UEE Physics key terms

A measure of how strongly a body resists angular acceleration, equal to the sum of each mass element multiplied by the square of its distance from the rotation axis. It depends on both mass distribution and axis choice.

The turning effect of a force about a chosen point, equal to the cross product of the position vector and the force. Its magnitude is force times perpendicular distance from the pivot.

The rotational analogue of linear momentum, equal to the cross product of position and linear momentum, or moment of inertia times angular velocity for a rigid body about a fixed axis. It is conserved when net external torque is zero.

A pair of equal, opposite and parallel forces whose lines of action are separated, producing a pure turning moment with zero net resultant force. The moment equals one force times the perpendicular separation.

A relation stating that the moment of inertia about any axis equals the moment of inertia about a parallel axis through the centre of mass plus mass times the squared separation of the axes.

The point where the mass-weighted average position of a body lies, and about which external forces act as if the whole mass were concentrated there for translational motion.

A state in which the net force and net torque on a body are both zero, so its translational and rotational acceleration vanish. Stability depends on how restoring forces behave after a small displacement.

Principles stating that certain total quantities of an isolated system remain constant over time, notably energy, linear momentum, angular momentum and electric charge. They allow problems to be solved without tracking every force.

A collision in which total kinetic energy is conserved as well as total momentum. In one dimension, relative speed of approach equals relative speed of separation.

The ratio of relative speed of separation to relative speed of approach for two colliding bodies along the line of impact. A value of one means perfectly elastic, zero means perfectly inelastic.

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

The progressive loss of oscillation energy, usually to heat, causing amplitude to decay over time. Light damping gives gradual decay, critical damping gives fastest return without oscillation, heavy damping returns slowly.

A large amplitude response occurring when a driven oscillator is excited near its natural frequency. At resonance the driving frequency matches the natural frequency and energy transfer is maximal.

The fraction of a cycle, usually expressed in radians or degrees, by which one oscillation leads or lags another. Two points a wavelength apart have a phase difference of 2π radians.

The principle that when two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements. It underlies interference and standing waves.

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 waves when they pass an obstacle or aperture, becoming pronounced when the wavelength is comparable to the aperture size. It limits the resolution of optical instruments.

The superposition of coherent waves producing regions of reinforcement and cancellation. Constructive interference occurs when path difference is a whole number of wavelengths, destructive when it is an odd half-integer.

A property of two waves having a constant phase relationship, so their interference pattern is stable over time. Lasers provide highly coherent light, unlike ordinary thermal sources.

Colour effects from light reflected at the two surfaces of a thin layer, where the reflected waves interfere. A phase reversal of half a wavelength commonly occurs at one boundary, altering the condition for reinforcement.

A resolution limit stating that two images are just resolvable when the central maximum of one diffraction pattern falls on the first minimum of the other. It gives the smallest resolvable angular separation as about 1.22 λ divided by aperture diameter.

The emission of electrons from a metal surface when light of sufficiently high frequency strikes it. It demonstrates that light delivers energy in discrete quanta, with maximum electron energy depending on frequency, not intensity.

The minimum energy needed to remove an electron from the surface of a particular metal. In the photoelectric effect it equals the photon energy minus the maximum kinetic energy of emitted electrons.

A quantum of electromagnetic radiation carrying energy proportional to frequency and momentum equal to energy divided by the speed of light. Photons have zero rest mass and travel at light speed in vacuum.

The wavelength associated with a particle, equal to Planck's constant divided by its momentum. It explains electron diffraction and why wave behaviour is negligible for everyday objects.

A quantum restriction stating that certain pairs of quantities, such as position and momentum, cannot both be known with arbitrarily small uncertainty; the product of their uncertainties is bounded below by a constant of order Planck's constant.

A mathematical description of a quantum system whose squared magnitude at a point gives the probability density of finding the particle there. It evolves deterministically but yields probabilistic measurement outcomes.

The spontaneous transformation of unstable nuclei into more stable ones, emitting alpha, beta or gamma radiation. The number of undecayed nuclei falls exponentially with a characteristic half-life.

The time taken for half the nuclei in a radioactive sample to decay, or for activity to fall to half its initial value. It is constant for a given isotope and independent of sample size.

The energy required to separate all nucleons in a nucleus completely, equal to the mass defect multiplied by the square of the speed of light. Binding energy per nucleon peaks near iron, indicating maximum nuclear stability.

The difference between the total mass of separate nucleons and the mass of the assembled nucleus. This missing mass appears as the nucleus's binding energy according to mass-energy equivalence.

A statement of energy conservation for thermal systems: the heat added to a system equals the increase in internal energy plus the work done by the system. Sign conventions must be stated explicitly.

The total kinetic and potential energy of the particles making up a system, excluding bulk motion and external potential energy. For an ideal gas it depends only on temperature.

A change in which no heat is exchanged with the surroundings, so work done comes entirely from internal energy. Rapid compression approximates this because heat transfer is too slow to matter.

A change occurring at constant temperature, so internal energy of an ideal gas stays constant and heat absorbed equals work done by the gas. It requires slow heat exchange with a reservoir.

A model explaining gas properties from the random motion of many molecules, deriving pressure and temperature from molecular collisions and average kinetic energy. It links macroscopic quantities to microscopic averages.

The square root of the mean of squared molecular speeds, a measure of typical molecular speed in a gas. It rises with the square root of absolute temperature and falls with molar mass.

The ratio of stored charge to potential difference across a conductor arrangement, measured in farads. For parallel plates it increases with plate area and permittivity and decreases with separation.

The force per unit positive charge at a point, a vector quantity pointing away from positive charge and towards negative charge. Field lines indicate direction and their density indicates strength.

The magnetic force per unit current per unit length on a conductor placed perpendicular to the field, measured in teslas. It describes the strength and direction of a magnetic field.

A law stating that the induced electromotive force in a circuit equals the negative rate of change of magnetic flux linkage through it. The minus sign expresses Lenz's law opposition to the change.