NEB Class 12 Physics key terms

The straight-line distance and direction from an object's starting point to its current position. Unlike distance travelled, it is a vector, so returning to the start gives zero displacement.

The rate of change of displacement with time. It is a vector quantity, so its sign or direction matters; average velocity uses total displacement, while instantaneous velocity describes a single moment.

The rate at which velocity changes with time, including speeding up, slowing down, or changing direction. It is a vector measured in metres per second squared (m/s²).

Two-dimensional motion of an object launched into the air and then moving only under gravity. Horizontal and vertical motions are treated separately; the horizontal velocity stays constant while vertical velocity changes at g.

Motion under gravity alone, with air resistance ignored, so every object accelerates downward at g regardless of mass. Near Earth's surface g is about 9.8 m/s².

The turning effect of a force, equal to force multiplied by the perpendicular distance from the pivot to the line of action. It is measured in newton metres (N m).

A body's resistance to change in its rotational motion, depending on mass and how that mass is distributed about the axis. It plays the role in rotation that mass plays in straight-line motion.

The rotational analogue of linear momentum, equal to moment of inertia times angular velocity for a rigid body. In the absence of external torque it remains constant, which explains effects such as a spinning skater speeding up when arms are pulled in.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed towards it. Its displacement follows a sine or cosine curve with constant amplitude and period.

A large increase in oscillation amplitude when a system is driven at its natural frequency. Energy transfers most efficiently from the driver to the oscillator at this frequency.

The progressive loss of oscillation amplitude due to dissipative forces such as friction or air resistance. Overdamping prevents oscillation entirely, while critical damping gives the fastest return to equilibrium.

The property of a material that lets it return to its original shape after a deforming force is removed. Behaviour is described by stress, strain, and the elastic modulus.

The ratio of tensile stress to tensile strain for a material within its elastic limit. A larger value means the material is stiffer and stretches less under the same load.

The force per unit length acting along a liquid surface, arising because surface molecules have unbalanced attractive forces. It makes droplets spherical and supports light objects such as insects.

A fluid's internal resistance to flow, caused by friction between adjacent layers moving at different speeds. Thick liquids such as honey have high viscosity, which generally falls as temperature rises.

The rise or fall of a liquid inside a narrow tube due to surface tension and adhesion between liquid and tube. Water rises in glass because adhesion beats cohesion; mercury is depressed.

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

A change in a gas at constant temperature, so internal energy stays fixed for an ideal gas and all heat supplied appears as work done. It is slow enough for thermal equilibrium with the surroundings.

A change in a gas with no heat exchange with the surroundings, so work done comes from internal energy and temperature changes. Rapid compression or expansion approximates this.

A measure of the disorder or spread of energy in a system. In any spontaneous process the total entropy of an isolated system never decreases, which gives direction to natural change.

Heat cannot flow spontaneously from a colder body to a hotter body without external work. It limits the efficiency of engines and explains why entropy of an isolated system cannot decrease.

A device that converts thermal energy into mechanical work by taking heat from a hot reservoir, doing work, and rejecting remaining heat to a cold reservoir. Its efficiency is work output divided by heat input.

The maximum possible efficiency of an engine operating between two temperatures, equal to one minus the cold absolute temperature divided by the hot absolute temperature. No real engine between the same reservoirs can exceed it.

The bending of light as it passes from one medium into another because its speed changes. Light bends towards the normal when entering a denser medium and away when leaving it.

The ratio of light's speed in vacuum to its speed in a medium, or equivalently the sine of the angle of incidence divided by the sine of the angle of refraction. It measures how strongly a medium bends light.

Complete reflection of light at a boundary when it travels from a denser to a rarer medium at an angle greater than the critical angle. It carries light along optical fibres.

The superposition of two or more coherent waves, producing regions of reinforced and cancelled amplitude. Constructive interference needs a path difference of a whole number of wavelengths.

The spreading of waves around obstacles and through narrow gaps, most noticeable when the gap is comparable to the wavelength. It limits the resolving power of optical instruments.

Restriction of a transverse wave's oscillations to one plane, showing that light is transverse. It occurs by reflection, by scattering, or on passing through a polarising filter.

A fixed phase relationship between waves, giving a stable interference pattern. Laser light is highly coherent, whereas ordinary sources emit randomly and cannot sustain visible fringes.

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation. The force acts along the line joining them.

The force per unit positive charge at a point in space, a vector pointing away from positive charge and towards negative charge. Field lines map its direction and relative strength.

The work done per unit positive charge in bringing a charge from infinity to a point, a scalar measured in volts. Potential difference drives current through a conductor.

The ability of a system to store charge per unit potential difference, measured in farads. For parallel plates it increases with plate area and decreases with separation.

The production of an electromotive force in a conductor when the magnetic flux through it changes. The induced emf depends on the rate of change of flux and the number of turns.

The product of magnetic flux density and the area perpendicular to it, measured in webers. A change in flux, through motion or a changing field, induces an emf.

An induced current flows in a direction that opposes the change producing it. This follows from energy conservation and gives the negative sign in Faraday's law.

The emission of electrons from a metal surface when light of sufficiently high frequency falls on it. It demonstrates that light delivers energy in discrete quanta called photons.

The minimum energy needed to remove an electron from the surface of a particular metal. It sets the threshold frequency below which no photoelectrons are emitted, however intense the light.

The lowest light frequency that can eject photoelectrons from a given metal surface. It equals the work function divided by Planck's constant, and below it no emission occurs.

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

The time taken for half the nuclei in a radioactive sample to decay. It is constant for a given nuclide and is used to date materials and plan radiation safety.

The spontaneous emission of particles or radiation from unstable nuclei as they change to a more stable state. Alpha, beta, and gamma emissions differ in charge, mass, and penetrating power.

The number of nuclear decays occurring per unit time in a sample, measured in becquerels. It falls exponentially with time, halving at each half-life.

The difference between the total mass of a nucleus's separate nucleons and the actual mass of the nucleus. This missing mass appears as binding energy, released when the nucleus forms.

The energy required to split a nucleus completely into its separate protons and neutrons. It equals mass defect times the square of the speed of light and measures nuclear stability.

A p-n junction that allows current mainly in one direction, acting as a rectifier. Forward bias lowers the barrier; reverse bias widens it, permitting only a tiny leakage current.

Conversion of alternating current into direct current using diodes. A half-wave rectifier uses one diode; a full-wave circuit uses two or four to use both halves of the cycle.

A three-terminal semiconductor device in which a small base current controls a much larger collector current. This current gain enables amplification and switching in electronic circuits.