absolute zero — lowest possible temperature matter can have, corresponding to the minimum energy that its atoms or molecules can have; the zero point of the Kelvin temperature scale.

absorption — (1) process by which a wave loses energy to a material as it passes through it;
(2) process by which the electron of an atom takes in the energy of a photon of light, thereby increasing the electron’s energy and moving it to a higher energy level.

acceleration — rate of change in velocity or the change in velocity divided by the change in time. Units of acceleration are units of speed divided by units of time, or m/s2 (sometimes written as m/s/s).

accuracy — how well a measurement’s value agrees with a commonly accepted value.

additive primary colors (RGB) — three colors (red, green, and blue) that, when added together in various combinations, can create any other color. Also called the additive color process.

adiabatic process — thermodynamic process in which no heat is added or removed from a system. Because no heat is exchanged, an adiabatic process is reversible.

alpha decay — radioactive decay in which a nucleus emits an alpha particle (consisting of two protons and two neutrons), thereby reducing the original atom’s atomic number by two.

ampere (A) — unit of electric current, equivalent to one coulomb of charge per second.

amplitude — maximum displacement of an oscillation from its equilibrium, or average, value.

angle of incidence — angle between a light ray that strikes a surface and the normal to the surface at the point of contact.

angle of reflection — angle between a light ray that reflects off a surface and the normal to the surface at the point of reflection.

angular momentum — momentum of an object due to its rotation or spin. Angular momentum is the product of the radius of rotation and the momentum, or L = r × mv. Angular momentum has units of kg m2/s.

angular velocity — rate at which a body rotates about an axis or center. Units are radians per second (rad/s).

antinode — point of maximum amplitude on a standing wave.

armature — rotating part of an electric motor containing electromagnets.

atom — smallest particle of an element that can exist in isolation or in a combination with other atoms. Consists of a heavy nucleus surrounded by one or more electrons.

atomic mass unit (amu) — standard unit of measure for expressing the mass of an atom. One amu is defined as one-twelfth of the mass of a carbon-12 atom or 1.661×10−27 kg.

atomic number — number associated with an atom that is equal to the number of protons in its nucleus.

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