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

This guide covers statements P8.3d, P8.3e, P8.3f, P8.3g and P8.3h of the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), first assessment 2018. They sit in Topic P8.3 Beyond Earth. Every statement here is Higher Tier only. Red-shift, the Big Bang and the formation of stars are taught on separate pages, so they are not repeated here.

Related pages: the OCR GCSE Physics hub, the printable checklist, the revision notes and the practice questions. For circular motion in general, see Newton’s second law, inertia, momentum and circular motion. To find your weak spots first, try the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do	Tier
P8.3d	Explain that all bodies emit radiation, and that the intensity and wavelength distribution depend on temperature	Higher Tier only
P8.3e	Recall the main features of the solar system: planets, moons, artificial satellites	Higher Tier only
P8.3f	Explain how gravity changes the velocity but not the speed of a body in a circular orbit	Higher Tier only
P8.3g	Explain why the radius of a stable orbit must change if the speed changes	Higher Tier only
P8.3h	Explain a body’s temperature as a balance of absorbed and emitted radiation, with Earth as the example	Higher Tier only

P8.3d: all bodies emit radiation (Higher Tier only)

Every object emits electromagnetic radiation, whatever its temperature. You are not a source of visible light, but you emit infrared all the time. A hot object emits a continuous range of wavelengths, not one single value. At each wavelength it emits a different amount of energy, and that pattern is the wavelength distribution of the emission.

Two things depend on temperature.

• Intensity. The hotter the body, the more energy it emits each second from each square metre of its surface. Intensity rises as temperature rises.
• Wavelength distribution. As temperature rises, the whole distribution moves towards shorter wavelengths, so the strongest emission is at a shorter wavelength. A cooler body emits mainly at longer wavelengths.

Everyday evidence helps you describe this. A metal rod heated in a flame first feels hot with no visible glow, because it is emitting infrared. Hotter, it glows dull red, then orange, then yellow-white. The colour changes because the strongest emission has moved to shorter wavelengths and more of the output now falls in the visible range. Stars show the same pattern. A cooler star looks reddish. A hotter star looks blue-white. The Sun’s surface is a few thousand kelvin, and its emission is strongest in the visible part of the spectrum. Earth’s surface is far cooler, so it emits mainly infrared.

A rough sketch in words: draw wavelength across the bottom and intensity up the side. A cool body gives a low, wide hump far to the right (long wavelength). A hot body gives a tall hump further left (short wavelength). Both curves are smooth and continuous. The hot curve is higher at every wavelength.

Worked example. Two lamps, A and B, are heated until A is at a higher temperature than B. State two differences in the radiation they emit.

1. A emits more energy per second from each square metre, so its intensity is greater.
2. The strongest emission from A is at a shorter wavelength than for B.

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

The solar system is the Sun and everything held in orbit around it by gravity. The Sun is a star, and it contains almost all of the mass.

The eight planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Group	Planets	Features
Inner, rocky	Mercury, Venus, Earth, Mars	Small, dense, solid surface, few or no moons
Outer, gas giants and ice giants	Jupiter, Saturn, Uranus, Neptune	Much larger, low density, thick atmospheres, many moons, ring systems

All planets orbit in the same direction, in paths close to one flat plane. Orbits are close to circular for most planets. Minor planets are smaller bodies that also orbit the Sun. Many lie in the asteroid belt between Mars and Jupiter. Dwarf planets such as Pluto and Ceres are examples.

Moons are natural satellites that orbit planets. Earth has one. Mars has two small ones. The large outer planets have many. Artificial satellites are made by people and launched to orbit Earth or another body. Gravity provides the inward force on both natural and artificial satellites.

Similarities and differences between natural and artificial satellites:

• Both stay in orbit because of gravity and follow the same rules.
• Moons formed naturally. Artificial satellites are built, launched and given a chosen orbit.
• Artificial satellites are much smaller, and they can carry instruments, be controlled from Earth and have a limited working life.

The specification names two Earth orbits for artificial satellites.

Orbit	Features	Typical uses
Geostationary	Directly above the equator, one orbit takes 24 hours, same direction as Earth spins, so it stays above one point on Earth. About 36 000 km above the surface	Communications, television, some weather monitoring
Polar	Passes over the north and south poles at a much lower height. Each orbit takes roughly one and a half hours. Earth turns beneath it, so over a day it covers the whole surface	Mapping, monitoring the environment, weather, surveillance

Worked example: orbital speed. The speed of a body in a circular orbit is the circumference divided by the time for one orbit: speed = 2πr / T, where r is the orbit radius measured from the centre of Earth. This is only a use of speed = distance / time.

A polar satellite has orbit radius 7.171 × 10⁶ m and period 6040 s.

• Distance in one orbit = 2π × 7.171 × 10⁶ = 4.506 × 10⁷ m
• Speed = 4.506 × 10⁷ / 6040 = 7460 m/s (3 significant figures)

A geostationary satellite has orbit radius 4.22 × 10⁷ m and period 24 hours = 86 400 s.

• Distance in one orbit = 2π × 4.22 × 10⁷ = 2.652 × 10⁸ m
• Speed = 2.652 × 10⁸ / 86 400 = 3070 m/s (3 significant figures)

The satellite in the lower orbit is moving much faster. This links to P8.3g below.

P8.3f: gravity changes velocity, not speed (Higher Tier only)

Velocity is a vector: it has a size (speed) and a direction. A body in a circular orbit at constant speed is still accelerating, because its direction keeps changing. A change in velocity is an acceleration.

By Newton’s second law, an acceleration needs a resultant force. For a planet or satellite in a circular orbit, that force is gravity, and it acts towards the centre of the orbit. Gravity is at right angles to the direction of motion at every moment. A force at right angles to the motion changes the direction but does not speed the body up or slow it down.

So the explanation has three parts:

1. Gravity acts towards the centre, at right angles to the velocity.
2. It changes the direction of the velocity but not its size.
3. The speed stays the same while the velocity changes continuously, so the body follows a circle.

If gravity switched off, the body would travel in a straight line at constant speed along the tangent to the orbit.

P8.3g: speed and radius of a stable orbit (Higher Tier only)

A stable circular orbit needs the right speed for its radius. Gravity is weaker further from the planet. At a smaller radius gravity is stronger, so the inward force is greater and a higher speed is needed to keep the body moving in a circle instead of falling in.

Three consequences, all qualitative:

• A satellite in a lower orbit travels faster than one in a higher orbit. The numbers in the P8.3e example show it: 7460 m/s at the polar orbit and 3070 m/s at geostationary height.
• If a satellite in a stable orbit speeds up, its orbit radius must change. It will not stay at the same radius. It moves into a higher orbit. A body that slows down moves into a lower orbit.
• Because the higher orbit is also longer, the time for one orbit increases with radius.

Worked example. Satellite A orbits at radius 7.0 × 10⁶ m with period 5830 s. Satellite B orbits at 1.0 × 10⁷ m with period 9950 s. Both orbits are circular and stable.

• Speed of A = 2π × 7.0 × 10⁶ / 5830 = 7540 m/s
• Speed of B = 2π × 1.0 × 10⁷ / 9950 = 6310 m/s

B is further out and slower. If B were given the speed of A, a stable orbit at that radius would no longer be possible, so its radius would have to change.

P8.3h: temperature as a balance of radiation (Higher Tier only)

A body absorbs radiation that arrives and emits radiation of its own. Its temperature depends on the balance of the two.

• Absorbing more than it emits: net energy gain, so the temperature rises.
• Emitting more than it absorbs: net energy loss, so the temperature falls.
• Absorbing and emitting at the same rate: temperature stays constant. This is equilibrium.

As a body warms it emits more, so the emission rate climbs until it matches the absorption rate. Then the temperature levels off.

Everyday examples: a car parked in sunshine warms until the energy it loses matches what it absorbs. A dark surface absorbs more radiation than a light, shiny one, so it ends up warmer. A cup of tea cools because it emits and loses more energy than it absorbs from a room.

Worked example. A metal plate absorbs radiation at 150 W and emits at 90 W.

• Net power = 150 − 90 = 60 W gained
• Energy gained in 10 minutes = 60 × 600 = 36 000 J

The plate warms. As it warms, it emits more. When the emission reaches 150 W, the temperature stops rising.

The Earth example

The factors that fix Earth’s temperature:

• Radiation from the Sun. Earth absorbs some of the incoming radiation, which is mostly visible light with some ultraviolet and infrared.
• Reflection. Clouds, ice and light surfaces reflect part of it back to space, so it is not absorbed.
• Emission. Earth’s surface and atmosphere are much cooler than the Sun, so they emit mainly infrared.
• The atmosphere. It affects the radiation from the Sun passing through it. Ozone absorbs much ultraviolet. Greenhouse gases such as carbon dioxide, methane and water vapour absorb infrared from Earth’s surface and re-emit it in all directions, including back down. Less infrared escapes directly to space.

Worked reasoning: suppose that, out of every 100 units of Sun’s energy reaching Earth, 30 are reflected. Then 70 are absorbed. For a steady temperature Earth must emit 70 units. If extra greenhouse gas means only 66 escape, there is a net gain of 4 units, and Earth warms. As it warms, it emits more, until 70 units escape again. The new equilibrium is at a higher temperature.

Common errors

• Saying the speed of a planet in a circular orbit changes. Speed is constant; the velocity changes.
• Writing that gravity “pulls the satellite forward”. It acts at right angles to the motion, towards the centre.
• Saying cooler objects do not radiate. All bodies emit.
• Describing a hotter body as emitting only shorter wavelengths. It emits a continuous range, with the strongest emission at shorter wavelength.
• Describing the greenhouse effect as the atmosphere “trapping heat” with no mention of absorbed infrared from the surface.
• Saying a geostationary satellite is not moving. It orbits once in 24 hours, in step with Earth’s spin.

Official syllabus

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