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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.

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*.

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*.

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.

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.

A negatively-charged ion*.

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

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

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

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

A positively-charged ion*.

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.

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.

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

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.

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

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

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

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 represented by the symbol, I.

Deceleration is acceleration* in the opposite direction to the direction of motion - in other words, slowing down, or negative acceleration.

Displacement is distance moved in a particular direction. Its unit is m.

One of the three fundamental particles* that make up atoms*. Electrons are by far the smallest in mass of the three fundamental particles. They have a negative charge of -1.6 × 10-19 C.

See voltage*.

element (chemical)

A chemical element is something that cannot be split into smaller components through chemical reactions.

element (electric)

An electric element is a resisitor designed to generate heat (for an electric fire, say).

Energy enables something to do physical work involving the application of a force.

The splitting of an atomic nucleus.This is what is done in nuclear reactors. A neutron is absorbed by the nucleus (making the nucleus unstable) which then splits into two smaller nuclei. For nuclei larger than iron, this process releases energy.

Something that can flow. Often mistaken as being another word for liquid, in fact both gases and liquids are fluids.

Forces are pushes and pulls that make things move or change shape.

How many times something occurs in a given time, normally per second. Frequency is measured in hertz (Hz) where 1 Hz = 1 per second.

Friction is a resisting force between two surfaces rubbing against each other.

Fusion is the joining of atomic nuclei to form a larger nucleus. Fusion powers the Sun and stars. It is hoped that we will one day be able to use the energy from hydrogen fusion to provide for our energy needs. Fusion only releases energy for nuclei that are smaller than iron.

A very large cluster of stars in space, normally hundreds of millions of stars, but sometimes as many as hundreds of billions. Note that the stars in the night sky are all relatively nearby in our own galaxy, the Milky Way*.

A hemisphere is half a sphere or globe.

A charged atom* or molecule*. A positive ion (or cation) will have lost one or more electrons*. A negative ion (or anion) will have gained one or more electrons. Ions cannot be formed by the loss or gain of protons*.

An atom or molecule is said to be ionised when it has gained or lost one or more electrons. If an atom or molecule gains an electron, it is negatively charged (see anion*); losing an electron makes it positive (see cation*).

An insulator is a medium through which an electrical current* will not flow.

Kelvin is the absolute temperature scale and it is measured from absolute zero*. A difference in temperature of 1 kelvin is the same as a difference of 1° C. However, 0 kelvin is approximately -273°C so, on this scale, water boils at 373 K and freezes at 273 K. Note however that there is no ° symbol used to indicate temperature on the kelvin scale.

The international unit of mass. A platinum archetype of 1 kilogram of mass is held in Paris.

The kilowatt-hour is a unit of energy It is equivalent to 3·6 megajoules.

The distance travelled in a year at the speed of light.

Magnetism is a force that can be attractive or repulsive.

A measure of the amount of matter in an object. (Do not confuse mass with weight*.)

Matter is the stuff that everything is made from. Where there is something, there is matter. Otherwise, there is a vacuum.

The Milky Way is our galaxy. It contains about 100 billion stars, of which the Sun is just one. The Milky Way rotates like a large catherine wheel.

A molecule is the least amount of a compound that can exist and still remain a compound.

The motor effect is the result of two things. Firstly, a current is passed through a conductor: this forms a magnetic field around it. Secondly, if an external magnetic field is present, the fields will repel and a force will be experienced by the conductor. This results in movement.

One of the three fundamental particles* that make up atoms*. Neutrons are the largest in mass of the three fundamental particles. They have no electrical charge.

The unit of force. 1 newton (N) is defined as being the force required to give a mass of 1 kg an acceleration of 1 m/s/s.

Newton metres2 per kilogram2 is the unit of the Universal Gravitational Constant. It is an esoteric unit so it's not one you'll encounter much.

pertaining to an atom:

The centre of an atom. The nucleus is where neutrons* and protons* are tightly bound. Nuclei are many thousands of times smaller than the atom itself. For example, if an atom was the size of a football stadium, the nucleus would be comparable to a pea.

pertaining to a comet:

The solid part of a comet.

Any of two particles normally found in an atomic nucleus. i.e. protons and neutrons.

The unit of electrical resistance*.

Ohm's Law quantifies the relationship between voltage*, resistance*, and current* and it most simply translates to volts = amps × ohms.

In a parallel electrical circuit, the components share an energy source but do not share the same current.

A particle is a small piece of something.

A white hot gas made only of ions*.

See voltage*.

The amount of energy transferred per second. Power is measured in watts* and is represented by the symbol, P.

One of the three fundamental particles* that make up atoms. Of the three fundamental particles, protons are the second largest by mass. They have a positive electrical charge of +1.6 × 10-19 C.

Resistance is the extent to which a conductor* hinders the flow of an electric current. Resistance is measured in ohms* and is represented by the symbol, R.

A resistor is a circuit component designed to reduce current.

A body that orbits another, larger body. An artificial satellite is a man-made vessel that orbits a planet or moon.

A series electrical circuit is a circuit where the components share the current flowing around the circuit.

Speed is the rate of change in distance. In other words, how quickly something moves through a given distance. It is normally measured in m/s and its formula is speed = distance ÷ time. See also velocity*.

A synthetic polymer is a plastic.

G is a constant required by Newton's Law of Gravitation. It has the value 6.67 × 10-11 Nm2/kg2*.

Velocity is speed in a stated direction. The unit of velocity is m/s. Velocity may alternatively be defined as rate of change in displacement. The formula for velocity is velocity = displacement ÷ time. See also speed.

The unit of electromotive force* or potential difference*.

The measure of energy available to drive a current*. Voltage is measured in volts* and is represented by the symbol, V.

A device for measuring electromotive force* or potential difference*.