IB Physics Tutor. Definitions and explanations. How to answer correctly and precisely.

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All definitions are taken from IB Past Papers (neither books nor just from google) hence are correct, precise and acceptable from IB point of view.

Topic 1

State what is meant by a systematic error.

error that is identical for each reading / error caused by zero error in instrument.

Which of the following will reduce random errors in an experiment?

Repeating readings.

Topic 2

State the difference between average speed and instantaneous speed.

average speed is the speed over a period of time/distance; instantaneous speed is the speed at a particular instant in time/point in space.

Which statement applies to an object in translational equilibrium?

the resultant force acting on the object must be zero.

Define linear momentum.

mass×velocity; (allow mv with symbols defined).

State the law of conservation of linear momentum.

State, in terms of momentum, Newton’s second law of motion.

the rate of change of momentum of a body is equal to/directly proportional to the force acting on the body.

total momentum does not change/is constant; (do not allow “momentum is conserved”)
provided external force is zero / no external forces / isolated system.

State Newton’s third law of motion.

) if object A exerts a force on object B, then object B (simultaneously) exerts an equal and opposite force on object A / every action has an equal and opposite reaction.

In the collision between two bodies, Newton’s third law

always applies.

The net force on a body is F. The impulse of F is equal to the

change in momentum of the body.

In an inelastic collision

momentum and kinetic energy are both conserved.

Topic 3

Two objects are in thermal contact, initially at different temperatures. What does determine the transfer of thermal energy between the objects?

The temperature of the objects only.

Define specific heat capacity.

the energy required to change the temperature (of a substance) by 1K/°C/unit degree;
of mass 1 kg / per unit mass.

(note: thermal capacity is energy required to raise the temperature of a whole object not 1 kg by 1 degree and hence is measured in J/K)
(note: energy density is energy per unit volume and measured in J per meter cubed but specific energy is energy per unit mass and is measured in J/kg)

Define the specific latent heat of fusion of a substance.

the energy (absorbed/released) when a unit mass/one kg;
of liquid freezes (to become solid) at constant temperature / of solid melts (to become liquid) at constant temperature.

Explain, in terms of the molecular model of matter, the relative magnitudes of the specific latent heat of vaporization of water and the specific latent heat of fusion of water.

potential energy changes during changes of state / bonds are weakened/broken during changes of state;
potential energy change is greater for vaporization than fusion / more energy is required to break bonds than to
weaken them;
SLH vaporization is greater than SLH fusion.

Water at constant pressure boils at constant temperature. Outline, in terms of the energy of the molecules, the reason for this.

temperature is a measure of the (average) kinetic energy of the molecules;
at the boiling point, energy supplied (does not increase the kinetic energy) but (only) increases the potential energy of the molecules/goes into increasing the separation of the molecules/breaking one molecule from another

The specific latent heat is the energy required to change the phase of

one kilogram of a substance at constant temperature.

Explain, in terms of the energy of its molecules, why the temperature of a pure substance does not change during melting

energy supplied/bonds broken/heat absorbed;
increases potential energy;
no change in kinetic energy (so no change in temperature);

Which of the following is numerically equal to the specific heat capacity of the substance of a solid body?

The thermal energy required to increase the temperature of unit mass of the body by 1K

What is the definition of the mole?

The amount of substance that contains as many elementary entities as the number of atoms in 12 g of carbon-12.

Molar mass is defined as

the mass of one mole of a substance.

Which of the following correctly identifies the properties of the molecules of a substance that determine the substance’s internal energy?

The total potential energy and random kinetic energy.

Distinguish between the concepts of internal energy and temperature.

internal energy:
total energy of component particles;
comprises potential energy + (random) kinetic energy;
measure of average kinetic energy of particles;
indicates direction of (natural) flow of thermal energy;
internal energy measured in J and temperature measured in K/°C.


State what is meant by an ideal gas

a gas in which there are no intermolecular forces
OR a gas that obeys the ideal gas law/all gas laws at all pressures, volumes and temperatures
OR molecules have zero PE/only KE

Assumptions of the kinetic model of an ideal gas

point molecules / negligible volume;
no forces between molecules except during contact; (note: therefore, the intermolecular potential energy of the molecules is zero)
motion/distribution is random;
elastic collisions / no energy lost;
obey Newton’s laws of motion;
collision in zero time;
gravity is ignored;

(note: to remember 2 is enough)
Under what conditions of density and pressure is a real gas best described by the equation of state for an ideal gas?

Low density and low pressure
(note: high or moderate temperature if question involves temperature)

Describe how the ideal gas constant R is defined

defined from the equation of state of an ideal gas PV=nRT;
all symbols (PVnT) correctly identified.

The volume of the gas in is increased to … at constant temperature. Explain, in terms of molecular motion, decreasing in pressure.

«volume has increased and» average velocity/KE remains unchanged
«so» molecules collide with the walls less frequently/longer time between collisions with the walls
«hence» rate of change of momentum at wall has decreased
«and so pressure has decreased»

An ideal gas is contained in a thermally insulated cylinder by a freely moving piston. The gas is compressed by the piston and as a result the temperature of the gas increases. What is the explanation for the temperature rise?

Energy is transferred to the molecules by the moving piston.

An ideal gas has an absolute temperature T. The average random kinetic energy of the molecules of the gas is

proportional to T.

With respect to a gas, explain the meaning of the terms thermal energy and internal energy.

thermal energy: (non-mechanical) energy transferred between two objects (at different temperatures);
internal energy: (total) potential energy and (random) kinetic energy of the molecules/particles (of the gas).

(note: for ideal gas potential energy is zero, hence total (internal) energy = total random kinetic energy)

Describe, with reference to the energy of the molecules, the difference in internal energy of a piece of iron and the internal energy of an ideal gas.

the internal energy of the iron is equal to the total KE plus PE of the molecules; the molecules of an ideal gas have only KE so internal energy is the total KE of the molecules.

Topic 4

State the principle of superposition.

net displacement of the medium;
equals the resultant/sum of individual displacements.

Define simple harmonic motion (SHM).

(periodic) motion in which acceleration/restoring force is proportional to the displacement from a fixed point;
directed towards the fixed point / in the opposite direction to the displacement.

How a standing wave can be formed.

travelling wave reflects at…
at certain fixed frequencies
incident wave and reflected wave
superpose (or interfere)
to produce a standing wave
(note: these are key phrases which could be used in explanation of formation of standing waves for different situations (problems), but you can add something according to the question, e.g. producing nodes and antinodes, etc.»)

Describe the formation of standing waves in a string fixed at both ends.

wave travels down string and is reflected;
incident and reflected waves interfere/add/superpose to give a standing wave.

A tuning fork is sounded above the tube. For particular values of L, a standing wave is established in the tube. Explain how a standing wave is formed in this tube.

travelling waves move down the tube;
which then interfere with the reflected waves (from the closed end of the tube/surface of the water);
(OR superposition as an alternative to interference).

Travelling vs standing waves

Energy propagation
T: energy (crests/disturbances) moves
S: no propagation
T: all particles oscillate with same amplitude
S: particles oscillate with different amplitudes
T: particles separated by integer number of wavelengths are in phase
S: all particles within two consecutive nodes are in phase

Outline the nature of electromagnetic waves.

a varying magnetic and electric field at right angles to each other;
vibration of E and B fields at right angles to the direction of propagation of the wave;
transverse wave;
same speed in a vacuum.

State what is meant by polarized light.

light in which the electric field vector oscillates on one plane/direction.

Electromagnetic waves propagating in a medium suffer dispersion. Describe what is meant by dispersion.

waves of different wavelength/frequency;
travel at different velocities;
the index of refraction of the medium depends on the wavelength/frequency.

A charge moves backwards and forwards along a wire. Outline, with reference to the motion of the charge, why electromagnetic radiation is produced by the moving charge.

during simple harmonic motion the charge oscillates/accelerates;
(oscillating/accelerating) charges radiate/produce (varying) electric/magnetic fields / produce electromagnetic waves.

Unpolarized light is incident on the surface of a transparent medium. The reflected light is completely plane polarized. The refracted light will be

partially plane polarized.

Describe, in terms of the propagation of energy, what is meant by a longitudinal travelling wave.

the direction of oscillation of the particles of the medium;
(must be “particles”)
is in the direction of energy propagation;

State what is meant by the terms ray and wavefront and state the relationship between them.

ray: direction of wave travel / energy propagation;
wavefront: line that joins points with same phase/of same crest/trough;
ray normal/at right angles/perpendicular to wavefront.

Describe the difference between transverse waves and longitudinal waves.

mention of perpendicular/right angle/90° angle for transverse and parallel for longitudinal;
clear comparison between direction of energy propagation and direction of vibration/oscillation of particles for both

The phenomenon of diffraction is associated with

all waves.

State what is meant by coherence.

constant phase difference.

Diffraction is

spreading out a wave when it meets an aperture/gap/slit/obstacle.
note: the greater the wavelength is the greater diffraction is. Why do TV’s require high antennas, whereas radio does not? Wavelength for radio waves is much greater, hence radio signals easily bend around buildings.
The insect will not be detected (by sound wave, for example) if the length of the insect is much smaller then the wavelength of sound wave. Also there is a limit to the magnification of a light micriscope. Wavelength of light about micrometer and if length of object is (of order or) less then micrometer there will be very little reflection. Light waves just bend around the object. To see something light must be reflected/

Topic 5

State Coulomb’s law.

the force between two (point) charges;
is inversely proportional to the square of their separation and (directly) proportional to (the product of) their

Coulomb’s law refers to electric charges that are

point charges.

Define electric field strength.

The force exerted per unit charge;
on a positive small/test charge.

What is the definition of electric current?

The rate of flow of electric charge.

Define electromotive force (emf). State how the emf of the battery can be measured.

the work done per unit charge in moving a quantity of charge completely around a circuit / the power delivered per unit current / work done per unit charge made available by a source;
place voltmeter across battery.

Define the electrical resistance of a component.

ratio potential difference across the component to current in the component.

Ohm’s law

The resistance of a conductor is constant provided that the temperature is constant.
OR potential difference across the component is directly proportional to the current in the component providing the temperature/physical conditions are constant.

The ampere is defined in terms of

. force per unit length between parallel current-carrying conductors.

Tungsten is a conductor used as the filament of an electric lamp. The filament of the lamp is surrounded by
glass which is an insulator. Outline, in terms of their atomic structure, the difference between the electrical properties of tungsten and of glass.

conduction is due to movement of the free electrons (transferring charge around circuit);
tungsten is a good electrical conductor with large numbers of free electrons;
glass is a poor electrical conductor with few/no free electrons.

Topic 6

Which single condition enables Newton’s universal law of gravitation to be used to predict the force between the Earth and the Sun?

The Earth and the Sun behave as point masses.

Explain why astronauts inside the spaceship would feel “weightless”, even though there is a force acting on them.

) astronauts and spaceship have the same acceleration;
acceleration is towards (centre of) planet;
so no reaction force between astronauts and spaceship;
astronauts and spaceships are both falling towards the (centre of the) planet;
at the same rate;
so no reaction force between astronauts and spaceship.

What is the definition of gravitational field strength at a point?

Force acting per unit mass on a small mass placed at the point.

s A car is moving at a constant speed over a curved bridge. Explain why the car is accelerating even though it is moving with a constant speed.

direction changing;
velocity changing so accelerating.

State Newton’s universal law of gravitation.

the (attractive) force between two (point) masses is directly proportional to the product of the masses; and inversely proportional to the square of the distance (between their centres of mass).
Use of equation is acceptable if all five quantities defined.

Topic 7

Explain how atomic spectra provide evidence for the quantization of energy in atoms.

atomic spectra have discrete line structures / only discrete frequencies/wavelengths;
photon energy is related to frequency/wavelength;
photons have discrete energies;
photons arise from electron transitions between energy levels;
which must have discrete values of energy.

State what is meant by the term isotope.

nuclides/atom/element/nucleus/nuclei that have different nucleon/neutron numbers but same proton number/are same element

Define the term unified atomic mass unit.

one twelfth of the mass of a carbon-12 atom;
Do not allow nucleus!

Define the term radioactive half-life.

time for the activity of a sample to halve / time for half the radioactive nuclei to decay.

State what is meant by the term elementary particle.

particle with no internal structure / cannot be broken down further.

State what is meant by an exchange particle.

a particle that mediates/carries/transmits one of the fundamental forces / a particle that is exchanged between two particles when undergoing one of the fundamental interactions

Identify another elementary particle other than the quark.

Electron / neutrino / any lepton / any named exchange particle.

Identify the boson that mediates the strong interaction.


Outline a laboratory procedure for producing and observing the atomic absorption spectrum of a gas.

shine white light through;
a tube of the gas;
then observe with spectroscope/grating/prism.

Describe the appearance of an atomic absorption spectrum. Explain why the spectrum provides evidence for quantization of energy in atoms.

continuous spectrum crossed by dark lines;
dark lines formed by the absorption of photons;
the absorbed photons have specific/discreet wavelengths;
indicating discreet differences in energy;
which can only be explained by existence of energy levels.


(a species of atom that is characterized by) the constitution of its nucleus / the number of protons and neutrons in the nucleus.

Evidence for the existence of isotopes can come from analysis of

. the paths taken by ions in a (Bainbridge) mass spectrometer.

Describe the phenomenon of natural radioactive decay.

emission of (alpha/beta/gamma) particles/photons/electromagnetic radiation;
nucleus becomes more (energetically) stable;
constant probability of decay (per unit time);
is random process;
activity/number of unstable nuclei in sample reduces by half over constant time intervals/exponentially;
not affected by temperature/environment / is spontaneous process.

State what is meant by an antiparticle.

a particle with the same mass but opposite quantum numbers/charge.

Outline how interactions in particle physics are understood in terms of exchange particles.

exchange particles are virtual particles/bosons;
that mediate/carry/transmit the weak/strong/em force between interacting particles.

State the name of a particle that is its own antiparticle.

photon / graviton / Z / Higgs.

Define binding energy of a nucleus.

the (minimum) energy required to completely separate the nucleons of a nucleus / the energy released when a nucleus is assembled.

A pure sample of a known element has a very long half-life. What measurement(s), together with the initial activity of the sample, must be made in order to measure the half-life of the element?

The mass of the sample.

Topic 8


State the Stefan-Boltzmann law for a black body.

power/energy per second emitted proportional to surface area;
and proportional to fourth power of absolute temperature / temperature in K.
(Equation with symbols defined is acceptable.)

Describe what is meant by the greenhouse effect in the Earth’s atmosphere.

effect caused by gas such as H O/NH /CH /CO /greenhouse gas in the atmosphere;
gas absorbs outgoing (long wave) radiation from Earth; (note: due to process called resonance)
gas re-radiates in all directions including some of the energy back to Earth.

Outline how an increase in the concentration of greenhouse gases in the atmosphere may lead to global warming

more greenhouse gases means that there is more absorption of outgoing radiation;
therefore more energy returns to Earth;
leading to a further/greater increase in the temperature of the surface (of Earth).

Distinguish between photovoltaic cells and solar heating panels.

photovoltaic cells generate emf/electricity;
solar panels generate thermal energy/heat.

Which of the following is the primary function of the moderator in a nuclear power station?

. To slow down neutrons.

In a nuclear power station, in order to increase the chances of a chain reaction

kinetic energy is removed from the neutrons.

The original source of the electrical power produced by a wind generator is

. the Sun’s radiated energy.

Outline in terms of energy changes how electrical energy is obtained from the energy of wind.

kinetic energy of wind transferred to (rotational) kinetic energy of turbine/blades;
kinetic energy changed to electrical energy in generator/dynamo.
Generator/dynamo must be mentioned.

State two reasons why it is impossible to convert all the available energy of the wind to electrical energy.

the speed of the air/wind cannot drop to zero;
wind turbulence / frictional losses in turbine/any moving part / resistive heating in wires.

Outline one advantage and one disadvantage of using wind turbines to generate electrical energy, as compared to using fossil fuels.

wind is renewable so no resources used up / wind is free / no chemical pollution / no carbon dioxide emission / does
not contribute to greenhouse effect / is “scalable” i.e. many sizes of turbine possible;
expensive initial cost / large land area needed / wind not constant / effect on movement of birds / aesthetically
unpleasant / noise pollution / high maintenance costs / best locations far from population centres / low energy density.

Outline why uranium ore needs to be enriched before it can be used successfully in a nuclear reactor.

U-238 is much more common than U-235 in ore;
U-235 is more likely to undergo fission / critical amount of U-235 required to ensure fission / OWTTE;
U-238 absorbs neutrons;
U-238 reduces reaction rate in reactor.

What is the phenomenon that best explains why greenhouse gases absorb infrared radiation?


Which of the following best defines non-renewable fuels?

Their rate of consumption is much greater than the rate at which they are being produced.

State why the energy transferred to the surroundings is said to be degraded.

it can no longer be used to do work / not available in useful form.

State two advantages of power production using fossil fuels compared to using nuclear fuels.

no radioactive waste;
no radiation risks to users;
lower expense of decommissioning / easier to decommission / easier to install / lower set-up cost;
transportation and storage less hazardous/safer;
simpler technology;
cannot be used for military purposes;
fossil fuels can be extracted/found more easily;
no chance of catastrophic accident/meltdown/Chernobyl.

Topic 9


Describe what is meant by the Doppler effect.

a change in the observed frequency/wavelength of a wave;
when there is relative motion of observer and source.

A thin film of oil lies on a puddle of water. White light from above shines on the film at normal incidence. Outline the process by which coloured fringes are formed.

light reflects from the top surface of the oil and the top surface of the water;
mention of interference/superposition;
path difference exists between both reflected rays;
different wavelengths interfere constructively for different positions/angles (hence colours appear/shift).

State the wave phenomenon that limits the resolution of the eye. State the Rayleigh criterion for determining if the images of two objects are just resolved.

the first minimum of one diffraction pattern;
falls on central maximum of other diffraction pattern.

Topic 10

What is the definition of gravitational field strength at a point?

Force acting per unit mass on a small mass placed at the point.

Explain what is meant by escape speed.

(minimum) speed of object to escape gravitational field of a planet/travel to infinity;
at surface of planet;
without (further) energy input.

Define electric field strength at a point in an electric field.

electric force per unit charge;
acting on a small/point positive (test) charge.

Define gravitational potential at a point in a gravitational field.

work done per unit mass;
in bringing (test) mass from infinity to point;
reference to small/point (test) mass.

Define electric potential at a point in an electric field.

the work done per unit charge;
when a small/test/point positive charge; (charge sign is essential)
is moved from infinity to the point.

Topic 11

Define magnetic flux.

the product of (the magnitude of) the normal component of magnetic field strength;
and area through which it passes/with which it is associated;
φ = BA cos θ ;
all terms defined/shown on a diagram.

Faraday’s law of electromagnetic induction states that the electromotive force (emf) induced in a conductor is proportional to

. the rate of change of magnetic flux linkage.

The capacitance of a device is defined as the

charge stored by the device for a potential difference of 1V across the device.

Capacitance of a capacitor is defined as the

ratio of charge stored to potential difference.

State Lenz’s law.

induced emf/induced current acts so as to oppose the change causing it.

Topic 12

Which phenomenon provides evidence for the wave nature of an electron?

Scattering of electrons by a crystal.

State what is meant by the photoelectric effect.

ejection of electron from metal surface following absorption of em radiation/photon.

Define work function.

minimum energy/work required to remove an electron (from the surface of the substance).

State what is meant by the wavefunction of an electron.

measure of the probability of finding an electron (at a particular place and time).

With reference to Schrödinger’s model, state the meaning of the amplitude of the wavefunction for the electron.

the (square of the) amplitude gives the probability of finding the electron at a given point in space.

Describe the de Broglie hypothesis.

all particles have an associated wavelength/behave like waves;
lambda=h/p with and symbols defined/described using terms.

With reference to Schrödinger’s model, state the meaning of the amplitude of the wavefunction for the electron.

the (square of the) amplitude gives the probability of finding the electron at a given point in space.

Define decay constant.

the probability of decay of a nucleus per unit time.

Describe the concept of a photon.

light consists of discrete packets/quanta/bundles of energy/particle;
each photon has an energy of hf (where h is the Planck constant and f is the frequency of light);

In the photoelectric effect there exists a threshold frequency below which no emission of photoelectrons takes place. Outline how the wave theory of light is unable to account for this observation. Concepts of the photon and work function are able to account for this observation.

the energy of a (em) wave depends on amplitude (not frequency);
so increasing the intensity should have resulted in electrons being emitted (at any frequency);
the work function is the minimum energy required to eject an electron from a metal surface;
if the photon energy (hf) is less than the work function then no emission will take place;

Monochromatic light is incident on a metal surface and electrons are emitted instantaneously from the surface. Explain why the emission of the electrons is instantaneous; the energy of the emitted electrons does not depend on the intensity of the incident light.

mention of photons;
of quantized energy / energy is hf;
one to one correspondence with electrons and photons;
(so arrival of light causes emission straightaway);
intensity is a measure of the number of photons not the individual photon energy.

The spectrum of electron energies emitted in a typical β-decay is continuous. Describe how this observation led physicists to propose the existence of the particles now called neutrinos.

idea that total energy released in the decay is fixed;
beta particle energies are less than this value/continuous;
the neutrino is postulated to account for this “missing” energy.

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