OCR GCSE Physics P1 - P7

The solar system has a star called the Sun at its centre. It includes all the objects that travel around the Sun in it paths called orbits. Distances in space stretch very far, so they are measured in light years. Light years measures distance, not time. The objects that you will need to know that orbit the Sun are:

- Planets - Eight spherical planets travel around the Sun in near circular orbits.

- Dwarf planets - They are much smaller than normal planets and travel in very similar orbits to normal planets.

- Asteroids - These are lumpy pieces of rock in near circular orbits,

- Comets - These objects have very elongated orbits that stretch far from the Sun and may also approach very close to it.

- Moons - These are balls of rock that orbit a planet in near circular orbits.

The Universe contains everything that exists. It contains thousands of millions of galaxies. Each galaxy contains thousands of milions of stars. Our nearest star is the Sun which is in our galaxy, the Milky Way.

Due to stars being so far away, people only find out about them due to the radiation they emit. Distant stars are very faint, or invisible to our eyes. Scientists use telescopes to detect different forms of radiation from the stars.

Sizes and distances in the Universe are enormous, so light years are used to measure them. This is the order of size: Earth's diameter - Sun's diameter - Diameter of solar system - Distance to next star - Diameter of Milky Way - Distance to next galaxy.

Some stars are brighter than others. This is because these stars are generally bright, or they are close to us. To measure the distance to a star, scientists compare its brightness with another similar star with a known distance. They need to know: the real brightness (how bright a star really is) and the relative brightness. (how bright they appear to be)

It is difficult to make accurate observations of stars. This is due to light pollution. This problem can be removed by putting telescopes in space.

As the Earth moves around the Sun, we look at stars from different angles. The nearby star stays in the same place but the background stars seem to change position. This parallax is used to measure the distance to nearby stars. If an object is nearby, its parallax is greater.

Disadvantages of parallax:

- Hard to get light from very dim stars and galaxies.

- At long distances, parallax measurements are too tiny to measure accurately.

- Direct measurements are not made.

The Sun gives out huge amounts of energy in the form of heat and light. Its energy comes from hydrogen. Hydrogen nuclei are jammed together so hard they combine to form different elements. In this case helium. This process releases masses amounts of energy and is called nuclear fusion.

In a star the temperature is so high and the density is so great that nuclei collide at enormous speeds. When they hit each other two or more nuclei are crushed together in a fusion reaction to make a heavier nucleus. This means a different element forms. All nuclear fusion gives out heat and energy. In each reaction a neutron is also released as well as energy and the newly created element.

Stars more than 8 times bigger than our Sun end their life as a massive explosion called a supernova. The explosion is so massive lots of different elements are formed as well as dust and particles. These are spread throughout the galaxy.

Starlight coming from galaxies to Earth changes during its journey. The wavelength of the light waves increases. This stretching of the light waves is evident that distant galaxies are moving away from us.

Red light has a longer wavelength than blue light. This change in colour is caused by a stretching of light waves, which is called redshift. Light from all near galaxies are red shifted. Light from galaxies even further are red shifted by a greater amount. The amount of redshift depends on how fast the galaxy is moving away.

A spectrometer is used to analyses the light coming from galaxies. This tells us if light has been redshifted.

About 5 billion years ago, our Sun formed at the centre of a huge swirling cloud of gases and dust which collapsed in on itself. The remaining matter swirling around the Sun formed into planets, asteroids and moons. We think the Earth was formed around 4.5 billion years ago.

Evidence for these ideas:

- Telescopes in space can see how other new planetary systems are formed.

- Scientists can date meteorites that have fallen to Earth. These rocks were formed when the solar system formed. Oldest was 5 billion years old.

The Big Bang theory predicts that an 'echo' of the initial rapid expansion could be detected now, as radiation is coming from all directions. The detection of cosmic background radiation convinced scientists that the Big Bang took place.

It is hard to predict what will happen to the universe, due to the enormous distances that need to be measured. Another thing we cannot measure is the total of matter in the universe. The total mass is crucial due to the effect of gravity.

If the Universe is higher than the critical mass gravity will start to pull everything together again. The Universe reaches a maximum size then begins to shrink.

If the Universe's mass is equal to the critical mass the Universe will reach a fixed size.

If the Universe is below the critical mass gravity is not strong enough to stop galaxies moving apart. The Universe expands forever.

Weathering and erosion break down and remove the surface rock, changing the size and shape of mountains over millions of years. Eroded rock fragments are transported by wind, water and ice, which are broken down further then deposited on river beds and in the sea. This is called sedimentation. Over time the sediments are crushed together to form new sedimentary rock.

Geologists can study mountains and large land masses to measure their age. This is because it is likely the sedimentation process has been taking place for the whole of the Earth's existence. The oldest rock found was 4 billion years old, this implies that the Earth must be atleast 4 billion years old for this rock to exist.

A scientist called Wegener suggested that continents were once joined to each other in the past and they have drifted apart over millions of years. He published his theory in 1915. His ideas were:

- The shape of continents looked as if they could interlock.

- Similar fossils and rock were found on continents separated by oceans.

- Mountain chains made from similar rock are found on the edges of continents.

Disadvantages of his theory:

- He could not explain how continents moved.

- Movement of continents were too small to detect.

- Data about rocks and fossils were not found for all continents.

The Earth's core is extremely hot and heats rock in the mantle. This causes convection, which moves the rocks in the mantle. Sections of the Earth's mantle are forced apart, making the seafloor spread. This is called seafloor spreading.

Every year, seafloors spread by a few centimentres. We do not see a gap because fresh rock from the mantle comes to the surface to fill the space. This forms underwater mountains called oceanic ridges.

Over millions of years, the Earth's magnetic field changes direction. Every time fresh rock rises to surface, it is magnetised in the direction of the Earth's field at that time. This causes a pattern to appear that is symetrical.

Tectonic plates float on the mantle, where two plates meet it is called a plate boundary. Earthquakes, volcanoes and mountains form on plate boundaries.

Volcanoes:

When plates move apart, there is a gap in the Earth's crust. Magma is forced through the cracks and piles up. This formes a volcanic mountain.

Mountains:

Mountains form when two plates come together and one plate is forced under the other. This could also cause a volcano to be formed.

Earthquakes:

Earthquakes occur when tectonic plates suddenly slide past each other. This releases lots of energy in a sudden jerk.

This links to the sedimentation process but there are specific steps to the rock cycle:

1) Mountains are weathered and fragments of rock break off.

2) They are transported away and deposited on the seabed as sediments.

3) At plate boundaries, movement of tectonic plates causes one to be subducted. When an oceanic plate is forced below sediments are dragged down as well.

4) As the plate moves deeper into the mantle the rock melts and becomes magma.

5) Pressure can cause the magma to rise. This either solidifies into rock beneath the land surface or escapes to form mountains.

Process repeats.

A wave is a series of disturbances that carry energy in the direction of the wave without transferring matter.

For wave motion you can use: distance = wave speed x time. This can be rearranged to find how fast a wave can travel.

Amplitude - The height of the wave from the peak to the normal. (Line in the middle)

Wavelength - Distance between 2 peaks or 2 troughs.

The number of waves passing a point per second is called the frequency. This is measured in hertz.

Another way of finding wave speed if you are given wavelength and frequency is:

wave speed (m/s) = frequency (hz) x wavelength (m)

Light is a form of electromagnetic radiation. Sources of light include things that glow such as the Sun, light bulbs and fires. These sources emit radiation that travel outwards in all directions. We can only see objects that emit or reflect light. Our eyes are examples of detectors because they respond to light. Many objects absorb light but are not detectors.

Almost all light passes through glass and air. It is transmitted through transparent materials. Light is partly absorbed by translucent materials. Shiny objects reflect most light that falls on them.

Light belongs to the electromagnetic spectrum, a family of electromagnetic waves that travel at the speed of a vacuum. The behavour of the waves depend on its frequency. Waves with higher frequencies carry more energy. They are grouped in ranges of frequencies.

Lowest --> Highest:

Radio Waves. Micro Waves. Infrared. Visible Light. Ultra Violet. X-Rays. Gamma Rays.

Photons are packets of energy and they have no mass. The energy carried by each photon depends on the frequency of the electromagnetic radiation. The higher the frequency, the more energy each photon transfers when it is absorbed.

Solar cells produce electricity and they work by absorbing energy from sunlight and then transfers it to electrical energy. The amount of energy absorbed by the solar cell depends on the intensity of the radiation arriving at its surface.

The intensity of a beam of radiation is a measure of radiation transferred each second. The intensity depends on:

- Number of photons arriving each second.

- Energy transferred by the individual photons.

As stated before, the energy carried by individual photons depends on the type of radiation, Photons of ultra violet have a higher frequency and more energy of photons than infrared.

The intensity of electromagnetic radiation gets less as you move further away from the source. This is because it gets spread over a larger area until it reaches you.

Intensity = energy transferred per second per metre squared.

If you move a torch closer to a surface, its light spreads over a smaller area and becomes brighter. The total energy from the torch each second is the same, but the intensity of the light on the surface increases. This happens because the distance decreases.

Everything is made up from the basic building blocks of matter: atoms, molecules and ions. These basic building blocks are formed from even smaller particles which include electrons.

Atoms are the smallest particles of an element. Molecules are the smallest part of a substance and are formed when more than one atom join together. Ions are charged particles formed when atoms or molecules are broken into smaller pieces. When an ion forms, either electrons are knocked out of an atom or molecule, or electrons join into an atom or molecule. Charged particles are formed because electrons have a negative charge. This is called ionisation.

All ions have a charge. Positively charged ions have lost electrons and negatively charged ions have gained electrons.

When an atom absorbs a gamma ray photon, the photon may have enough energy to knock an electron out of the atom. Gamma rays are a type of ionising radiation. Other types of ionising radiation are x-rays and high frequency ultra violet radiation. Ionisation can also happen when a molecule absorbs high frequency radiation, as long as each photon has enough energy to knock an electron.

Ionisation does not happen with visible light, microwaves, infrared or radio waves because the photon energy is too low to knock electrons. If molecules in our cells are ionised, the processes that occur within the cell can change.

-Ultraviolet rays from the sun can damage skin cells and excess exposure to sunlight can lead to skin cancer.

-Too much exposure to x-rays can cause cancers to develop.

The damage of cells is greater when exposed to ionising radiation for longer periods of time or to a higher intensity of radiation.