Cambridge IGCSE Physics 6 Space Physics Exam Questions

About Cambridge IGCSE Physics 6 Space Physics

Space physics in Cambridge IGCSE Physics 0625 covers the Earth and the Solar System, and stars and the Universe. You need to explain day and night, seasons and the phases of the Moon, describe the planets and other bodies in the Solar System, and calculate orbital speed. The second part covers the Sun as a star, the life cycle of stars, galaxies and the light-year, and evidence for the Big Bang from redshift. Extended candidates also use the Hubble constant to estimate the age of the Universe. Examiners often ask data analysis questions on planets and short explanations of the life cycle of stars.

What you need to know

• 6.1 The Earth The Earth rotates on its axis once in about 24 hours, causing day and night and the apparent daily motion of the Sun. It orbits the Sun in about 365 days, and its tilted axis causes the seasons. The Moon orbits the Earth in about one month, giving its phases.
• 6.1 Orbital speed Average orbital speed v = 2πr / T, where r is the average radius of the orbit and T is the orbital period. Make sure units are consistent, e.g. metres and seconds.
• 6.1 The Solar System The Sun, eight planets in order (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), minor planets including dwarf planets and asteroids, moons and comets. The inner planets are small and rocky; the outer planets are large and mostly gas. The Solar System formed from a cloud of gas and dust by accretion.
• 6.1 Gravity in the Solar System The Sun's gravitational attraction keeps the planets in orbit. Its gravitational field strength decreases with distance, so planets further out orbit more slowly and have longer periods. Light from the Sun takes about 500 s to reach the Earth.
• 6.2 The Sun and stars The Sun is a medium-sized star made mostly of hydrogen and helium, radiating most of its energy as infrared, visible light and ultraviolet. Stars are powered by nuclear fusion of hydrogen into helium. Galaxies contain many billions of stars; the Sun is in the Milky Way. One light-year is about 9.5 x 10^15 m.
• 6.2 Life cycle of stars A star forms from a cloud of gas and dust (nebula) that collapses to a protostar, then becomes a stable star. A star like the Sun becomes a red giant, then a planetary nebula with a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova and leaves a neutron star or black hole; the nebula formed contains heavier elements.
• 6.2 The Universe Light from distant galaxies is redshifted, showing they are moving away; this supports the Big Bang theory that the Universe is expanding. Extended candidates describe cosmic microwave background radiation, define the Hubble constant H0 = v / d (about 2.2 x 10^-18 per second) and estimate the age of the Universe as d / v = 1 / H0.

Common mistakes in 6 Space Physics

• Saying seasons are caused by the Earth being closer to the Sun in summer. They are caused by the tilt of the Earth's axis.
• Saying the Sun moves across the sky. The apparent movement is caused by the Earth's rotation.
• Using days or years in v = 2πr / T without converting when the answer is needed in m/s. Convert the period to seconds.
• Muddling the life cycles of different stars. Only stars much more massive than the Sun become supernovae, neutron stars or black holes.
• Calling a light-year a unit of time. It is the distance light travels in one year.

Try three 6 Space Physics exam-style questions