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

This guide is written for the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. It covers sections 3.8.2 (solar system and orbital motion) and 3.8.3 (red shift and the expanding universe). The qualification is untiered. Section 3.8.3 is for the full Physics award only.

Use the course hub for the other topics and the printable checklist to tick off each outcome. For recall and testing, use the revision notes, the practice questions and the 10-minute diagnostics. Revise the wave equation in the waves guide and resultant force in the Newton’s laws guide.

What this unit covers

Section	What you must be able to do
3.8.2 a	Describe the solar system and compare planets, moons, the Sun, comets and asteroids by relative size and motion
3.8.2 b	State what the universe is made of: galaxies of stars, and our Milky Way
3.8.2 c	Distinguish planets, moons, and artificial satellites in geostationary and low polar orbits
3.8.2 d, e	Explain that gravity is the centripetal force, and why it changes velocity but not speed
3.8.2 f, g	Link orbital speed and orbital radius, and apply this to placing satellites
3.8.3 a	Describe the Doppler effect for light, sound and microwaves
3.8.3 b	Explain red shift and how it supports the Big Bang model
3.8.3 c, d	State what CMBR is, and the age of the universe

The specification says the equation for centripetal force is not required, so orbits are explained in words. The calculation tool you need is speed = distance / time.

The solar system (3.8.2 a)

The Sun is a medium-sized star. It holds almost all the mass of the solar system. Eight planets orbit the Sun, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four nearest are small and rocky. The four furthest are much larger.

Smaller objects also orbit the Sun.

Object	Size	Motion
Sun	A star, far larger than anything else in the solar system	Centre of the solar system; the planets orbit it
Planet	Large and roughly spherical	Orbits the Sun on a near-circular path
Moon	Smaller than the planet it orbits (Earth’s Moon is about a quarter of Earth’s diameter)	Orbits a planet, and travels round the Sun with it
Dwarf planet	Smaller than a planet, for example Pluto or Ceres	Orbits the Sun
Asteroid	Small rocky lumps, mostly in a belt between Mars and Jupiter	Orbit the Sun
Comet	Small balls of ice and dust	Orbit the Sun on long, stretched-out paths, passing close to the Sun only briefly

In a “describe the differences” answer, give a size point and a motion point for each body, and name what it orbits.

Galaxies and the Milky Way (3.8.2 b)

The universe contains thousands of millions of galaxies. Each galaxy contains thousands of millions of stars. Our Sun is just one of the many thousands of millions of stars in our own galaxy, the Milky Way. Distances between galaxies are given in light-years. One light-year is the distance light travels in one year: 3.00 × 10⁸ × 365.25 × 86 400 = 9.47 × 10¹⁵ m.

Natural and artificial satellites (3.8.2 c)

A satellite is any object that orbits a larger one. A moon is a satellite that formed naturally around a planet. Artificial satellites are made by people and launched to orbit the Earth. The specification names two orbits.

Orbit	Description	Typical uses
Geostationary	Above the equator, about 36 000 km above the surface, moving in the same direction as Earth spins. One orbit takes 24 hours, so it stays above the same point on the ground	Communications, satellite television
Low polar	Much lower, a few hundred kilometres up, passing over the north and south poles. One orbit takes roughly an hour and a half	Weather monitoring, mapping, observing the whole surface

The Earth turns beneath a polar satellite, so each orbit sees a fresh strip of ground and the whole surface is covered over time. A dish pointing at a geostationary satellite never has to move.

Worked example 1: speed from circumference and period

A satellite in a low polar orbit has an orbital radius of 6.78 × 10⁶ m, measured from the centre of the Earth. It takes 5560 s for one orbit. Find its speed and the number of complete orbits in one day.

1. Distance in one orbit = 2πr = 2π × 6.78 × 10⁶ = 4.26 × 10⁷ m.
2. Speed = distance / time = 4.26 × 10⁷ / 5560 = 7660 m/s (3 s.f.).
3. Orbits in a day = 86 400 / 5560 = 15.5, so 15 complete orbits.

Worked example 2: the Moon

The Moon orbits at a radius of 3.84 × 10⁸ m and takes 27.3 days. Convert the time: 27.3 × 86 400 = 2.36 × 10⁶ s. Distance = 2π × 3.84 × 10⁸ = 2.41 × 10⁹ m. Speed = 2.41 × 10⁹ / 2.36 × 10⁶ = 1020 m/s (3 s.f.).

The satellite is moving about seven and a half times faster than the Moon. Section 3.8.2 f explains why.

Gravity as the centripetal force (3.8.2 d, e)

A body moving in a circle needs a resultant force towards the centre. That inward force is the centripetal force. It is not a new kind of force. For planets and satellites it is the force of gravity. The Sun’s gravity keeps planets in orbit, a planet’s gravity keeps its moons and satellites in orbit, and the Earth’s gravity does this for the Moon.

The key explanation, in steps:

1. Gravity pulls towards the middle of the orbit, at right angles to the direction of travel.
2. Velocity is speed in a given direction. A force at right angles changes the direction but does not speed the body up or slow it down.
3. A change in velocity is an acceleration, so the body accelerates towards the centre.
4. The speed stays constant while the velocity changes all the time, so the body follows a curve instead of a straight line.

With no gravity, the body would carry on in a straight line at constant speed.

Orbital speed and orbital radius (3.8.2 f, g)

The pull of gravity decreases as the separation of the two masses increases. Further out, the centripetal force is smaller. A smaller inward force can only keep a body on a circle if it moves more slowly. So:

• a larger orbital radius goes with a lower orbital speed
• a smaller orbital radius goes with a higher orbital speed

For any particular radius there is one speed that gives a stable orbit. If the speed changes, the stable radius changes too.

Worked example 3: comparing two circular orbits

Satellite A orbits at 6.78 × 10⁶ m (example 1). Satellite B orbits at 1.25 × 10⁷ m and takes 13 900 s for one orbit.

1. Distance = 2π × 1.25 × 10⁷ = 7.85 × 10⁷ m.
2. Speed = 7.85 × 10⁷ / 13 900 = 5650 m/s (3 s.f.).
3. B has the larger radius and the lower speed (5650 m/s compared with 7660 m/s), and it takes longer for one orbit.

This matches the rule: further from the centre, weaker gravity, lower orbital speed.

Placing a satellite

To put a satellite in a chosen orbit, its speed has to be the speed for that radius. At the wrong speed it cannot stay on that circle, and its radius changes. A geostationary satellite must be at the height where its period is 24 hours, and that fixes its speed.

The Doppler effect (3.8.3 a)

If a wave source moves relative to an observer, the observed wavelength and frequency change. This is the Doppler effect. It happens with sound, light and microwaves.

Source is…	Observed wavelength	Observed frequency
Moving away	Increases	Decreases
Moving towards	Decreases	Increases

A siren sounds higher in pitch as the vehicle approaches and drops as it moves off.

Worked example 4: a siren

A siren emits 700 Hz. Sound travels at 340 m/s in air. A stationary listener hears 760 Hz as the vehicle approaches and 650 Hz as it leaves. Find the wavelength heard in each case, using wave speed = frequency × wavelength.

1. Emitted: 340 / 700 = 0.486 m.
2. Approaching: 340 / 760 = 0.447 m (shorter).
3. Receding: 340 / 650 = 0.523 m (longer).

Red shift and the Big Bang (3.8.3 b)

Atoms emit and absorb light at fixed wavelengths, which show up as lines in the light from a galaxy. For most distant galaxies, every line is found at a longer wavelength than in the laboratory. The whole pattern moves towards the red end. This is red shift, the Doppler effect for light from a source moving away.

The observations:

• Light from most distant galaxies is red-shifted.
• The further away the galaxy, the larger the increase in wavelength.
• A larger red shift means a faster speed of recession, so the further away the galaxy, the faster it is moving away.

Worked example 5: reading the evidence

A line has a wavelength of 434.0 nm in the laboratory. It is measured in four galaxies.

Galaxy	Distance (million light-years)	Recession speed (km/s)	Observed wavelength (nm)
P	40	900	435.3
Q	90	2000	436.9
R	150	3300	438.8
S	260	5700	442.2

Shifts: P 1.3 nm, Q 2.9 nm, R 4.8 nm, S 8.2 nm. The shift grows steadily with distance, and so does the speed. Every galaxy is moving away from us, and the most distant is moving fastest.

This suggests that space itself is expanding, not that we are at the centre. Every galaxy moves away from every other, and more distant ones have more space between them to stretch. Dots on an inflating balloon model this.

If space is expanding now, then in the past everything was closer together. The Big Bang model says the universe started as one tiny point and has been expanding since. Red shift supports it, because it shows the expansion that the model predicts.

A strong answer states that distant galaxies show red shift, links bigger shift to greater distance and speed, and concludes that space is expanding from a small starting point.

CMBR and the age of the universe (3.8.3 c, d)

Cosmic microwave background radiation (CMBR) is electromagnetic radiation, in the microwave region, that fills the whole universe. It is left over from radiation that existed soon after the universe began. The specification states that no other current theory explains CMBR, only the Big Bang theory.

So there are two separate pieces of evidence:

Evidence	What it shows
Red shift of distant galaxies	The universe is expanding
CMBR	Radiation left over from soon after the beginning

Scientists believe the universe started in a big bang about 14 thousand million years back, which is 1.4 × 10¹⁰ years.

Common errors

• Saying the speed of a planet in a circular orbit changes. The speed is constant. The velocity changes.
• Writing that gravity “pulls the satellite forward”. It acts towards the centre, at right angles to the motion.
• Saying a geostationary satellite is not moving. It orbits once every 24 hours.
• Mixing up the Doppler effect for approaching and receding sources. Approaching: shorter wavelength, higher frequency.
• Saying galaxies “explode outwards from a point in space”. The model is that space itself expands.
• Quoting the age of the universe as 14 million years. It is 14 thousand million.

The exam preparation page covers general technique.

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.