satellite

A satellite is any object that orbits a planet. The Moon is the Earth's only natural satellite. There are at least 60 natural satellites in the solar system. Some of these, such as Ganymede, the largest satellite of Jupiter, are larger than the smallest planets, Pluto and Mercury.

The planet with the greatest number of natural satellites is Saturn with 18.

A man-made satellite is known as an artificial satellite. Orbits can be equatorial, polar or geostationary.

scalar

A quantity that has magnitude (or size) but not direction; e.g. . The following quantities are scalar: average speed, energy, power, time, temperature, potential difference.

See also vector

scattering

The name given to the deflection of particles of matter or electromagnetic radiation by the particles that make up the material through which they pass.

Scattering of alpha particles by the nuclei of gold atoms enabled Ernest Rutherford (1871-1937), the New Zeland physicist to discover the structure of the atom. The experiment, performed in Manchester University in 1911 by Hans Geiger (1882-1945), a German physicist, working for Rutherford, is known as alpha-particle scattering.

Scattering of visible light by molecules of gas that make up our atmosphere is the reason why a clear daytime sky is blue.

second

The second is the unit of time. There are 60 x 60 (or 3,600) seconds in one hour.

WWW: SECOND

second law of motion

Newton's second law of motion states that the rate of change of momentum of a body is proportional to the unbalanced force action on it.

In other words, an unbalanced force is needed to change the velocity of an object.

This definition leads to the formula:

secondary waves

Secondary waves (also known as S-waves) are transverse seismic waves that are caused by earthquakes. S-waves can't travel through liquid, which is why they can't be detected over a wide region of the Earth's surface directly opposite their source. This fact was used by geologists to show that the Earth must have a liquid core.

seismic waves

Seismic waves are powerful vibrations due to earthquakes. There are two types of seismic wave: P-waves and S-waves.

These vibrations travel through the Earth until they reach the surface. They are the most useful information that scientists have with which to study the Earth's inner structure.

WWW: SEISMIC WAVES

semiconductor

A substance that has electrical properties that lie between a conductor and an insulator. Silicon and germanium are natural semiconductors.

If a small amount of a foreign substance added to a semiconductor (a process known as doping), the original material can become a normal conductor.

series circuit

An electrical circuit in which components are connected one after the other. The current through each component is the same as its neighbours.

short circuit

When a short circuit occurs in an electrical circuit the current does not flow around the whole circuit. Instead it bypasses some or all the components and returns to the supply by a path of low resistance either because the uninsulated leads from the supply have made contact or because a wire is used to bridge the components.

A short circuit often results in a large current being drawn from the supply, which can damage either the supply or the leads through which the current flows, or both. The leads may heat up enough to cause a fire.

solar constant

The solar constant is the amount of energy that reaches each square metre of the Earth's surface from the Sun per second. The average value of the solar constant at the top of the atmosphere is 1400 W/m² .

solar power

Solar power is the general name given to the Sun's radiant energy. This can be a source of thermal energy, when it is used to heat water in passive solar panels, or of electrical energy in photovoltatic cells.

solar system

The system of planets, moons, asteroids, comets & dust centred on the Sun.

WWW: ABOUT PLANETS

solid

A state of matter in which atoms are in close contact and exert forces on one another when they move from their equilibrium position. Atoms in the solid state therefore vibrate about a mean position. The amplitude of vibration depends on temperature: the hotter the solid, the larger the amplitude of vibration. This is why solids expand when heated.

sound

Sound is the vibration of a medium that is audible to the human ear. The perfect human ear is sensitive to frequencies between 20 hz and 20,000 hz. In practice, few people are sensitive to the highest frequencies. Furthermore, one loses sensitivity to high frequency as one gets older.

Vibrations with frequencies less than 20 hz are called infrasound, and those with frequencies above 20,000 hz are called ultrasound. Humans are unable to hear either infrasound or ultrasound.

spark

A spark is a brief electrical current that flows between charged bodies or a charged body and the earth when the electrical insulation of the air gap between them breaks down due to ionisation of the air. Dry air is ionised when an electrical field reaches about 30kV/cm, i.e. 3,000,000 Volts per metre. Since water vapour is always present in the atmosphere to some degree, sparks usually occur at much lower electric field strengths.

specific heat capacity

Specific heat capacity (or s.h.c.) is the quantity of internal energy required to change the temperature of one kilogram of a substance by one degree Celsius or Kelvin. The unit of s.h.c. is 'Joules per kilogram per degree Kelvin'. Each substance has its own unique specific heat capacity. Water has the largest s.h.c. (4200 J/kgK). Metals have low s.h.c. compared to liquids. This means that it takes less heat to warm up an object made of metal than one which is a liquid (given that they have the same mass)..

specific latent heat

Specific latent heat (or s.l.h.) is the quantity of energy required to change the state of 1 kg of a substance.

spectroscopy

The science of producing and analysing spectra using spectroscope, which is an instrument that forms a spectrum of a source of light or other electromagnetic radiation.

spectrum

A spectrum is a range of properties arranged in order of size.

A spectrum of white light is created when a narrow beam of white light passes through a prism or a diffraction grating. The resulting spectrum consists of the following colours in order: red, orange, yellow, gree, blue and violet.

The inclusion of the 'seventh' colour, indigo, is an artifice introduced to satisfy Isaac Newton's numerological fancy. Indigo is not visible in the spectrum of white light.

speed

The average velocity at which an object moves between two points. Speed is total distance covered / total time taken. Speed is a scalar quantity.

speed of light

All electromagnetic waves travel through a vacuum at the same speed, 300,000,000 metres per second. This is known as the speed of light, and it is the greatest speed at which electromagnetic waves can travel. Electromagnetic waves travel more slowly through transparent materials such as glass and water. The speed of light in a medium other than a vacuum can be calculated from the refractive index of the medium.

300,000,000 m/s is approximately the same as 1,000,000,000 km/hr (one billion kilometres per hour), which in some ways is a more useful way to think about the speed of light. For example, Saturn is about 1.5 billion kilometres from the Sun so light takes about 1.5 hours to reach Saturn from the Sun.

WWW: SPEED OF LIGHT

star

A star is a large body composed mainly of hydrogen and helium that emits visible light and other forms of electromagnetic energy due to a thermonuclear reaction at its core.

Stars differ from one another in mass and brightness: the more massive a star the brighter it is. The Sun is a star. Most stars are smaller and less hot then the Sun.

WWW: STAR

stellar parallax

The apparent shift in the position of a nearby star relative to more distant stars due to changes in the Earth's position as it orbits the Sun. Stellar parallax is greatest when a star is observed 6 months apart. Stellar parallax is imperceptible to the naked eye.

See also parsec

stopping distance

The shortest distance in which a moving vehicle (motorcycle, car, bus, truck etc.) can be brought to rest by its brakes.

On the web:

1: government information of stopping distance

2: The physics of moving cars

strain energy

When an object is stretched or squeezed elastically, the work done in deforming it is stored as energy. This energy is known as strain energy.

A catapult is an example in which strain energy is stored by stretching rubber. When the elastic cord is released, the strain energy is transferred into kinetic energy in the projectile being launched.

stratosphere

The stratosphere is the second major layer of air in the atmosphere. It lies above the troposphere and stretches from 10 to 50 km above the Earth's surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. Then it increases gradually to 200-220 degrees Kelvin (K). Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region.

Sun

The Sun is a star. It has a diameter of 1,394,000 km (about 110 times the diameter of Earth) and a mass of 1.99 x 10³⁰ kg ( about 99.9% of the total mass of the Solar System.)

The source of the Sun's energy is nuclear fusion that takes place within its core (which makes up the innermost 10% of the Sun.) The core temperature is about 15 million Kelvin , and it's surface temperature is 6000 K. The total lifetime of the Sun is estimated as 10 billion years. It is about half way through its life now.

The Sun emits radiant energy from its surface mainly as infrared, visible light and ultraviolet.

The Sun will become a white dwarf after it has exhausted its nuclear fuel.

supernova

The final stage in the life of a massive star. This happens when it has exhausted its nuclear fuel and so fusion reactions are no longer possible. The star then first collapses and this is followed by an explosion in which so much energy is released that the supernova can outshine an entire galaxy of several billion stars, if only for a few minutes. The remains of the exploded star becomes a neutron star or, in the case of the most massive stars, it becomes a black hole.

Systeme International

The Systeme International (or SI) has been adopted for most scientific work. It is based on the metric system. There are seven base units for seven base quantities

Length	Mass	Time	Current	Temperature	Luminous intensity	amount of substance
metre	kilogram	second	Ampere	Kelvin	candela	mole

WWW: System International