Key Terms: Letter A

Absorption

The process by which a material takes in energy from a wave or particle and converts it into another form, such as thermal energy.

Absorption Spectra

The range of wavelengths of light that are absorbed by a substance, appearing as dark lines in a continuous spectrum due to the absorption of specific energies by atoms or molecules.

Acceleration

The rate of change of velocity of an object with respect to time, expressed as a vector quantity.

Acceleration due to Gravity

The constant acceleration experienced by an object in free fall near surface of the Earth, with a standard value of 9.8 m/s² downward.

Action-Reaction Pair

Consists of two equal and opposite forces exerted by two objects on each other, as described by Newton’s Third Law of Motion.

Adiabatic Process

A thermodynamic process in which no heat is exchanged with the surroundings, meaning all energy transfer occurs as work.

Air Resistance

A type of frictional force that opposes the motion of an object moving through air, increasing with speed and surface area.

Alpha Decay

A type of radioactive decay in which an unstable nucleus emits an alpha particle (two protons and two neutrons) to become a more stable nucleus.

Alpha Particle

A positively charged particle consisting of two protons and two neutrons, identical to a helium-4 nucleus, and emitted during alpha decay.

Alternating Current

An electric current that periodically reverses direction, typically following a sinusoidal waveform.

Ammeter

A device used to measure the electric current in a circuit, typically connected in series with the circuit element.

Ampere

The SI unit of electric current, defined as one coulomb of charge passing through a conductor per second.

Amplitude

The maximum displacement of a wave from its equilibrium position, representing the wave’s energy or intensity.

Angle of Incidence

The angle between an incoming wave or ray and the normal to the surface it strikes.

Angle of Reflection

The angle between the reflected ray and the normal to a surface, equal to the angle of incidence according to the law of reflection.

Angle of Refraction

The angle between the refracted ray and the normal to the surface at the point of refraction, determined by Snell’s Law.

Area

The measure of the extent of a two-dimensional surface or shape, typically expressed in square units such as square meters (m²).

Area Charge Density

The amount of electric charge per unit area on a surface, typically measured in coulombs per square meter (C/m²).

Atomic Mass Unit

A unit of mass equal to 1/12 the mass of a carbon-12 atom, approximately 1.66 × 10⁻²⁷ kg.

Atomic Number

The number of protons in the nucleus of an atom, which determines the element’s identity and its position in the periodic table.

Atomic Structure

Refers to the arrangement of subatomic particles—protons, neutrons, and electrons—within an atom, determining its properties and behavior.

Atomic Theory

The scientific concept that matter is composed of indivisible particles called atoms, which combine in specific ways to form substances and undergo various physical and chemical processes.

Atoms

The fundamental building blocks of matter, consisting of a nucleus of protons and neutrons surrounded by a cloud of electrons.

Aurora

A natural light display in the Earth’s sky, caused by the interaction of charged particles from the solar wind with the Earth’s magnetic field and atmosphere.