms the basis of a system of units. These are: metres; kilograms; seconds; amperes; kelvin;  moles; candela.


Derived units:

Units which may be expressed as a simple combination of fundamental units. Newtons, for example, may be expressed as kg m s-2 (from F = ma).


Random errors/uncertainties:

Random errors produce a spread of reading either side of the true value. If only random errors are present then the mean (of sufficient readings) will lie close to the true value.


Systematic errors:

Systematic errors result in all readings being out by the same amount to one side of the true value. They often stem from faults in the experimental method or apparatus. Averaging has no effect upon systematic errors.


Zero error:

A common example of a systemic error. Zero errors are caused by instruments giving readings when no quantity is present. E.g. A balance which has not been zeroed may give a reading when unladen.



Measurements are precise if they are closely clustered (small spread): i.e. the uncertainty is small



Measurements are accurate if their average is close to the true value: i.e. the systematic error is small.


Absolute uncertainties:

An uncertainty in a quantity expressed in the units of that quantity.


Fractional uncertainties:

The ratio of the absolute uncertainty in a quantity to the mean value.


Percentage uncertainties:

The fractional uncertainty expressed as a percentage.



A quantity that has both a magnitude (size) and direction. Examples include: velocity; acceleration; force.



A quantity that has only magnitude (not direction). Examples include: mass; energy; time.



Topic 2: Mechanics



The distance of a point in a particular direction.



The rate if change of displacement of an object with respect to time (change in displacement per unit time).



The distance traveled per unit time.



The rate of change of velocity with respect to time.


Newton's first law of motion:

A body will remain at rest or moving at constant velocity unless acted upon by an unbalanced force.


Newton's second law of motion:

The rate of change of momentum of a body is directly proportional to the unbalanced force acting on that body and takes place in the same direction.


Newton's third law of motion:

If body A exerts a force on body B then body B will exert an equal and opposite force on body A.


Linear momentum:

The product of mass and velocity.


Law of conservation of linear momentum:

For a system of isolated bodies the total momentum is always the same.



The product of a force and the time for which it acts. The impulse is equal to the change in momentum that is produced.



The product of force (acting on a body) and the distance moved (by its point of application) in the direction of the force. Work involves a transfer of energy equal to the work done.


Kinetic energy:

The energy a body has due to its movement.


Change in gravitational potential energy

The energy a body gains or loses due to a change in its position in a gravitational field.


Principle of conservation of energy:

Energy can neither be created nor destroyed. It can only be changed from one form to another.


Elastic collision:

A collision in which both momentum and kinetic energy is conserved.


Inelastic collision:

A collision in which momentum is conserved but kinetic energy is not.



The rate of working (work done divided by time taken).



The efficiency of a system is the ratio of useful energy out to total energy in.



Topic 3: Thermal Physics


Internal energy:

The sum of the total potential energy and random kinetic energy of the molecules of a substance.


Thermal energy (Heat):

The amount of energy transfer from one body (or system) to another as a result of a difference in temperature.



A measure of how hot an object is; temperature determines the direction of heat flow. It is related to the average kinetic energy of each molecule in a substance. Temperature is usually measured in kelvin (or degrees celsius).


The Mole:

An amount of a substance that contains as many elementary units (molecules) as there are atoms in 12 g of Carbon-12.


Molar mass:

The mass of one mole of a substance.


The Avogadro constant:

The number of molecules in one mole of a material.


Specific Heat Capacity, c:

The amount of thermal energy required to raise the temperature of 1 kg of a particular material by 1 K. Units = J kg-1 K-1.


Thermal capacity, C:

The amount of thermal energy required to raise the temperature of a body by one kelvin. Units = J K-1.


Specific Latent heat, L:

The amount of thermal energy required to change the state of 1 kg of a particular material without change of temperature. Units = J kg-1.

Specific latent heat of fusion refers to a change from solid to liquid.

Specific latent heat of vaporization refers to a change from liquid to gas.



A gradual change in state of a material from liquid to vapor at a temperature below the boiling point of a liquid. Evaporation occurs at the surface of a liquid.



A change in state of a material from liquid to gas: Boiling takes place throughout a liquid and always happens at the same temperature (for a given pressure).



The force per unit area acting upon a surface.



Topic 4: Oscillations and waves


Equilibrium position:

The position where an oscillator would rest if not disturbed.


Displacement (of an oscillator):

The distance moved in a particular direction from the equilibrium position.



The maximum displacement from the equilibrium position.



The number of oscillations that an oscillator makes per unit time.



The time taken for one complete oscillation.


Phase angle:

An angle between zero and 2π radians that is used to describe the different stages of an oscillatory cycle.


Phase difference:

The difference in phase angle between two different stages of an oscillatory cycle.


Simple Harmonic Motion (SHM):

A form of periodic motion the restoring force is proportional to the displacement and directed towards the equilibrium position.



The gradual loss of energy of an oscillator due to frictional or other resistant forces.



The increase in amplitude of an oscillator that occurs when it is forced to oscillate at its natural frequency.


Natural frequency of vibration:

The frequency of the free oscillation of a system.


Forced vibrations:

The vibration of an object due to the application of a periodic driving force. Under such circumstances the frequency of the oscillations will generally be equal to the frequency of the driving force.


Transverse wave:

A wave for which the displacement of points along the wave is at 90 degrees to the direction of energy propagation.


Longitudinal wave:

A wave for which the displacement of points along the wave is parallel to the direction of energy propagation.




The highest point of a transverse wave. The crest has the maximum positive displacement.



The lowest point of a transverse wave. The trough has the greatest negative displacement.



A point of highest density or pressure along a longitudinal wave.



A point of lowest density or pressure along a longitudinal wave.



A line joining points along a wave that are in phase (usually the crests).



Lines (with arrows) showing the direction of travel of a wave. Rays are always at right angles to wavefront.


Displacement (of a wave):

The distance in a particular direction of a point along a wave from its equilibrium position.


Amplitude (of a wave):

The maximum displacement of points along a wave from the equilibrium position.


Frequency (of a wave):

The number of waves passing a fixed point per unit time.


Period (of a wave):

The time taken for one complete wave to pass a fixed point.



The distance between two consecutive points which have the same phase.


Wave speed:

The distance traveled by the wave profile in the direction of energy flow per unit time.



The power per unit length (or area) carried by a wave.


Principle of superposition:

When two or more waves overlap the net displacement of the medium through which the waves travel is equal to the sum of individual displacements produced by each wave.


Constructive interference:

The creation of a large resultant wave which occurs at a point where two superposing wave are in phase.


Destructive interference:

The creation of a wave of minimum (or zero) amplitude which occurs at a point where two superposing waves are in anti-phase.


Path difference:

The difference in the distances traveled by two waves meeting at a single point.



Topic 5: Electric currents


Electric potential difference:

The electric potential energy transferred by (or to) each coulomb of charge passing between two points in an electric circuit.


Electron volt:

The work done on an electron in moving it through a potential difference of one volt.


Electric current:

The charge flowing past a point per unit time. Current is always defined as going from positive to negative.



The opposition to the flow of charge. Resistance is defined as the ratio of the potential difference across electrical component to the current passing through it.


Ohm's law:

The current in an ohmic conductor is proportional to the potential difference across it, provided temperature and other physical conditions remain constant.


Electromotive force (EMF):

The EMF of a power supply is the amount of energy converted to electrical energy by that power supply per unit charge.


Internal resistance:

The resistance within a source of electric current, such as a cell or generator.


Potential divider:

Two resistors connected in series such that a fraction of the total voltage may be tapped off by connecting across one of the resistors.



Topics 6: Fields and forces


Newton's law of universal gravity:

Every single point mass attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separations.


Gravitational field strength:

The force per unit mass experienced by a small test mass due to the attraction of other masses.


Law of conservation of charge:

The total charge of a closed system is constant (i.e. Charge cannot be created or destroyed).


Coulomb's law:

The force experienced by a point charge due to the presence of another point charge is directly proportional to the product of their charges and inversely proportional to the square of their separations.


Electric field strength:

The force per unit charge experienced by a small positive test charge due to the presence of other charges.


Magnetic field:

A region in space where a small magnet experiences a turning force.

The magnitude and direction of a magnetic field are expressed in terms of the Magnetic Flux Density, B (units = Tesla): the direction is the same as that of the field lines (flux) and the magnitude is related to their separation.



Topics 7: Atomic and nuclear Physics



A particular combination of protons and neutrons that form a nucleus.



Nuclei with the same number of protons but different numbers of neutrons.



A particle found in the nucleus (i.e. a proton or neutron).


Nucleon number (A)

The number of protons plus neutrons in a nucleus


Proton number (Z):

The number of protons in a nucleus.


Neutron number (N):

The number of neutrons in a nucleus.


Radioactive half-life:

The time taken for half the original nuclei in a sample to decay.


Artificial (induced) transmutation:

The change of a nucleus from one form to another due to the bombardment of a material with high energy particles.


Unified atomic mass unit:

One atomic mass unit is defined as being one twelfth the mass of a carbon-12 atom.


Mass defect:

The difference between the rest mass of an atomic nucleus and the sum of the rest masses of its individual nucleons in the unbound state.


Binding energy:

The amount of work required to pull apart the constituent particles in a nucleus.


Binding energy per nucleon:

The binding energy of a nucleus divided by nucleon number of the nucleus. The binding energy per nucleus gives the average energy lost by each nucleon when brought together to form the nucleus.



Topic 8: Energy, power and climate change


Degraded energy:

Energy that has become more spread out or disordered, and hence difficult to convert into mechanical energy.


Energy density:

The energy that can be extracted from each kg of fuel.


Controlled nuclear fission (power production):

A chain reaction in which one, on average, neutron from each fission will go on to produce a further fission. In such a reaction the rate of fission in the reactor is approximately constant.


Uncontrolled nuclear fission (nuclear weapons):

A chain reaction in which more than one neutron, on average, goes on to crate a further fission. This caused the rate of fission to increase at an accelerating rate.


Fuel enrichment:

A process which is used to increase the proportion of Uranium-235 in the nuclear fuel. Enrichment typically increases the amount of Uranium-235 from around 0.7% to 3%.


Photovoltaic cell:

A semiconducting device that is used to convert solar radiation directly into electrical energy.


Solar heating panel:

A panel that uses solar radiation to heat water for the purpose of central heating or hot water supply.


The greenhouse effect:

The effect by which solar radiation is able to pass through a planet's atmosphere, but the longer wavelength electromagnetic radiation, such as infra red, emitted from the planet's surface is reflected by gases in the planet's atmosphere. This results in a raising of the equilibrium temperature for the planet.


Enhanced greenhouse effect:

The raising of a planet's temperature above its usual equilibrium due to an increase in the amount of greenhouse gases in the planet's atmosphere.



The ratio of reflected radiation to incident radiation upon a surface.

White objects have albedos close to one. Dark objects have albedos close to zero.


Black-body radiation:

The spectrum of thermal radiation given off by an ideal (black) emitter.


Surface heat capacity (Cs):

The amount of heat required to raise the temperature of 1m2 of ground by 1 K.


Coefficient of volume expansion:

The proportional increase in volume of a liquid per unit rise in temperature




AHL only


Topic 9: Motion in fields


Gravitational potential:

The work done in bringing a unit mass from infinity to a particular point in a gravitational field.


Gravitational potential energy:

The work done in bringing a mass from infinity to a particular point in a gravitational field.


Electric potential:

The work done in bringing a unit charge from infinity to a particular point in an electric field.


Electric potential energy:

The work done in bringing a charge from infinity to a particular point in an electric field.


Kepler’s third law:

For any object orbiting a given body the ratio of the square of the time period to the cube of the average distance is a constant.


Topic 10: Thermal Physics


Ideal gas:

A hypothetical gas which obeys the gas laws perfectly. The ideal gas model makes the following assumptions: molecules are identical spheres which occupy a negligible volume compared to the volume of the gas; no forces act on the molecules, except for when they collide, and when they do collide they do so elastically (total kinetic energy is conserved).


Absolute zero:

The lowest possible temperature. A material at absolute zero contains no internal energy: Measured in Celsius absolute zero is approximately -273˚c.


Kelvin scale of temperature:

The Kelvin temperature scale is defined such that 0 K is equal to absolute zero and a change of 1 K is equal to a change of 1 ˚c. Temperatures measured in Celsius may be converted to Kelvin by adding 273, and vice versa.


First law of thermodynamics:

The amount of heat, Q, added to a gas is equal to the increase in internal energy of a gas, ΔU., plus the work done by the gas, W.


Second law of thermodynamics:

In any thermodynamic process the total entropy always increases. The second law of thermodynamics places restrictions on what is possible: It is not possible, for example, to convert heat completely into work.



A measure of the inability of a system’s energy to do work. Entropy may be thought of as a measure of the amount of disorder of a system.


Isochoric (isovolumetric):

An isochoric change is one where the volume remains constant.



An isobaric change is one where the pressure remains constant.



An isothermal change is one where temperature remains constant. Despite this, isothermal changes usually involve a transfer of heat to or from the gas, which is needed to offset the work done by or on the gas. Because it takes time for heat to be transferred isothermal changes usually have to happen slowly



An adiabatic change is one which involves no transfer of heat. Despite this, the temperature of a gas may change as a result of work done on or by the gas. In reality, because it is impossible to perfectly insulate a system, some heat is always transferred and so perfect adiabatic changes are impossible. But providing a change is rapid (to minimize heat exchange)  a change can be approximated as adiabatic.



Topic 11: Waves Phenomena


Standing wave:

The result of the superposition of two waves of equal amplitude and wavelength traveling in opposite directions. Standing waves consist of a fixed pattern of nodes (points of zero motion) and anti-nodes (points of maximum motion). They often occur when waves are reflected.


Doppler effect:

The observed change in the frequency of a wave as a result of the relative motion of an observer with respect to the wave source.


Resolution (of a telescope):

The smallest angular separation at which two objects can just be discerned. The resolution of a telescope is limited by the diffraction of light as it enters the telescope, which causes point sources to spread out as a blob.


Rayleigh criterion:

According to this criterion the limit of resolution for two point object occurs when the centre of the diffraction pattern for one of the points falls upon the first minimum for the other source. From this the angular resolution (in radians) is found to be 1.22 λ/d.






A property of some transverse waves: the direction of the displacement of points along the wave (e.g. horizontal or vertical). A wave is said to be polarized if all points are constrained to oscillate in the same plane.


Optically active substance:

A substance which is able to rotate the plane of polarization of light passing through it.



Topic 12: Electromagnetic Induction


Magnetic flux:

May be thought of as being the number of magnetic field lines (lines of flux) passing through a given area. It is defined as the product of the normal component of magnetic field strength and the area that it links.


Magnetic flux linkage:

The product of the magnetic flux (through a coil) and the number of turns on the coil. The flux linkage is the most relevant quantity for electromagnetic induction since a changing flux will have an electromagnetic effect upon each turn it passes through.


Faraday’s law of electromagnetic induction:

The EMF induced is equal to the rate of change of flux linkage.


Lenz’s law:

The direction of the induced EMF is such that it will oppose the change creating it.



Topics 13: Quantum and nuclear Physics


Decay constant:

The probability of a specified nucleus decaying in one second.



Topic 14: Digital technology



The ratio of the charge stored on a device to the potential difference across it. The units of capacitance are farads although, typically, capacitances are measured in nano- or even pico-farads.




Quantum efficiency (of a pixel):

The ration of the number of electrons emitted to the number of photons incident upon a pixel.



The ratio of image height to object height.


Resolution (of a digital image):

The size of the smallest thing that can be seen (resolved) in an image. Two objects may just be resolved if they are at least two pixels apart.