New Zealand Scholarship Physics key terms

A rotating vector whose projection onto a fixed axis gives a sinusoidally varying quantity, so alternating currents and voltages can be added and analysed using vector geometry rather than trigonometry.

The total opposition a circuit presents to alternating current, combining resistance with reactance as a complex quantity; its magnitude equals peak voltage divided by peak current.

The frequency-dependent opposition to alternating current from a capacitor or inductor, equal to 1/(ωC) for a capacitor and ωL for an inductor, measured in ohms.

The condition in a driven oscillating system where the driving frequency matches a natural frequency, producing maximum amplitude response; in an LCR circuit it occurs when inductive and capacitive reactances cancel.

A dimensionless measure of how sharply a resonant system peaks, equal to resonant frequency divided by the width of the response curve at half maximum power; high values mean low damping.

The progressive loss of oscillation amplitude in a system due to dissipative forces such as friction or electrical resistance; classified as underdamped, critically damped, or overdamped by behaviour.

Oscillation in which the restoring force is directly proportional to displacement and directed toward equilibrium, giving a sinusoidal displacement-time graph with constant period independent of amplitude.

The rate of change of phase in a sinusoid, equal to 2π times the ordinary frequency and measured in radians per second; symbol ω.

Power transmitted per unit area perpendicular to the direction of propagation, measured in watts per square metre; for a point source in three dimensions it falls off as the inverse square of distance.

The principle that when two or more waves overlap, the resultant displacement at any point equals the algebraic sum of the individual displacements, producing interference.

The reinforcement or cancellation that occurs when coherent waves meet; constructive interference gives larger amplitude, destructive interference gives smaller amplitude, depending on path difference.

A fixed, constant phase relationship between two waves, necessary for a stable observable interference pattern; laser light is highly coherent, ordinary lamps are not.

The difference in distance travelled by two waves from their sources to a common point; it determines whether they arrive in phase or out of phase.

A wave pattern formed by two identical waves travelling in opposite directions, with fixed nodes of zero displacement and antinodes of maximum displacement, and no net energy transport.

The spreading of a wave as it passes through an aperture or around an obstacle, becoming more pronounced when the wavelength is comparable to the aperture or obstacle size.

The ability of an optical instrument to distinguish two closely spaced objects; limited by diffraction, with the minimum resolvable angle roughly proportional to wavelength divided by aperture diameter.

The change in observed frequency of a wave when the source and observer move relative to each other; approaching motion raises frequency, receding motion lowers it.

The shift of spectral lines toward longer wavelengths from a receding source, used in astronomy to infer recession velocity and cosmological distance.

The emission of electrons from a metal surface when light above a threshold frequency strikes it; the maximum electron energy depends on frequency, not intensity, supporting the photon model.

The minimum energy needed to remove an electron from the surface of a particular metal, equal to Planck's constant times the threshold frequency.

A quantum of electromagnetic radiation carrying energy equal to Planck's constant times frequency, with zero rest mass and momentum equal to energy divided by the speed of light.

The wavelength associated with a moving particle, equal to Planck's constant divided by its momentum; observable for electrons and other low-mass particles.

A mathematical function describing a quantum system, whose squared magnitude gives the probability density of finding the particle at a location.

The quantum restriction that certain paired quantities, such as position and momentum, cannot both be known precisely; the product of their uncertainties is at least the reduced Planck constant divided by two.

A discrete allowed energy value that a bound quantum system such as an atom or nucleus can occupy, with transitions between levels emitting or absorbing photons of specific energy.

The time required for half the nuclei in a radioactive sample to decay; it is constant for a given isotope and independent of sample size or chemical state.

The probability per unit time that a given unstable nucleus will decay, related to half-life by the natural logarithm of two divided by the half-life.

The energy required to separate a nucleus into its constituent protons and neutrons, equal to the mass defect multiplied by the square of the speed of light.

The difference between the total mass of a nucleus's separate nucleons and the actual nuclear mass, appearing as binding energy through the mass-energy equivalence relation.

The principle that total momentum of an isolated system remains constant in the absence of external forces, applied to collisions, explosions, and particle decays.

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

The point at which a body's mass can be treated as concentrated for analysing translational motion under external forces; it moves as if all external forces acted there.

The rotational analogue of mass, measuring resistance to angular acceleration; it depends on mass distribution relative to the axis of rotation.

The rotational analogue of linear momentum, equal to moment of inertia times angular velocity for a rigid body; conserved when no external torque acts.

The turning effect of a force, equal to the product of the force and its perpendicular distance from the pivot, causing angular acceleration.

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

The minimum speed needed at a body's surface for an object to escape its gravitational field without further propulsion, equal to the square root of twice gravitational parameter over radius.

Three empirical rules for orbital motion: orbits are ellipses with the central body at one focus; equal areas are swept in equal times; and the square of the period is proportional to the cube of the semi-major axis.

A region in which a charged object experiences a force; the field strength at a point is the force per unit positive charge placed there, measured in newtons per coulomb.

The electrical potential energy per unit positive charge at a point in an electric field, measured in volts; it is a scalar quantity.

The principle that the total electric flux through a closed surface is proportional to the enclosed charge, useful for finding fields of symmetric charge distributions.

The ability of a system to store charge per unit potential difference, equal to charge divided by voltage and measured in farads.

The measure of magnetic field passing through a given area, equal to the field component perpendicular to the surface multiplied by the area; measured in webers.

The principle that a changing magnetic flux through a circuit induces an electromotive force proportional to the rate of change of that flux.

The rule that an induced current flows in a direction whose magnetic effect opposes the change that produced it, a consequence of energy conservation.

A measure of the number of microscopic arrangements consistent with a system's macroscopic state, and of energy unavailable for useful work; it never decreases in an isolated system.

The principle that heat flows spontaneously from hotter to colder bodies and that no cyclic engine can convert heat entirely into work, limiting efficiency of heat engines.

The fraction of heat input converted to useful work by a heat engine; the Carnot limit depends only on the absolute temperatures of the hot and cold reservoirs.

The equation of state relating pressure, volume, amount, and absolute temperature of a gas through the universal gas constant, valid for dilute gases.

The model explaining gas properties from the random motion of molecules, deriving pressure and temperature from particle collisions with container walls.

The relation expressing total energy of a particle as the square root of the sum of the squares of its momentum times the speed of light and its rest mass times the speed of light squared.

The slowing of a clock's measured rate as observed from a frame in relative motion, given by the Lorentz factor; confirmed by muon lifetimes and atomic clock experiments.

The shortening of an object's measured length along its direction of motion as observed from a frame moving relative to it, by the Lorentz factor.