Beyond Earth — OCR GCSE Physics

The Big-Bang model says the universe began from a very small, hot, dense region and has expanded ever since. Red-shift supports this because nearly all galaxies show light shifted to longer wavelengths, indicating they recede, and more distant galaxies recede faster, which fits expansion from a common origin. A second line of evidence is the cosmic microwave background radiation (CMBR): faint microwave radiation arriving from every direction, interpreted as cooled remnant radiation from the early hot universe. Together these observations are explained by an initial hot dense state followed by expansion and cooling. A useful method is to state each piece of evidence, then say what the Big-Bang model predicts and how the observation matches that prediction.

A cloud of dust and gas (a nebula) is pulled together by gravity. As the material falls inward, gravitational potential energy becomes thermal energy, so the core heats up. When the core is hot enough, hydrogen nuclei fuse to form helium, releasing energy. That energy raises the pressure and pushes outward, while gravity pulls inward. When the two balance, the star is in equilibrium and stays stable for a long time. The Sun formed this way. In the wider lifecycle, a star of the Sun's mass later expands into a red giant, sheds its outer layers and leaves a white dwarf; more massive stars may end as neutron stars or black holes.

Every object with a temperature above absolute zero emits electromagnetic radiation. The hotter the object, the greater the intensity of radiation it emits at every wavelength. Temperature also changes the wavelength distribution: a cool object emits most strongly at long infrared wavelengths, while a hotter object peaks at shorter wavelengths and can emit visible and ultraviolet radiation. Because a hot object emits a continuous range of wavelengths, it emits photons with a continuous range of energies; since energy is proportional to frequency (E = hf) and frequency is inversely proportional to wavelength, higher-energy photons correspond to shorter wavelengths.

Our solar system contains the Sun and eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Minor planets, such as asteroids in the asteroid belt and dwarf planets beyond Neptune, also orbit the Sun but are much smaller than planets. Most planets have natural satellites (moons) that orbit them, for example the Moon orbiting Earth. Artificial satellites are human-made objects placed in orbit, often around Earth. A geostationary satellite orbits above the equator with a period of 24 hours, so it stays above the same point on Earth's surface; a polar satellite passes over the poles in a lower orbit and scans the whole surface as Earth rotates beneath it. Both kinds of artificial satellite are held in orbit by gravity, just as natural satellites are, but they differ in origin, orbit and purpose.

For a planet moving in a circular orbit, the gravitational force from the Sun acts towards the centre of the orbit. Velocity is a vector: it has both a size (speed) and a direction. Because the planet's direction of motion changes continuously around the circle, its velocity changes even though its speed stays constant. The gravitational force is always perpendicular to the planet's motion, so it changes the direction of motion rather than the speed. This is a qualitative explanation only: you need to state that the force acts towards the centre, that the direction of motion changes, and that velocity is a vector so a change of direction means a change of velocity while speed remains the same.

A satellite stays in a stable circular orbit when gravity supplies exactly the centripetal force it needs. For a given central body, the orbital speed and the orbital radius are inversely related: a faster stable orbit must have a smaller radius, and a slower stable orbit must have a larger radius. So if a satellite's speed is increased and it is to settle into a new stable orbit, its radius must decrease; if its speed is reduced, its radius must increase. This is qualitative only at Higher tier: you reason about the direction of the change, not a numerical calculation. A useful check is that a satellite close to a planet orbits quickly, while a distant satellite orbits slowly.

A body's temperature depends on the balance between the radiation it absorbs and the radiation it emits. If it absorbs more than it emits, its temperature rises; if it emits more than it absorbs, its temperature falls; when the two rates are equal, its temperature stays constant. Everyday examples include a cup of hot drink cooling because it emits more infrared than it absorbs, and a car left in sunshine warming because it absorbs more than it emits. For the Earth, incoming short-wavelength radiation from the Sun passes through the atmosphere, while the Earth emits longer-wavelength infrared; greenhouse gases absorb and re-emit some of this outgoing radiation, so the balance sets the surface temperature. Factors such as the atmosphere's composition and cloud cover therefore affect how much solar radiation reaches the surface and how much escapes.

Seismic waves and sound waves behave differently in different materials, and these differences let us probe places we cannot see. P waves are longitudinal and travel through solids and liquids; S waves are transverse and cannot pass through liquids. Both change speed at a boundary, and some energy reflects back. By timing arrivals at seismometers, scientists work out the depth of boundaries and detect the liquid outer core, because S waves are blocked and a P-wave shadow zone appears. In deep water, sonar sends a sound pulse downward; the time for the echo to return, combined with the speed of sound in water, gives the depth. Reflection from the seabed or from objects detects them; absorption limits range. Answers should reason qualitatively about velocity, absorption and reflection for detection and exploration.