# OCR GCSE P1

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## The Solar System

The Sun is the star at the centre of our solar system. There are 8 planets which have near circular orbits around the sun. The four planets closest to the sun are solid rock and the four outer planets are gas giants.

Asteroids are irregular lumps of rock, mostly in near-circular orbits around the sun.

Comets are small objects made of rock and ice with very elongated orbits around the sun.

Dwarf planets are small spherical lumps of rock in orbit around the sun.

99% of the solar system's mass is the sun.

Planets in weight order: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury

Planets closest to sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Distances to objects outside the solar system are measured in light years the distance light travels in a year). Light travels at 300,000 km/s (3x10^5)

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## Finding out the distance to stars

All the evidence that we have about distant stars and galaxies comes from radiation that we can detect from earth.

REAL BRIGHTNESS: the amount of light emitted by the star, taking into account its distance
RELATIVE BRIGHTNESS: the amount of light that can be seen from earth from that star

Relative brightness: 2 stars which have the same real brightness have a different relative brightness. Knowing the distance to one star allows you to calculate the distance to the other using their difference in relative brightness. The disadvantages of this method are: it is based on the assumption that similar types of stars have the same real brightness; it is based on estimating the distance to one of the stars; dust, rain, clouds and light pollution can obstruct someone's view of the star.

Parallax: can only be used to measure nearby stars and uses angles from them to calculate their distance. They appear to move against the fixed background of distant stars (the parallax effect). These small angles can tell us how far away they are from us.

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## Fusion of elements in stars

Inside a star, hydrogen nuclei are jammed together so hard that they combine in pairs to form helium. This process releases lots of energy and is called nuclear fusion. All chemical elements with atoms heavier than helium are made in stars.

Conditions for nuclear fusion:

-High densities-at high enough densities, nuclear fusion can make heavier elements up to iron

-High temperatures

Heavy stars end their lives as a supernova. This is a massive explosion where all the different chemical elements are made, including those heavier than iron.

The solar system was made from a collapsing cloud of dust 5000 milllion years ago. Apart from hydrogen, evidence from the elements in the sun suggests that all the other materials in the cloud came from the explosions of large stars.

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## Redshift

Most galaxies appear to be moving away from us. This motion of the galaxies increases the wavelength of the light that we receive from them. This is called redshift. Mostly, the amount of redshift increases with distance from earth. Generally, the further away the galaxy is, the faster it is moving away from us. The evidence for redshift tells us that the universe is expanding.

The universe's fate depends on how it continues to expand. If there is enough mass in the univesre, gravity will slow down the rate of expansion and make it collapse again.

The fate of the universe is difficult to predict because:

-We can only measure the mass of those parts of the universe that emit radiation.

-Precise measurements of the speed and distance of galaxies is difficult because their radiation has to travel such a long way to reach us.

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## The Big Bang and the Changing Earth

The universe started expanding from a single point 14,000 million years ago.

The sun was created about 5000 million years ago.

The earth was created about 4500 million years ago.

Scientists believe that the universe began with a Big Bang. Evidence from cosmic background radiation supports this.

The surface of the earth is always changing. Sedimentation is the settling of eroded rock fragments at the bottom of the sea and in riverbeds. The build up of sedimentary rock layers eventually makes the seas shallower.

Continental drift-Alfred Wegener's theory:

Wegener's theory was based on the evidence that continents appear to fit together and that similar fossils and rocks are found on continents now separated by oceans. At the time, people did not accept Wegener's theory because they already had simpler theories; no one could explain or measure the movement of continents; Wegener was not a trained geologoist and his theory was very different to the others.

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## Seafloor spreading and the earth's structure

The seafloor between continents moving apart can increase by a few centimetres each year. This is called seaflooor spreading. Oceanic ridges form on the expanding seafloor where liquid rock from the mantle fills the gap. The solidifying rock in ocean ridges is magnetised by the earth's field. The earth's magnetic field changes direction over millions of years. Each time the field reverses, so does the magnetism of the ocenanic ridges. Therefore, the seafloor has strips of reversed magnetism parallel to the gap where the new rock was created.

The structure of the earth:

The inner core of the earth is mostly liquid iron. The outer core is a layer of liquid nickel and iron about 2200km thickSemi-liquid rock in the mantle floats on the core. A thin layer of solid crust floats on the mantle.

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## Seismic waves

Two types of sesimic wave are generated when tectonic plates suddenly move.

P-waves move quickly through solids and liquids. They are longitudinal which means that the particles vibrate in the same direcation as the wave's motion.

S-waves travel more slowly and can only pass through solids. They are transverse which means that they vibrate at right angles to the direction of the wave's motion.

From this, we can work out the structure of the earth. For example, the core of the earth must be liquid since only P-waves are able to pass though it.

Seismic waves speed up and change direction when they enter denser regions of the earth's core.

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## Waves and their properties

A wave transfers energy from a vibrating source. It creates a series of disturbances as it moves, vibrating material that it passes through. There is no overall transfer of matter in the direction of motion of the wave, just energy. The amplitude of the wave is the maximum height of the disturbance from the undisturbed position. The wavelength is the distance from one maximum disturbance to the next. The frequency of a wave is the number of vibrations of the source in one second. The unit of frequnecy is hertz. 1 hertz means one vibration per second.The speed of the wave is how fast each maximum disturbance moves away from the source.

Wave speed = distance traveled (m) / time taken (s)

Wave speed = frequency (hz) x wavelength (m)

The higher the frequency, the shorter the wavelength (inversely proportonal)

An oscillope is a machine that displays waves on a screen. A grid on the screen lets you compare the wavelength and amplitude of waves. Shorter wavelength=higher pitch sound. Larger amplitude=louder osund.

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