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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Glossary of physics | 2/13 | https://en.wikipedia.org/wiki/Glossary_of_physics | reference | science, encyclopedia | 2026-05-05T07:51:11.278375+00:00 | kb-cron |
alternating current (AC) A form of electric current in which the movement of electric charge periodically reverses direction. Contrast direct current.
ammeter An instrument used to measure electric current.
amorphous solid A type of solid which does not have a definite geometric shape.
ampere (A) Often abbreviated as amp. The SI base unit of electric current, defined as one coulomb of electric charge per second.
amplifier Also electronic amplifier or (informally) amp. An electronic device that can increase the power of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. By definition, an amplifier is any circuit that has a power gain greater than one.
amplitude The height of a wave as measured from its center (normal) position.
angle of incidence In geometric optics, the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal. The ray can be formed by any wave: optical, acoustic, microwave, X-ray, etc.
angle of reflection The change in direction of a wavefront at an interface between two different media such that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound, and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.
ångström (Å) A unit of length primarily used to measure subatomic particles that is equal to 10−10 metres (one ten-billionth of a metre) or 0.1 nanometres.
angular acceleration The time rate of change of angular velocity. In three dimensions, it is a pseudovector. In SI units, it is measured in radians per second squared (rad/s2), and is usually denoted by the Greek letter alpha (α). Just like angular velocity, there are two types of angular acceleration: spin angular acceleration and orbital angular acceleration, representing the time rate of change of spin angular velocity and orbital angular velocity, respectively. Unlike linear acceleration, angular acceleration need not be caused by a net external torque. For example, a figure skater can speed up her rotation (thereby obtaining an angular acceleration) simply by contracting her arms inwards, which involves no external torque.
angular displacement The angle (in radians, degrees, or revolutions) through which a point revolving around a centre or line has been rotated in a specified sense about a specified axis.
angular frequency (ω) Also angular speed, radial frequency, circular frequency, orbital frequency, radian frequency, and pulsatance. A scalar measure of rotation rate. It refers to the angular displacement per unit time (e.g. in the rotation of an astronomical body) or the rate of change of the phase of a sinusoidal waveform (e.g. in oscillations and waves), or as the rate of change of the argument of the sine function. Angular frequency (or angular speed) is the magnitude of the vector quantity that is angular velocity. The term angular frequency vector
ω
→
{\displaystyle {\vec {\omega }}}
is sometimes used as a synonym for the vector quantity angular velocity.
One revolution is equal to 2π radians, hence
ω
=
2
π
T
=
2
π
f
,
{\displaystyle \omega ={{2\pi } \over T}={2\pi f},}
where: ω is the angular frequency or angular speed (measured in radians per second), T is the period (measured in seconds), f is the ordinary frequency (measured in hertz) (sometimes symbolised with ν).
angular momentum Also (rarely) moment of momentum or rotational momentum. The rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity–that is, the total angular momentum of a closed system remains constant.
angular velocity (ω) A measure of the rate at which an object rotates or revolves relative to another point, i.e. how fast the angular position or orientation of an object changes with time. There are two types of angular velocity: spin angular velocity refers to how fast a rigid body rotates with respect to its centre of rotation, whereas orbital angular velocity refers to how fast a rigid body's centre of rotation revolves about a fixed origin, i.e. the time rate of change of its angular position relative to the origin. Angular velocity is generally expressed as an angle or arc per unit time; e.g. the SI unit of angular velocity is radians per second (rad/sec), with the radian having a dimensionless value of unity, so that the unit is often written as 1/sec. Angular velocity is usually represented by the Greek letter omega (ω, sometimes Ω). By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.
anion A negatively charged ion. Contrast cation.
annihilation In particle physics, the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy and momentum of the initial pair are conserved in the process and distributed among a set of other particles in the final state. Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of such an original pair are zero. Hence any set of particles may be produced whose total quantum numbers are also zero as long as conservation of energy and conservation of momentum are obeyed.