Cambridge IAL Physics key terms

A physical quantity fully described by magnitude alone, with no direction. Examples include mass, temperature, speed, energy and time. Scalars are added using ordinary arithmetic.

A physical quantity needing both magnitude and direction to be fully specified, such as displacement, velocity, acceleration and force. Vectors combine by geometric rules rather than simple addition.

The single vector that has the same effect as two or more vectors acting together. Found by tip-to-tail drawing or by resolving components.

Splitting a single force into two perpendicular components, usually horizontal and vertical, so each can be treated independently. Uses trigonometry: adjacent component equals F cos θ.

The turning effect of a force about a pivot, calculated as force multiplied by the perpendicular distance from the pivot to the line of action of the force. Unit is the newton metre.

A pair of equal, opposite and parallel forces whose lines of action do not coincide. It produces rotation only, with torque equal to one force times the perpendicular separation.

The state of a body when the net force and net moment on it are both zero, so it stays at rest or moves with constant velocity.

The single point through which the entire weight of a body may be considered to act, whatever the body's orientation. For a uniform object it coincides with the geometric centre.

The upward buoyant force a fluid exerts on an object immersed in it, equal to the weight of fluid displaced. Arises from the pressure difference between the bottom and top of the object.

Force acting normally per unit area on a surface. Measured in pascals, where one pascal equals one newton per square metre. In fluids it increases with depth.

Mass per unit volume of a material, symbol ρ, measured in kilograms per cubic metre. Used to predict whether an object floats or sinks in a given fluid.

The constant maximum speed reached by a falling object when the upward drag force equals its weight, giving zero acceleration. Depends on shape, mass and the fluid's properties.

The resistive force a fluid exerts on a body moving through it, acting opposite to the motion. It increases with speed and with the cross-sectional area of the body.

A measure of a fluid's resistance to flow, arising from internal friction between its layers. Higher viscosity means slower flow and greater drag on objects moving through it.

The net 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. Area under a force–time graph gives impulse.

In a closed system with no external resultant force, total momentum before an interaction equals total momentum after it. Applies to collisions and explosions in any direction.

A collision in which total kinetic energy is conserved as well as momentum. No energy is transferred to internal energy, deformation or sound.

A collision in which momentum is conserved but total kinetic energy is not, some being converted to internal energy, sound or permanent deformation.

The minimum energy needed to remove an electron from the surface of a metal. Depends on the metal only, not on the frequency of incident light.

The emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency strikes it. Provides evidence that light arrives as discrete photons.

The minimum frequency of incident radiation that can cause photoelectric emission from a given metal. Below it, no electrons are released however intense the beam.

A quantum of electromagnetic radiation, carrying energy equal to Planck's constant times frequency. Photons have zero rest mass and travel at the speed of light.

The energy required to remove an electron completely from an isolated gaseous atom or ion. Measured in electronvolts or joules, it reveals electron energy levels.

A discrete allowed energy value that an electron can occupy within an atom. Transitions between levels emit or absorb photons of specific frequencies.

The charge stored per unit potential difference across a capacitor, measured in farads. Depends on plate area, separation and the dielectric between the plates.

For a capacitor discharging through a resistor, the time taken for the charge or potential difference to fall to about 37 percent of its initial value. Equals the product RC.

An insulating material placed between capacitor plates. Its polarisation reduces the field strength for a given charge, increasing capacitance by a factor equal to its relative permittivity.

The force per unit current per unit length on a conductor placed at right angles to a magnetic field. Measured in teslas; it describes the strength of the field.

The product of magnetic flux density, cross-sectional area and number of turns for a coil in a field. Changes in it induce an electromotive force.

The induced electromotive force in a circuit equals the rate of change of magnetic flux linkage through it. Faster changes or more turns give larger induced emf.

The direction of an induced current always opposes the change producing it. A consequence of energy conservation, it explains the opposing force felt when a magnet approaches a coil.

The large-amplitude oscillation that occurs when a system is driven at its natural frequency. At resonance the driving frequency matches the natural frequency and amplitude peaks.

Oscillation in which acceleration is directly proportional to displacement from equilibrium and directed towards it. The defining relation is a equals minus omega squared x.

The progressive loss of energy from an oscillating system, usually through resistive forces, reducing amplitude over time. Described as light, critical or heavy.

A circuit arrangement in which two or more components share a supply voltage in proportion to their resistance or reactance. Used to provide a chosen output voltage.

The resistance within a source of electromotive force that causes its terminal potential difference to fall as current increases. Modelled as a resistor in series with an ideal source.

The energy transferred from chemical, mechanical or other form into electrical energy per unit charge passing through a source. Measured in volts, it is not a force despite the name.

Force applied per unit cross-sectional area of a material, measured in pascals. It describes the internal load carried by the material, not the resulting deformation.

The fractional change in length of a material under stress, calculated as extension divided by original length. It has no units.

The ratio of tensile stress to tensile strain for a material in its linear elastic region. A property of the material itself, measured in pascals.

The greatest stress a material can bear and still return to its original shape when the load is removed. Beyond it, permanent plastic deformation occurs.

The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in joules per kilogram per kelvin. Determined experimentally by heating and measuring temperature rise.

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

A model gas whose molecules have negligible volume and exert no forces on each other except during elastic collisions. It obeys the equation of state relating pressure, volume and temperature.

The lowest possible thermodynamic temperature, zero kelvin, at which particles have minimum kinetic energy. Equivalent to minus 273 degrees Celsius.

An estimate of the range within which the true value of a measurement lies. May arise from the instrument's resolution, random variation or systematic error.

How closely repeated measurements of the same quantity agree with each other, reflecting the spread of readings. High precision does not guarantee accuracy.

How close a measured value is to the true value of the quantity. Systematic errors reduce accuracy, while random errors mainly reduce precision.

The absolute uncertainty of a measurement expressed as a fraction of the measured value, multiplied by 100. Used to judge which measurement most limits an experiment's reliability.

A consistent bias in measurements, always in the same direction, caused by faulty instruments, zero errors or poor technique. It shifts all readings by a similar amount.

Unpredictable variation in measurements caused by fluctuating conditions or reading difficulties. Reduced by taking repeated readings and averaging.

A straight line drawn through a set of plotted data points so that the points are distributed evenly about it, minimising the sum of squared deviations. Used to extract a gradient and intercept.

The slope of a graph, found from the change in the vertical coordinate divided by the change in the horizontal coordinate between two points far apart on the line of best fit.

A measurement in which the instrument is adjusted until it reads zero, so the unknown quantity is balanced against a known one. Examples include the metre bridge and balanced potentiometer.

The digits in a measured value that carry meaning about its precision. Answers should be quoted to the same or one more significant figure than the least precise input.