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Absorption
It happens when wave passes through a medium and gives up some of its energy.

Acceleration
Acceleration is the rate of increase of velocity. Acceleration tells you how much faster or slower a moving object gets every second. A negative acceleration is called a deceleration. The SI Unit of acceleration is the metre per second per second (ms-2)

Acceleration due to Gravity

This is the acceleration caused by the gravitational pull of a planet. On Earth, the strength of the acceleration due to gravity is roughly 10 ms-2. The acceleration due to gravity is often given the symbol 'g'.

Accommodation

Accommodation relates to the ability of the eye to change its focal length and so focus on objects at a range of distances. To focus on a close object, the eye lens will be fat (having a short focal length). To focus on a far object, the lens will be pulled into a thin shape having a long focal length. Failure of accommodation gives rise to long-sightedness and short-sightedness.

Activity

The activity of a sample of radioactive material is the number of disintegrations per second taking place in that particular sample. The SI unit of activity is the becquerel (Bq).

Aerodynamic

Aerodynamic objects have minimal air resistance. This means that they can move at greater speed through the air. Remember that as objects go faster the aerodynamic drag increases. Therefore, an aerodynamic object will be able to travel faster than a non-aerodynamic one!

Alpha (a) Radiation

a (alpha) radiation consists of slow moving helium nuclei. These nuclei are released during the disintegration of an unstable nucleus. a radiation will be stopped by a few centimetres of air, or a sheet of paper. It is a strongly ionising radiation, due in part to its relatively large mass - it is about 8000 times as massive as the electrons in b radiation! The helium nuclei are positively charged.

Alternating Current (a.c.)

Electrical current continually changes direction in an a.c. power supply (as opposed to direct current (d.c.))


Alternator

This is a device for generating a.c. electricity. It consists of rotor coils (with a d.c. supply) and stator coils which produce the a.c. electricity.

Ampere

The ampere is the SI unit of electrical current. At a simple level it can be defined as the number of coulombs of electrical charge flowing per second.

Amplifier

An amplifier is a component of an electronic system, such as a radio or TV which makes all the input signals louder. Ideally, an amplifier will increase the amplitude of all input signals by the same factor. Amplifiers require a power supply which can be battery or mains. The voltage gain or the power gain can be used to describe how much amplification is taking place.

Amplitude

This is the height of a wave, measured vertically from the centre line to a crest or a trough. The SI unit is the metre, although in some contexts amplitudes may be given in volts.

Amplitude Modulation

Amplitude modulation refers to the changing of the amplitude of a radio-frequency or microwave frequency carrier wave. The other method of modulation is called frequency modulation.


Analogue

Analogue signals should be compared with digital signals. Analogue signals are continuous. As an example, a microphone is an analogue input device. This is because it can produce a range of voltage levels.

AND Gate

An AND gate is a logical device. It usually takes two inputs (although more are possible) and produces only one output. The logic state of the output depends on the logic state of the inputs.


Angle of Incidence

The angle between the normal line and a ray or wave incident to a surface.

Angular magnification
The factor by which an image's apparent angular size is increased (or decreased). Cf. magnification.

Angular momentum
A measure of rotational motion; a conserved quantity for a closed system.

Angle of Reflection

The angle between the normal line and a ray or wave reflected from a surface.


Asteroid

An asteriod is a lump of rock which orbits around the sun. (The biggest asteroids are about 100km across, although many are 1km or less across. Asteroids orbit the Sun in the asteroid belt, between the orbits of Mars and Jupiter.)

Atom

The basic unit of one of the chemical elements.

Average Speed

The average speed of an object is defined as the distance traveled divided by the time elapsed.

Remember that average speed is different from instantaneous speed.

Audio Wave

A sound wave, but usually in the context of modulation and radio transmission


Axis
An arbitrarily chosen point used in the definition of angular momentum. Any object whose direction changes relative to the axis is considered to have angular momentum. No matter what axis is chosen, the angular momentum of a closed system is conserved.