STPM Physics 960 key terms

The scatter in repeated measurements caused by unpredictable variations in the measuring process; reduced by averaging many readings and estimated from the spread of values about the mean.

A consistent shift in all readings caused by a fault in the instrument or method, such as a zero error; it does not average out on repetition and must be corrected or calibrated away.

The absolute uncertainty of a quantity expressed as a fraction of its measured value, multiplied by 100; added when quantities are multiplied or divided, and multiplied by the power when raised to a power.

The estimated range, in the same unit as the measurement, within which the true value is expected to lie; usually quoted to one significant figure alongside the mean reading.

A reading shown by an instrument when the true value is zero, arising from imperfect calibration or worn parts; it shifts every reading by the same amount and must be subtracted.

A scalar has magnitude only, such as mass or temperature; a vector has both magnitude and direction, such as displacement or force, and vectors combine by the parallelogram or triangle rule.

Splitting a vector into two perpendicular components, usually horizontal and vertical, using trigonometry so that each component's separate effect can be calculated and then recombined.

The single vector that produces the same effect as two or more vectors acting together, found by vector addition and having both a magnitude and a direction.

The requirement that every term in a valid physical equation has the same base units; used to check equations and to derive possible relationships between quantities.

The seven SI base units (kg, m, s, A, K, mol, cd) from which all other units are formed by multiplication and division; derived units such as the newton are combinations of these.

Using the base-unit dimensions of quantities to test whether an equation is consistent or to predict the form of a physical relationship, up to a dimensionless constant.

The rate of change of velocity with time; a vector quantity, so an object moving at constant speed round a curve is accelerating because its direction changes.

Two-dimensional motion under gravity alone, analysed by treating horizontal motion as uniform velocity and vertical motion as uniform acceleration, with the two components independent of each other.

If body A exerts a force on body B, then B exerts an equal and opposite force on A; the pair acts on different bodies, so the forces never cancel each other.

A sketch showing a single object isolated with all external forces acting on it drawn as arrows from the body, used to apply Newton's second law correctly.

The product of a body's mass and velocity, a vector measured in kg m s⁻¹; 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; equal to the change in momentum produced, so a longer contact time reduces the force for a given momentum change.

A collision in which both total momentum and total kinetic energy are conserved, so the bodies separate with no loss of kinetic energy to heat, sound or deformation.

The net force directed towards the centre of a circular path that keeps a body moving in that circle; it is not a new kind of force but is provided by tension, gravity, friction or another real force.

The rate of change of angular displacement, measured in rad s⁻¹; related to linear speed by v = ωr and to period by ω = 2π/T.

Oscillation in which acceleration is directly proportional to displacement from equilibrium and directed towards it; the motion is sinusoidal with a period independent of amplitude.

The progressive loss of energy from an oscillating system to its surroundings, causing the amplitude to decrease; light damping reduces amplitude slowly, while heavy damping prevents oscillation altogether.

The large-amplitude response that occurs when a system is driven at a frequency close to its natural frequency, where energy transfer from the driver is greatest.

A model gas whose molecules have negligible volume, exert no forces except during brief elastic collisions, and obey pV = nRT; real gases approach this behaviour at low pressure and high temperature.

The explanation of gas pressure and temperature in terms of molecular motion, giving pV = ⅓Nm⟨c²⟩; pressure arises from the rate of change of momentum of molecules striking the walls.

The sum of the random kinetic and potential energies of all the particles in a system; for an ideal gas it depends only on temperature, since there are no intermolecular forces.

The energy balance for a system: the heat supplied equals the increase in internal energy plus the work done by the system, expressed as Q = ΔU + W.

The energy required to raise the temperature of one kilogram of a substance by one kelvin, measured in J kg⁻¹ K⁻¹; for gases the value differs at constant pressure and constant volume.

When two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements; this explains interference, stationary waves and beats.

Two waves are coherent when they have a constant phase difference, which requires the same frequency; only coherent sources produce a stable, observable interference pattern.

The difference in the distances travelled by waves from two sources to a common point; constructive interference occurs at a whole number of wavelengths, destructive at half-odd multiples.

A wave pattern formed when two identical waves travelling in opposite directions superpose, giving nodes of zero amplitude and antinodes of maximum amplitude fixed in position.

A plate with many closely spaced parallel slits that produces sharp interference maxima at angles given by d sin θ = nλ, used to measure wavelength precisely.

The charge stored per unit potential difference across a capacitor, measured in farads; for parallel plates C = ε₀εᵣA/d, so it increases with plate area and decreases with separation.

The product RC for a capacitor discharging through a resistor; it is the time taken for the charge to fall to 1/e of its initial value, about 37%.

The production of an e.m.f. in a conductor when the magnetic flux linking it changes; the induced e.m.f. equals the rate of change of flux linkage (Faraday's law).

The force per unit current per unit length on a conductor placed at right angles to a magnetic field, measured in tesla; it indicates the strength of the field.

The force per unit positive charge experienced at a point in an electric field, a vector measured in N C⁻¹ or V m⁻¹; field lines show its direction.

A high-gain differential amplifier used with negative feedback to build circuits such as inverting and non-inverting amplifiers, whose gain depends only on external resistors.

Returning a fraction of an amplifier's output to its inverting input in antiphase, which reduces gain but improves stability, bandwidth and linearity.

The condition at the inverting input of an op-amp with negative feedback where the potential is held at almost the same value as the non-inverting input, but no current flows into the input.

A circuit element that produces an output determined by its digital inputs according to a truth table; AND, OR and NOT are the basic gates, from which NAND and NOR are built.

A table listing every possible combination of inputs to a logic circuit alongside the corresponding output, used to define and check the behaviour of gates.

The emission of electrons from a metal surface when light of frequency above a threshold falls on it; the maximum kinetic energy depends on frequency, not intensity.

The minimum energy needed to remove an electron from the surface of a metal, measured in electronvolts or joules; it sets the threshold frequency for photoemission.

The lowest frequency of incident light that can cause photoemission from a given metal surface, equal to the work function divided by the Planck constant.

The principle that light and matter show both wave and particle behaviour depending on the experiment; the de Broglie wavelength of a particle is h/p.

One of the discrete allowed energies of an electron in an isolated atom; a photon is emitted or absorbed when an electron moves between levels, with energy equal to the difference.

The time for half the nuclei in a radioactive sample to decay; it is constant for a given nuclide and is related to the decay constant by t½ = ln 2 / λ.

The spontaneous and random disintegration of unstable nuclei, emitting alpha, beta or gamma radiation; the number of nuclei remaining falls exponentially with time.

Checking or adjusting an instrument against a known standard so that its readings correspond closely to true values, reducing systematic error before measurements are taken.

Precision describes how closely repeated readings agree with each other, while accuracy describes how close a reading is to the true value; a precise set may still be inaccurate.

The difference between an experimental value and an accepted value, expressed as a percentage of the accepted value, used to judge how well an experiment has worked.

A straight line drawn through a set of plotted data points so that the points are balanced above and below it; its gradient and intercept are then used with the equation being tested.