IB Physics SL key terms

Variation in repeated measurements caused by unpredictable factors; reduced by taking more readings and averaging, and estimated from the spread of results about the mean.

A consistent shift in all readings in the same direction, caused by faulty instruments or flawed method; it affects accuracy and is not reduced by repeating measurements.

How close a measured value is to the true or accepted value; degraded mainly by systematic error rather than by scatter between repeat readings.

How closely repeated measurements agree with each other, reflecting the fineness of the scale and the scatter of results; improved by better instruments and more readings.

The stated plus-or-minus range of a single measurement or result, expressed in the same unit as the quantity itself, for example 2.50 plus or minus 0.05 metres.

Absolute uncertainty divided by the measured value, multiplied by 100; used to compare the reliability of quantities with different units or very different sizes.

A physical quantity fully described by magnitude alone, with no direction, such as mass, time, speed, energy or temperature.

A physical quantity needing both magnitude and direction, such as displacement, velocity, acceleration or force; added by resolving into perpendicular components.

The part of a vector along a chosen axis, found by multiplying the magnitude by the sine or cosine of the angle to that axis.

The single vector that has the same effect as two or more vectors combined, found by tip-to-tail drawing or by adding perpendicular components.

A sketch of a single object showing only the external forces acting on it as labelled arrows from a point, used to find the resultant force.

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.

The product of a force and the time for which it acts; equal to the change in momentum produced, and equal to the area under a force-time graph.

For a system with no net external force, the total momentum before an interaction equals the total momentum after it, whether the collision is elastic or inelastic.

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

The energy transferred by a force acting through a displacement; equal to the force times the displacement times the cosine of the angle between them.

Energy cannot be created or destroyed, only transferred between stores or transformed between forms; the total energy of a closed system stays constant.

The ratio of useful energy or power output to total energy or power input, usually expressed as a percentage; the remainder is dissipated, often as internal energy.

The constant speed reached by a falling object when the upward drag force equals its weight, so the resultant force and acceleration are zero.

The sum of the randomly distributed kinetic and potential energies of the particles in a substance; it depends on temperature, state and number of particles.

The energy required to raise the temperature of one kilogram of a substance by one kelvin, with no change of state.

The energy required to change the state of one kilogram of a substance at constant temperature, without any change in temperature of the substance.

A model gas whose particles occupy negligible volume and exert no forces on each other except during brief collisions; it obeys the equation of state pressure times volume equals number of moles times gas constant times temperature.

Temperature measured in kelvin from absolute zero, the point at which particles have minimum kinetic energy; obtained by adding 273 to the Celsius value.

A wave in which particle oscillations are perpendicular to the direction of energy travel, as in light and waves on a string; it can be polarised.

A wave in which particle oscillations are parallel to the direction of energy travel, producing compressions and rarefactions, as in sound.

The restriction of oscillations of a transverse wave to a single plane; evidence that light is transverse, and used in polarising filters to control intensity.

When two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements; the basis of interference and standing waves.

The difference in distance travelled by two waves from their sources to a common point; constructive interference occurs at whole-number multiples of the wavelength.

A pattern formed by two identical waves travelling in opposite directions, with fixed nodes of zero amplitude and antinodes of maximum amplitude.

The change in observed frequency when a source, observer or medium moves relative to the other; frequency rises on approach and falls on separation.

The force per unit positive charge experienced at a point in an electric field; a vector quantity measured in newtons per coulomb or volts per metre.

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

The energy transferred per unit charge by a source such as a battery when converting other forms of energy into electrical energy; measured in volts.

The resistance within a source of emf that causes the terminal voltage to fall below the emf as current increases, dissipating energy as heat inside the source.

The net force directed towards the centre of a circular path that keeps an object moving in that circle; it does no work because it is perpendicular to velocity.

The gravitational force per unit mass at a point, equal to the local acceleration of free fall; measured in newtons per kilogram.

The time taken for half the nuclei in a sample of a particular radionuclide to decay; a constant for that nuclide, unaffected by temperature or chemical state.

The spontaneous and random transformation of an unstable nucleus into a more stable one, emitting alpha, beta or gamma radiation; the rate depends only on the number of undecayed nuclei.

The energy required to separate a nucleus completely into its individual nucleons; related to the mass defect by the equation energy equals mass defect times the square of the speed of light.