OxfordAQA International GCSE Physics: Solar system, orbits, red shift, Big Bang (9203) – Revision Notes

These notes condense sections 3.8.2 (solar system and orbital motion) and 3.8.3 (red shift and the expanding universe) of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. The qualification is untiered, and section 3.8.3 is for the full Physics award only. For full explanations and longer worked examples, read the study guide.

Related pages: the course hub, the printable checklist, the practice questions and the 10-minute diagnostics.

Solar system and universe (3.8.2 a, b)

• Eight planets orbit the Sun, a medium-sized star: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Other objects orbiting the Sun: asteroids, dwarf planets, comets.
• Moons orbit planets.
• The universe has thousands of millions of galaxies. Each has thousands of millions of stars.
• Our Sun is a single star among the thousands of millions in the Milky Way.

Object	Size	Motion
Sun	Star, by far the largest	Planets orbit it
Planet	Large	Orbits the Sun
Moon	Smaller than its planet	Orbits a planet
Dwarf planet	Smaller than a planet	Orbits the Sun
Asteroid	Small, rocky; many in a belt between Mars and Jupiter	Orbits the Sun
Comet	Small, ice and dust	Orbits the Sun on a long, stretched path

Memory aid for the order: use any sentence of your own whose words start M, V, E, M, J, S, U, N.

Satellites (3.8.2 c)

	Natural satellite	Artificial satellite
Example	Moon	Weather or communications satellite
Origin	Formed naturally	Built and launched
Orbits	A planet	Usually the Earth

Orbit	Key facts	Uses
Geostationary	Above the equator, about 36 000 km up, period 24 hours, same direction as Earth’s spin, stays over one point	Communications, television
Low polar	A few hundred kilometres up, over both poles, period about an hour and a half, Earth turns beneath it	Weather, mapping, whole-surface coverage

Gravity, centripetal force and acceleration (3.8.2 d, e)

• Gravity provides the centripetal force for planets, moons and satellites.
• The force acts towards the centre of the orbit, at right angles to the motion.
• This resultant force makes the body accelerate inwards.
• Direction changes, so velocity changes. Speed stays the same.
• Accelerating does not always mean speeding up.
• The equation for centripetal force is not required.

Method for an explanation question:

1. State the force: gravity, towards the centre.
2. State that it is at right angles to the motion.
3. Velocity is a vector, so a change of direction is a change of velocity.
4. A change of velocity is acceleration, with no change of speed.

Orbital speed and radius (3.8.2 f, g)

If the orbital radius is…	Gravity (centripetal force) is…	Orbital speed is…	Time for one orbit is…
Larger	Smaller	Lower	Longer
Smaller	Larger	Higher	Shorter

• One radius goes with one stable speed.
• A change in orbital speed means a change in orbital radius.
• Satellite placing: the speed fixes the radius of the final orbit.

Useful relation for any circular orbit (this is just speed = distance / time):

orbital speed (m/s) = 2 × π × orbital radius (m) / time for one orbit (s)

Worked reminder: a satellite orbits at radius 8.4 × 10⁶ m and takes 7660 s. Distance = 2π × 8.4 × 10⁶ = 5.28 × 10⁷ m. Speed = 5.28 × 10⁷ / 7660 = 6890 m/s (3 s.f.).

Choosing an orbit for a job:

Job	Orbit	Reason
Television broadcast to a fixed dish	Geostationary	The satellite stays over the same place, so the dish never moves
Mapping all of the surface	Low polar	The Earth turns beneath the orbit, so every region passes underneath
Weather images of the poles	Low polar	The orbit passes over both poles

Always use the radius from the centre of the Earth, not the height above the surface.

Doppler effect (3.8.3 a)

Source relative to observer	Observed wavelength	Observed frequency
Moving away	Increases	Decreases
Moving towards	Decreases	Increases

• Applies to sound, light and microwaves.
• Wave speed = frequency × wavelength still holds for the observed wave.
• Sound reminder: a siren at 500 Hz in air at 340 m/s has wavelength 340 / 500 = 0.68 m. If the observer hears 540 Hz, the wavelength heard is 340 / 540 = 0.630 m.

Red shift and the Big Bang (3.8.3 b)

1. Light from most distant galaxies has a longer wavelength than expected. This is red shift.
2. The further the galaxy, the bigger the increase in wavelength.
3. A bigger shift means a faster recession speed, so more distant galaxies move away faster.
4. This suggests space itself is expanding.
5. If space is expanding, everything was once closer together.
6. This supports the Big Bang model: the universe started as one tiny point.

Worked reminder: a spectral line has a laboratory wavelength of 486.0 nm. It is seen at 487.2 nm in galaxy A and at 489.9 nm in galaxy B. The shifts are 1.2 nm and 3.9 nm. Both are red shifts, because both wavelengths are longer. Galaxy B has the larger shift, so it is moving away faster and is probably further away.

Distinction to keep: red shift is evidence that the universe is expanding. CMBR is evidence of a hot early universe. Use both when asked about evidence.

CMBR and age (3.8.3 c, d)

• CMBR is electromagnetic radiation, in the microwave region, filling the universe.
• It comes from radiation left over from soon after the universe began.
• Currently only the Big Bang theory explains CMBR.
• Age of the universe: about 14 thousand million years (1.4 × 10¹⁰ years).

Must-know distinctions

Pair	Difference
Planet and moon	A planet orbits the Sun; a moon orbits a planet
Asteroid and comet	Asteroids are rocky and mostly in a belt; comets are ice and dust on stretched orbits
Geostationary and low polar	Geostationary stays over one point, high; polar is low and covers the whole surface
Speed and velocity	Speed has size only; velocity has size and direction
Natural and artificial satellite	Natural formed by itself; artificial built and launched
Red shift and Doppler effect	Red shift is the Doppler effect for light from a receding source

Quick self-test

1. Name the eight planets in order from the Sun.
2. A satellite orbits at radius 7.2 × 10⁶ m and takes 6080 s per orbit. Calculate its speed.
3. A second satellite orbits at 9.5 × 10⁶ m and takes 9220 s. Calculate its speed and say which satellite is in the higher orbit.
4. Explain why a planet in a circular orbit accelerates although its speed is constant.
5. Which type of orbit keeps a satellite above one point on the equator? State its period.
6. A vehicle moves away from you sounding a horn. State what happens to the wavelength and frequency you detect.
7. State what is meant by red shift.
8. What does a larger red shift tell you about a galaxy?
9. State two pieces of evidence for the Big Bang.
10. Write the age of the universe in years in standard form.

Answers

1. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
2. Distance = 2π × 7.2 × 10⁶ = 4.52 × 10⁷ m. Speed = 4.52 × 10⁷ / 6080 = 7440 m/s (3 s.f.).
3. Distance = 2π × 9.5 × 10⁶ = 5.97 × 10⁷ m. Speed = 5.97 × 10⁷ / 9220 = 6470 m/s (3 s.f.). The second satellite is in the higher orbit, and its speed is lower.
4. Gravity acts towards the centre, at right angles to the motion. It changes the direction of the velocity. A change in velocity is an acceleration. The speed stays the same.
5. Geostationary orbit. The period is 24 hours.
6. The wavelength increases and the frequency decreases.
7. The observed wavelength of light from a distant galaxy is longer than the wavelength emitted, because the galaxy is moving away.
8. The galaxy is moving away faster, and it is further away.
9. Red shift of light from distant galaxies, and the cosmic microwave background radiation.
10. 1.4 × 10¹⁰ years.

Where marks are usually lost

• Writing that the speed of an orbiting body changes, instead of the direction of its velocity.
• Using the height above the surface as the orbital radius in a speed calculation.
• Forgetting to convert days or hours to seconds before dividing.
• Saying “the satellite is stationary” for a geostationary orbit. It orbits, once every 24 hours.
• Describing red shift as the galaxies being redder because of their stars. It is a shift in wavelength of the light received.
• Writing that galaxies move away “through” space, with no mention of space expanding.
• Naming only red shift as evidence for the Big Bang when two pieces are asked for.
• Quoting the age as 14 million years, or giving it without a unit.
• Saying a larger orbit means a faster satellite. Further out means slower.

Official syllabus

OxfordAQA International GCSE Physics (9203), specification for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.8.2 and 3.8.3.