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absolute zero

Absolute zero is the theoretical temperature at which atoms stop vibrating. All atoms have vibrational energy, even in solids, but if we cool them, they vibrate less. In practice, absolute zero would seem to be impossible to reach - but we can get to within millionths of a degree of it.

acceleration

Rate of change in velocity. If a body increases its speed (the result of a force acting on it) then it is said to have positive acceleration. See also deceleration*.

ammeter

A device for measuring current. An ammeter is always connected directly into a circuit in series* with the components through which the current is flowing. A good ammeter will have virtually no resistance*.

ampere

The unit of electrical current, normally abbreviated to amp or A. 1 amp is equal to a flow of 1 coulomb* of charge* per second. This translates to the formula, amps = coulombs ÷ seconds.

amplitude

The maximum displacement that an oscillation or wave has from its rest position. A measure of the energy contained in the motion or wave, large amplitude sounds are louder; large amplitude light waves are brighter.

anion

A negatively-charged ion*.

astronomical unit

An astronomical unit (AU) is defined as the average distance between the Earth and the Sun (approximately 148 million kilometers).

atmosphere

A layer of gases that envelops a planet, moon or other celestial body.

atom

The building blocks of matter. Once thought to be indivisible particles*, atoms are now understood to consist of electrons*, protons* and neutrons*.

battery

A group of electrical cells connected in series to provide an energy source for a circuit*.

cation

A positively-charged ion*.

cell

An energy source providing an electrical potential difference between its two terminals such that a current* can flow between them. A cell's energy is stored internally as chemicals that react with each other.

charge (Q)

Electrical charge is a property or quality that some particles* (and hence bodies) can have which gives rise to forces of attraction or repulsion between those particles. Charge is measured in coulombs* and is represented by the symbol, Q.

circuit

An electrical circuit consists of an energy source connected by conductors to electrical components.

coma

The gas and dust surrounding the nucleus of a comet.. We see a comet's coma for two reasons. First, because the dust in the coma reflects sunlight and, second, because sunlight makes the gas in the coma fluorescent.

conductor

A conductor is a medium through which an electric current* will flow.

constellation

One of the 88 named patterns of stars. For example, the seven brightest stars in the constellation Ursa Major form the Plough.

coulomb

The unit of electrical charge*. 1 coulomb is abbreviated to C.

current (I)

Electrical current is the flow of charges*. In wires, the charges are carried by electrons*. In fluids*, the charges are carried by ions*. Current is measured in amperes* and is

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