IB Physics HL key terms

The straight-line distance and direction from an initial position to a final position; a vector quantity measured in metres, distinct from the scalar distance actually travelled along a path.

The rate of change of displacement with respect to time, including direction; the gradient of a displacement–time graph gives instantaneous velocity, measured in metres per second.

The rate at which velocity changes with time, including any change of direction; the gradient of a velocity–time graph gives acceleration, measured in metres per second squared.

The resultant force on a body equals the rate of change of its momentum; for constant mass this reduces to force equals mass times acceleration, with force and acceleration in the same direction.

A diagram showing a single body isolated with all external forces acting on it drawn as arrows from a point, used to find the resultant force before applying Newton's laws.

The product of a body's mass and velocity, a vector measured in kilogram metres per second; the total momentum of an isolated system is conserved in all collisions and explosions.

The product of a force and the time for which it acts; it equals the change in momentum, so a longer contact time reduces the peak force for the same momentum change.

A collision in which both total momentum and total kinetic energy are conserved; no energy is transferred to internal energy, deformation, or sound during the interaction.

A collision in which total momentum is conserved but total kinetic energy decreases, the lost energy appearing as internal energy, deformation, or sound.

The net force directed towards the centre of a circular path that keeps a body moving in that circle; its magnitude is mass times speed squared divided by radius.

The rate of change of angular displacement, equal to the angle swept per unit time, measured in radians per second and related to linear speed by v equals omega times radius.

The gravitational force per unit mass experienced by a small test mass at a point, a vector measured in newtons per kilogram that equals the local free-fall acceleration.

The work done per unit mass in bringing a small test mass from infinity to a point in a gravitational field; it is always negative and zero at infinity.

A surface on which every point has the same potential; no work is done moving a charge or mass along it, and field lines cross it at right angles.

The electric force per unit positive charge at a point, a vector measured in newtons per coulomb or volts per metre; field lines point away from positive charge.

The work done per unit positive charge in bringing a small test charge from infinity to a point in an electric field; it is a scalar measured in volts.

The magnetic force per unit current per unit length on a conductor placed at right angles to the field, a vector measured in tesla that describes field strength.

The product of magnetic flux density and the area perpendicular to the field, measured in webers; it quantifies the field passing through a surface.

The generation of an electromotive force in a conductor whenever the magnetic flux through it changes, whether by motion, changing field strength, or changing area.

The induced electromotive force equals the negative rate of change of magnetic flux linkage; faster flux change or more turns gives a larger induced emf.

The induced current flows in a direction that opposes the change producing it; this explains the negative sign in Faraday's law and follows from conservation of energy.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed towards it; the defining condition for a body to move with a sinusoidal displacement–time graph.

The time taken for one complete oscillation or cycle of a wave or oscillation, measured in seconds and equal to the reciprocal of frequency.

The number of complete oscillations or wave cycles passing a point each second, measured in hertz; it equals the reciprocal of the period.

The distance between two consecutive points in phase on a wave, such as adjacent crests, measured in metres; it equals wave speed divided by frequency.

The distance travelled by a wave crest per unit time; it equals frequency multiplied by wavelength and is set by the medium, not by the source alone.

The ratio of the speed of light in a vacuum to its speed in a medium; it determines how much light bends at a boundary by Snell's law.

The angle of incidence in the denser medium for which the refracted ray travels along the boundary; beyond it, total internal reflection occurs.

When two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements; this produces interference patterns of reinforcement and cancellation.

The difference in distance travelled by two waves from their sources to a point; a whole-number multiple of wavelength gives constructive interference, a half-number multiple gives destructive interference.

Two wave sources with a constant phase difference and the same frequency, required to produce a stable, observable interference pattern rather than shifting fringes.

A pattern formed by two identical waves travelling in opposite directions, with fixed nodes of zero displacement and antinodes of maximum displacement; energy is not transported along the wave.

A point in a standing wave where the displacement remains permanently zero because the two travelling waves always cancel there; adjacent nodes are half a wavelength apart.

A large-amplitude response occurring when a system is driven at a frequency matching its natural frequency; damping reduces the peak amplitude and widens the response curve.

The progressive loss of energy from an oscillating system to its surroundings, reducing amplitude; critical damping returns a system to equilibrium fastest without overshooting.

The sum of the randomly distributed kinetic and potential energies of the particles in a system; for an ideal gas it depends only on temperature.

A measure of the average random kinetic energy of particles in a substance; it determines the direction of net thermal energy transfer between bodies in contact.

The energy required to raise the temperature of one kilogram of a substance by one kelvin, without a change of state; measured in joules per kilogram per kelvin.

The energy required to change the state of one kilogram of a substance at constant temperature; it measures the energy needed to break or form intermolecular bonds.

A model gas whose particles have negligible volume and no intermolecular forces except during elastic collisions; it obeys the equation of state pressure times volume equals number of moles times gas constant times temperature.

The state in which two systems in thermal contact show no net energy transfer between them, so they share the same temperature and their macroscopic properties remain constant.

The time taken for half the nuclei in a sample of a particular isotope to decay, or for its activity to halve; it is constant and independent of sample size.

The probability that a single nucleus decays per unit time; it relates to half-life by the natural logarithm of two divided by half-life, and sets the exponential decay curve.

The number of nuclear decays occurring per unit time in a sample, measured in becquerels; it equals the decay constant multiplied by the number of undecayed nuclei.

The energy required to separate all nucleons in a nucleus completely; it equals the mass defect multiplied by the square of the speed of light.

The difference between the total mass of the separate nucleons and the mass of the assembled nucleus; it corresponds to the binding energy released when the nucleus forms.

The emission of electrons from a metal surface when illuminated by light above a threshold frequency; it shows light delivers energy in discrete quanta called photons.

The minimum energy required to remove an electron from the surface of a particular metal; it equals the threshold frequency multiplied by Planck's constant.

The lowest light frequency that can cause photoelectric emission from a given metal; below it no electrons are emitted regardless of intensity.

A quantum of electromagnetic radiation carrying energy equal to Planck's constant times frequency; its energy is proportional to frequency and inversely proportional to wavelength.

A discrete allowed energy that an electron can occupy within an atom; transitions between levels emit or absorb photons of energy equal to the level difference.

The passage of a particle through a potential barrier it classically lacks the energy to cross, arising because its wavefunction is non-zero beyond the barrier; probability falls rapidly with barrier width and height.

The quantity deliberately changed by the experimenter across several values in an investigation; it is plotted on the horizontal axis to test its effect on the dependent variable.

The quantity measured in response to changes in the independent variable; it is plotted on the vertical axis and should be recorded with its associated uncertainty.

A quantity held constant throughout an investigation so it cannot affect the dependent variable, ensuring any observed change is attributable to the independent variable.

The interval within which the true value of a measurement is expected to lie, arising from instrument resolution and random or systematic effects; it is often quoted as an absolute or percentage value.

Unpredictable variation in repeated measurements caused by fluctuating conditions or reading judgement; it scatters results about the true value and is reduced by averaging repeats.

A consistent shift of all measurements in one direction, caused by a faulty instrument or flawed method; it affects accuracy but not the spread of repeated readings.

How close repeated measurements are to each other, reflecting the spread of results and the resolution of the instrument; precise results may still be inaccurate.

How close a measured value is to the true value; it is reduced by systematic error and is not improved simply by repeating measurements or using a finer scale.