OCR GCSE Physics A: Radiation from bodies, the solar system and orbits (J249) – Revision Notes

These notes condense statements P8.3d to P8.3h of OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), first assessment 2018. All five statements are Higher Tier only. For full explanations and worked examples, read the study guide. Then test yourself with the practice questions. Planning links: the OCR GCSE Physics hub, the printable checklist and the free 10-minute diagnostics.

Key ideas at a glance

Statement	One-line summary
P8.3d	All bodies emit radiation. Hotter means greater intensity and a distribution shifted to shorter wavelengths
P8.3e	Eight planets, minor planets, moons, artificial satellites. Geostationary and polar orbits differ
P8.3f	Gravity acts towards the centre, so velocity changes while speed stays constant
P8.3g	A stable orbit has one speed for its radius. Change the speed and the radius must change
P8.3h	Temperature depends on absorbed radiation against emitted radiation. Equilibrium means they are equal

P8.3d: emitted radiation (Higher Tier only)

• Every body emits electromagnetic radiation at every temperature.
• The emission is a continuous range of wavelengths, not one value.
• Raise the temperature and:
  • the total energy emitted per second per square metre (intensity) increases
  • the strongest emission moves to a shorter wavelength
• Cool objects: mainly infrared. Very hot objects: visible light, then bluer light.
• Heated metal: no glow, then dull red, orange, yellow-white.

Sketch to remember: two smooth humps on one axis. The hot one is taller and sits further left. The cool one is lower and sits further right.

Method for a two-mark comparison: one mark for intensity (hotter body, greater), one for wavelength (hotter body, strongest emission at shorter wavelength).

P8.3e: solar system facts (Higher Tier only)

Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Feature	Inner planets	Outer planets
Names	Mercury, Venus, Earth, Mars	Jupiter, Saturn, Uranus, Neptune
Make-up	Rocky, solid surface	Gas giants and ice giants
Size and density	Smaller, denser	Larger, less dense
Moons	None to two	Many

• Minor planets: smaller bodies orbiting the Sun, for example in the asteroid belt between Mars and Jupiter. Dwarf planets such as Pluto and Ceres.
• Moon: a natural satellite of a planet.
• Artificial satellite: a human-made body in orbit. Gravity is the inward force on both types.

Orbit	Height and period	Position	Uses
Geostationary	About 36 000 km above the surface, 24 hours	Above the equator, appears fixed in the sky	Communications, television, weather
Polar	Low, around one and a half hours	Passes over both poles, Earth turns below	Mapping, environment monitoring, weather, surveillance

Similar to natural satellites: held by gravity, same laws. Different: built and launched, small, chosen orbit, working life.

P8.3f and P8.3g: circular orbits (Higher Tier only)

Remember the chain:

1. Gravity acts towards the centre.
2. Gravity is at right angles to the velocity.
3. So the direction changes, but not the speed.
4. Changing velocity is an acceleration, so a resultant force is needed.

Quantity	Constant in a circular orbit?
Speed	Yes
Direction	No
Velocity	No
Resultant force (size)	Yes
Resultant force (direction)	No, always towards the centre

Stable orbit rules (qualitative):

• Lower orbit: stronger gravity, faster speed, shorter period.
• Higher orbit: weaker gravity, slower speed, longer period.
• A satellite that speeds up moves into a larger orbit radius. A satellite that slows moves into a smaller one.
• A stable circular orbit cannot keep the same radius if its speed changes.

Useful relation from speed = distance / time: orbital speed = 2πr / T.

Worked reminder. The Moon orbits at radius 3.84 × 10⁸ m, period 27.3 days. T = 27.3 × 86 400 = 2.359 × 10⁶ s. Speed = 2π × 3.84 × 10⁸ / 2.359 × 10⁶ = 1020 m/s.

P8.3h: radiation balance (Higher Tier only)

Situation	Result
Absorbed greater than emitted	Warms
Emitted greater than absorbed	Cools
Absorbed equals emitted	Constant temperature (equilibrium)

• A warmer body emits more, so heating stops rising once emission matches absorption.
• Net power = absorbed power − emitted power. Energy = power × time.
• Dark, matt surfaces absorb well. Light, shiny surfaces reflect.

Earth’s temperature depends on:

• radiation arriving from the Sun
• the fraction reflected by clouds, ice and light surfaces
• infrared emitted by the surface and atmosphere
• atmosphere composition: ozone absorbs ultraviolet; greenhouse gases absorb infrared from the surface and re-emit it in all directions

Method for a greenhouse-gas question: (1) more greenhouse gas absorbs more outgoing infrared, (2) less escapes to space, so emitted is less than absorbed, (3) Earth warms, (4) it emits more until the balance returns at a higher temperature.

Must-know distinctions

Pair	Difference
Speed and velocity	Speed has size only. Velocity has size and direction
Intensity and wavelength	Intensity is how much energy is emitted per second per square metre. Wavelength says where in the spectrum the emission is strongest
Absorbed and reflected	Absorbed radiation raises the energy of the body. Reflected radiation does not
Emitted and reflected	Emitted radiation comes from the body’s own temperature. Reflected radiation just bounces off
Moon and artificial satellite	Natural against built and launched
Geostationary and polar	One fixed point, 24 hour period, higher; whole surface over a day, low, short period
Ozone and greenhouse gases	Ozone absorbs ultraviolet from the Sun. Greenhouse gases absorb infrared from Earth

Short worked examples

Orbit speed from data. Radius 4.22 × 10⁷ m, period 86 400 s.

1. Circumference = 2π × 4.22 × 10⁷ = 2.652 × 10⁸ m
2. Speed = 2.652 × 10⁸ / 86 400 = 3070 m/s

Comparing two orbits without numbers. A is closer to the planet than B. Gravity at A is stronger, so A needs a higher speed to stay in a stable circular orbit, and A completes an orbit in a shorter time.

Heating balance. A drinks can in sunshine absorbs 25 W and emits 10 W. Net gain = 15 W. In 4 minutes it gains 15 × 240 = 3600 J. It keeps warming until emission is 25 W.

Explaining a glow. Question: why does a hotter filament look whiter? Answer: higher temperature moves the strongest emission to shorter wavelength, so the visible output includes more of the shorter (blue and green) wavelengths as well as red, and the intensity is greater.

Quick self-test

1. State what happens to the wavelength of strongest emission when a body is heated.
2. Why does a metal rod glow red before white?
3. Name the four outer planets in order from the Sun.
4. State one similarity and one difference between a moon and an artificial satellite.
5. Why can a geostationary satellite stay above one place on Earth?
6. A satellite orbits with radius 8.0 × 10⁶ m and period 7100 s. Calculate its speed to 3 significant figures.
7. Explain why a planet in a circular orbit is accelerating at constant speed.
8. A satellite speeds up and keeps a stable orbit. What must happen to its radius?
9. A roof absorbs 400 W and emits 340 W. Calculate the energy gained in 5 minutes.
10. Explain why extra carbon dioxide warms Earth until a new equilibrium.

Answers

1. It gets shorter.
2. At lower temperature the strongest emission is at long wavelength (infrared) with only a little red light. As temperature rises, the distribution moves to shorter wavelengths, so more yellow, green and blue light is added and the glow turns white.
3. Jupiter, Saturn, Uranus, Neptune.
4. Similarity: both held in orbit by gravity. Difference: a moon is natural; an artificial satellite is built and launched.
5. Its period is 24 hours, it orbits above the equator and in the same direction as Earth spins.
6. Circumference = 2π × 8.0 × 10⁶ = 5.027 × 10⁷ m. Speed = 5.027 × 10⁷ / 7100 = 7080 m/s.
7. Its direction changes, so its velocity changes; a change in velocity is an acceleration, caused by the resultant force of gravity towards the centre.
8. The radius must increase (a larger orbit).
9. Net power = 60 W. Energy = 60 × 300 = 18 000 J.
10. More infrared from the surface is absorbed and re-emitted, so less escapes, emission falls below absorption, the surface warms and emits more, and balance returns at a higher temperature.

Where marks are usually lost

• Writing “the speed changes direction”. Speed has no direction; say velocity.
• Leaving out which way gravity acts. State it acts towards the centre of the orbit.
• Giving only one half of a two-part comparison of hot and cool bodies.
• Saying “heat” is emitted instead of radiation.
• Forgetting that the new equilibrium on a warming Earth is at a higher temperature, not a return to the old one.
• Mixing up geostationary and polar orbits. Geostationary: fixed above the equator. Polar: over the poles and covers the whole surface.
• Not converting days or hours to seconds before using speed = distance / time.
• Describing a satellite’s orbit as a straight line because “speed is constant”. Constant speed in a circle still means a changing velocity.
• Treating the Sun as a planet. It is a star, and the planets orbit it.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), first assessment 2018, published by OCR.