Physics · 2 Thermal physics

2.3.3 Radiation

Before you start

Thermal energy moves from a hotter place to a cooler one, and conduction (2.3.1) and convection (2.3.2) both pass it on through particles.
Light from the Sun crosses empty space to reach us. Infrared is another region of the same electromagnetic spectrum (studied fully in 3.3).
Power is energy transferred per unit time, $P = Δ E / t$ , measured in watts: $1$ W is $1$ J per second (1.7.4).
Specific heat capacity, $c = Δ E / (m Δ θ)$ , for one worked example (2.2.2).
Melting and boiling take in energy without a change in temperature (2.2.3).

Syllabus objectives covered (9)

Know that thermal radiation is infrared radiation and that all objects emit this radiation
Know that thermal energy transfer by thermal radiation does not require a medium
Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of infrared radiation
Know that for an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy
Know what happens to an object if the rate at which it receives energy is less or more than the rate at which it transfers energy away from the object
Know how the temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted from the Earth’s surface
Describe experiments to distinguish between good and bad emitters of infrared radiation
Describe experiments to distinguish between good and bad absorbers of infrared radiation
Describe how the rate of emission of radiation depends on the surface temperature and surface area of an object

Part 1 of 8

Thermal radiation is infrared, and every object emits it

You have met two ways thermal energy moves: conduction and convection. Both pass energy along through particles. Thermal radiation is the third way, and it works differently: the energy is carried by a wave.
That wave is infrared radiation, one region of the electromagnetic spectrum — the same family as light, radio waves and X-rays. In this topic, 'thermal radiation' and 'infrared radiation' mean the same thing.
All objects emit infrared radiation, all the time. Not only hot ones: a cup of tea, your hand, a wall and a block of ice are all emitting infrared right now.
What changes with temperature is how much. A hotter object emits infrared at a greater rate than a cooler one. A thermal imaging camera uses this: it detects infrared, so a warm body shows up clearly against cooler surroundings, even in the dark.
The trap here is 'cold objects do not emit radiation'. They do. An ice cube emits at a lower rate than your hand, but it still emits. 'All objects' means all objects.
Every object also absorbs some of the infrared that falls on it. So at any moment an object is both emitting and absorbing. Whether it warms up or cools down depends on which rate is bigger — you will come back to that balance later in this lesson.

Part 2 of 8

Radiation does not need a medium

Conduction needs particles that vibrate and pass energy to their neighbours. Convection needs a liquid or gas that can flow. Take away all the particles — a vacuum — and both of them stop.
Infrared radiation is an electromagnetic wave, and electromagnetic waves do not need a medium. They travel through a vacuum.
This is how energy from the Sun reaches the Earth. The space between them is almost a vacuum, so conduction and convection cannot carry energy across it. Radiation can.
When you explain this, give both halves: radiation does not need a medium (it can travel through a vacuum), and conduction and convection need particles. The contrast is what makes it an explanation rather than a bare statement.
'Does not require a medium' means it does not need one. Infrared also travels through air, and through some transparent materials. Do not write that radiation can only travel through a vacuum.

Part 3 of 8

Black or white, dull or shiny

When infrared falls on an opaque surface, it is either absorbed (the surface gains energy and warms up) or reflected (it bounces off). Infrared that is reflected is not absorbed, so a good reflector is a poor absorber.
Two features of the surface control how the infrared is split between these: its colour (black or white) and its texture (dull, also called matt, or shiny).
Colour: black surfaces are better absorbers and better emitters of infrared than white surfaces. White surfaces are better reflectors.
Texture: dull surfaces are better absorbers and better emitters than shiny surfaces. Shiny surfaces are better reflectors.
Put the two together. Dull black is the best absorber and the best emitter, and the worst reflector. Shiny white or shiny silver is the worst absorber and the worst emitter, and the best reflector.
Notice that absorbing and emitting go together. A good absorber is also a good emitter; a poor absorber is also a poor emitter. You need one list, not two.
Examples in sunlight: a dull black car seat gets much hotter than a white one, because it absorbs more of the Sun's radiation. A white roof keeps a house cooler, because it reflects more of the Sun's radiation and absorbs less.
The trap is thinking a shiny surface is a good emitter because it looks bright. It looks bright because it reflects well. Reflecting well and emitting well go in opposite directions.
Avoid 'black attracts heat'. Nothing is attracted. Write that black is a good absorber of infrared radiation.

Part 4 of 8

Experiment: telling good emitters from bad

Start from what could spoil the comparison. The rate of emission depends on the temperature of the surface as well as on its colour and texture. So the surfaces you compare must be at the same temperature, and only the surface may differ.
A Leslie cube solves this. It is a hollow metal cube whose vertical faces have different surfaces — for example, one face dull black and another shiny silver. Fill it with hot water and every face is in contact with the same water, so the faces are at the same temperature.
Place an infrared detector a fixed distance from one face, pointing straight at it, and record its reading. A larger reading means the face is emitting infrared at a greater rate.
apparatus diagram — not drawn yet
A hollow metal cube on a heat-proof mat, filled with hot water, with one visible face labelled 'dull black' and another labelled 'shiny silver'; an infrared detector connected to a meter points at the dull black face from a marked distance d.
Same water, so same temperature; same distance d for every face. Only the surface changes.
Turn the cube so that each face in turn is the same distance from the detector, and record each reading. Work quickly, so the water barely cools between readings.
Result: the dull black face gives the highest reading and the shiny silver face the lowest. Dull black is the best emitter; shiny silver is the worst.
The variables you keep the same, and can name in an answer: the temperature of the faces (same hot water), the distance from detector to face, the detector itself, and the area of each face (equal on a cube).
A second method: take two identical metal cans, one painted dull black and one shiny silver. Fill both with the same volume of hot water from the same kettle, at the same starting temperature, and put on identical lids. Record the temperature of each every minute.
The can whose temperature falls faster is the better emitter: the dull black one. Both cans also lose energy by conduction and convection, but they are identical apart from the surface, so the difference in how fast they cool comes from the surface.

Part 5 of 8

Experiment: telling good absorbers from bad

Now the surfaces are cool, and they must receive infrared at the same rate. You compare how quickly each one warms up.
Take two identical metal plates — same size, thickness and metal. Paint the front of one dull black and polish the front of the other to shiny silver. Stand them facing an electric radiant heater, one on each side, at the same distance from it.
Fix a drawing pin to the back of each plate with the same small amount of wax. Switch on the heater and time how long each pin takes to fall. A pin falls when its plate is hot enough to melt the wax.
apparatus diagram — not drawn yet
Top view: an electric radiant heater in the centre, with two identical vertical metal plates at equal distances d on either side, their front faces towards the heater, one labelled 'dull black' and one labelled 'shiny silver'; a drawing pin held by a blob of wax on the back of each plate.
Equal distances, so each plate receives infrared at the same rate. The plate that absorbs better warms faster.
Result: the pin on the dull black plate falls first. Dull black absorbs infrared at a greater rate, so its temperature rises faster and its wax melts sooner. The shiny silver plate reflects much of the infrared falling on it, so it warms more slowly.
The variables you keep the same: the distance from the heater, the size, thickness and metal of the plates, the amount of wax, and the starting temperature of the plates.
A second method: two identical cans of cold water, one dull black and one shiny silver, the same distance from the heater. Record the temperature of each every minute. The water in the dull black can warms faster.
Do not mix the two experiments up. For emitters, the surfaces are hot and you measure what they give out. For absorbers, the surfaces are cool and you measure how fast they heat up.

Part 6 of 8

What sets the rate of emission: surface temperature and surface area

Colour and texture are not the only things that decide how fast an object emits radiation. Two more matter.
Surface temperature. The higher the temperature of the surface, the greater the rate at which it emits radiation. A hot radiator emits more energy each second than an identical cool one.
Surface area. The larger the surface area, the greater the rate of emission, because there is more surface emitting. Two objects at the same temperature with the same surface finish: the one with twice the surface area emits at twice the rate.
This is why a part that must lose energy quickly, such as an air-cooled motorbike engine, has metal fins: they give it a much larger surface area. (A large area also helps it lose energy by convection.)
It works the other way too. An animal that curls up in the cold reduces the surface area it exposes, so it emits radiation at a lower rate.
Be careful with temperature. Doubling a temperature in °C does not double the rate of emission. At this level you need only the direction: hotter surface, greater rate of emission.

Part 7 of 8

Constant temperature means balanced rates

Every object is receiving energy (by absorbing radiation, and perhaps by conduction or convection too) and transferring energy away, all the time. What happens to its temperature depends on how those two rates compare.
Constant temperature: the object transfers energy away at the same rate as it receives energy. Energy still flows in and out. Nothing has stopped; in each second, energy in equals energy out.
Receives energy at a greater rate than it transfers energy away: it gains energy overall, so its temperature rises.
Receives energy at a smaller rate than it transfers energy away: it loses energy overall, so its temperature falls.
One exception you know from 2.2.3: while a substance is melting or boiling, it takes in energy with no change in temperature. Away from a change of state, the three rules above hold.
Rates are powers, in watts, so the difference between them is the net power. An object that receives $50$ W and transfers away $35$ W gains $15$ J every second. Using $Δ E = P t$ , in $60$ s it gains:
$Δ E = (50 - 35) \times 60 = 15 \times 60 = 900 J$
Now link this to surface temperature. Suppose an object starts to receive more energy than it transfers away. Its temperature rises — and a hotter surface emits radiation at a greater rate. So the outgoing rate climbs.
The outgoing rate keeps climbing until it equals the incoming rate. Then the temperature stops rising and stays constant at a new, higher value. The same reasoning in reverse explains why a cooling object levels off at a lower temperature.
graph diagram — not drawn yet
Temperature (°C) against time (s): a curve starting at a low temperature, rising steeply at first, then rising more slowly and levelling off to a horizontal line; the horizontal part labelled 'rate of energy received = rate of energy transferred away'.
The curve flattens because the hotter object emits faster, until out equals in.

Part 8 of 8

The Earth's temperature: the same balance on a planet

The Earth follows the same rule as any object. It receives energy by radiation from the Sun, and it loses energy by radiation into space. Its average temperature depends on how those two rates compare.
Not all the incoming radiation is absorbed. Some is reflected straight back into space by clouds, ice, snow and other light-coloured surfaces. The rest is absorbed, mostly by the surface, which warms up.
The Earth's surface, like every object, emits infrared radiation. Some of this infrared passes out through the atmosphere into space.
Some is absorbed by greenhouse gases in the atmosphere — carbon dioxide, methane and water vapour. These gases then re-emit infrared in all directions, so some of it goes back towards the surface instead of into space.
ray diagram — not drawn yet
The Sun on the left sending arrows of incoming radiation to the Earth; one arrow reflected back to space from a cloud or ice; outgoing infrared arrows leaving the Earth's surface, some passing into space and some absorbed in an atmosphere layer labelled 'greenhouse gases', with re-emitted arrows pointing both up to space and down to the surface.
In: radiation absorbed from the Sun. Out: infrared that escapes to space. Temperature is constant when the two rates are equal.
If the Earth radiates energy into space at the same rate as it absorbs energy from the Sun, its average temperature stays constant.
More greenhouse gas: less of the surface's infrared escapes into space each second. The Earth now absorbs energy faster than it loses it, so its temperature rises. The hotter surface emits at a greater rate, until the rates balance again — at a higher temperature.
Less ice and snow: less of the Sun's radiation is reflected, so more is absorbed. The Earth gains energy faster than it loses it, and its temperature rises. More ice and snow would do the opposite.
A change in the Sun's output: more incoming radiation raises the Earth's temperature; less incoming radiation lowers it.
Clouds work on both sides: they reflect some incoming radiation back into space, and they also absorb some of the infrared from the surface and re-emit part of it back down.
The trap: greenhouse gases do not reflect infrared back to the Earth. They absorb it and then re-emit it in all directions. Use those two words.

Worked examples

Example 1. Describe an experiment to show that a dull black surface is a better emitter of infrared radiation than a shiny silver surface.

Show the working

Decide first what must be the same. The rate of emission depends on temperature as well as on the surface, so the two surfaces must be at the same temperature. Only the surface may differ.
Choose apparatus that makes this happen: a Leslie cube with one dull black face and one shiny silver face, filled with hot water from a kettle. Both faces touch the same water, so they are at the same temperature.
Say what you measure, and how: place an infrared detector a fixed distance, say $10$ cm, from the dull black face and record the reading.
Turn the cube so the shiny silver face is the same distance from the same detector, and record the reading. Do it quickly, so the water has hardly cooled.
Finish with the result and what it shows. A larger reading means a greater rate of emission, so the dull black face's higher reading shows it is the better emitter.

Answer: Fill a Leslie cube (one face dull black, one shiny silver) with hot water so both faces are at the same temperature. Place an infrared detector a fixed distance from the dull black face and record the reading, then turn the cube so the shiny silver face is the same distance from the detector and record again, working quickly. The dull black face gives the higher reading, so it is the better emitter.

Example 2. A space probe of mass $5.0$ kg is in sunlight, far from any planet. It absorbs radiation from the Sun at a rate of $450$ W and emits infrared radiation at a rate of $300$ W. (a) State and explain what happens to the temperature of the probe. (b) The probe is made of a material with specific heat capacity $900$ J/(kg °C). Assuming both rates stay the same for the next $60$ s, calculate the rise in temperature of the probe in that time. (c) Explain why the temperature of the probe does not keep rising.

Show the working

First notice where the probe is. Space has almost no particles to conduct or convect, so radiation is the only way energy reaches the probe or leaves it. The two rates given are the whole picture.
(a) Compare the rates. Energy in at $450$ W is greater than energy out at $300$ W, so the probe gains energy overall and its temperature rises.
(b) The net rate of gain is the difference between the two rates. $150$ W means $150$ J gained every second.
$P_{net} = 450 - 300 = 150 W$
Multiply by the time to get the energy gained, using $Δ E = P t$ .
$Δ E = 150 \times 60 = 9000 J$
Rearrange $c = Δ E / (m Δ θ)$ to make $Δ θ$ the subject: multiply both sides by $Δ θ$ , then divide both sides by $c$ .
$Δ θ = \frac{Δ E}{m c} = \frac{9000}{5.0 \times 900} = 2.0^{\circ} C$
(c) As the probe warms, its surface temperature rises, so it emits radiation at a greater rate. The rate of emission climbs towards $450$ W. When it reaches $450$ W, energy out equals energy in and the temperature stays constant.

Answer: (a) The temperature rises, because the probe receives energy ( $450$ W) at a greater rate than it transfers energy away ( $300$ W). (b) $2.0$ °C. (c) A hotter surface emits radiation at a greater rate, so the rate of emission rises until it equals $450$ W; then the temperature stays constant.

Example 3. A vacuum flask has a double glass wall with a vacuum between the two walls. The glass surfaces facing the vacuum are silvered, so they are shiny. Explain why the vacuum alone would not stop a hot drink in the flask from cooling, and how the silvered surfaces reduce energy transfer by radiation.

Show the working

Start with what the vacuum does. It has no particles, so energy cannot cross the gap by conduction or by convection.
Now the gap in that protection: radiation does not need a medium. The hot inner wall still emits infrared, which can cross the vacuum and be absorbed by the outer wall. So a vacuum alone would still let the drink cool by radiation.
The silvered surface of the inner wall is shiny, so it is a poor emitter. It emits infrared at a lower rate than a dull surface would.
The silvered surface of the outer wall is a good reflector. Much of the infrared that does cross the gap is reflected back towards the drink instead of being absorbed by the outer wall.

Answer: The vacuum stops conduction and convection, but infrared radiation needs no medium and can still cross it. The shiny silvered inner surface is a poor emitter, so less infrared is emitted, and the shiny silvered outer surface is a good reflector, so much of the infrared that crosses is reflected back towards the drink.

Example 4. Explain, using the idea of a balance between incoming and outgoing radiation, how an increase in the amount of carbon dioxide in the atmosphere causes the Earth's average temperature to rise.

Show the working

Begin with the balance. The Earth's average temperature is constant when it radiates energy into space at the same rate as it absorbs energy from the Sun.
Name what is radiated: the Earth's surface emits infrared radiation.
Say what carbon dioxide does to it. Carbon dioxide is a greenhouse gas: it absorbs some of this infrared and re-emits it in all directions, so some goes back towards the surface.
With more carbon dioxide, less infrared escapes into space each second. The Earth now absorbs energy at a greater rate than it radiates energy away, so its temperature rises.
Finish the story. The warmer surface emits infrared at a greater rate, until the outgoing rate matches the incoming rate again. The balance is restored at a higher temperature.

Answer: The Earth's surface emits infrared. Carbon dioxide absorbs some of it and re-emits it in all directions, some back to the surface, so with more carbon dioxide less energy escapes into space each second. The Earth then absorbs energy faster than it radiates it away, and its temperature rises until the hotter surface emits enough for the rates to balance again, at a higher temperature.

Where people go wrong

Mistake: Saying only hot objects emit thermal radiation.

Instead: All objects emit infrared radiation. A hotter surface emits at a greater rate, but a cold one still emits.

Emission depends on temperature for its rate, not for whether it happens at all. The objective states 'all objects'.

Mistake: Calling a shiny surface a good emitter because it looks bright.

Instead: Shiny (and white) surfaces are poor emitters and poor absorbers, and good reflectors. Dull black is the best emitter and absorber.

A shiny surface looks bright because it reflects. Infrared that is reflected is not absorbed, and a poor absorber is also a poor emitter.

Mistake: Thinking an object at constant temperature has no energy going in or out.

Instead: Energy is still transferred in and out. The temperature is constant because the two rates are equal.

Every object is always absorbing and emitting radiation. A constant temperature is a balance, not a standstill.

Mistake: Saying a vacuum stops all thermal energy transfer.

Instead: A vacuum stops conduction and convection only. Radiation needs no medium and crosses a vacuum.

Conduction and convection need particles; infrared is an electromagnetic wave. That is why a vacuum flask also needs silvered surfaces.

Mistake: Saying greenhouse gases reflect infrared back to the Earth.

Instead: Greenhouse gases absorb some of the infrared emitted by the Earth's surface and re-emit it in all directions, some back towards the surface.

Reflection and absorption are different processes, and in this topic the keyword must be used in its correct scientific sense.

Check yourself

A block of ice at $- 5$ °C is in a freezer. Does it emit thermal radiation? Explain.
Answer
Yes. All objects emit infrared (thermal) radiation. The ice emits at a lower rate than a warmer object would, but it does emit.
Explain why energy from the Sun reaches the Earth by radiation and not by conduction or convection.
Answer
The space between the Sun and the Earth is almost a vacuum. Conduction and convection need particles, but thermal radiation is infrared, an electromagnetic wave, which does not need a medium.
Two identical cans of water, one dull black and one shiny white, stand in strong sunshine. Which can's water warms faster, and which surface is the better reflector?
Answer
The water in the dull black can warms faster, because dull black is the better absorber of the Sun's radiation. The shiny white surface is the better reflector.
An object receives energy at a rate of $40$ W and transfers energy away at a rate of $55$ W. State what happens to its temperature, calculate the net energy it loses in $10$ s, and state the rate at which it would have to transfer energy away for its temperature to stay constant.
Answer
Its temperature falls, because it transfers energy away faster than it receives it. Net energy lost $= (55 - 40) \times 10 = 150$ J. For a constant temperature it would have to transfer energy away at $40$ W, the same rate as it receives energy.
Two copper blocks have the same surface finish and are both at $80$ °C. Block X has twice the surface area of block Y. Compare the rates at which they emit radiation, and state one change to block Y that would make it emit at a greater rate.
Answer
Block X emits at twice the rate of block Y, because it has twice the surface area at the same temperature with the same surface. Raising the temperature of block Y (or making its surface dull black) would increase its rate of emission.
In a Leslie cube experiment, why is the cube filled with hot water rather than each face being heated separately?
Answer
So that every face is at the same temperature. The rate of emission depends on temperature as well as on the surface, so only the surface may differ for a fair comparison.
Two identical metal plates, one dull black and one shiny silver, stand the same distance from a radiant heater, each with a drawing pin waxed to its back. Which pin falls first, and why?
Answer
The pin on the dull black plate. Dull black is a better absorber of infrared, so that plate's temperature rises faster and its wax melts first.
State two changes that would make the Earth's average temperature rise, and for each say how it affects the balance of radiation.
Answer
Any two from: more greenhouse gas in the atmosphere (less of the surface's infrared escapes into space); less ice and snow cover (less of the Sun's radiation is reflected, so more is absorbed); an increase in the Sun's output (more incoming radiation). Each makes the Earth absorb energy faster than it radiates energy into space.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student has a hollow metal cube. One vertical face is painted dull black and the opposite face is polished shiny silver. She also has a kettle of hot water and an infrared detector connected to a meter.

(a) Describe how she could use this apparatus to find which surface is the better emitter of infrared radiation. 4 marks No calculator

Mark scheme

B1 fill the cube with hot water (so that both faces are at the same temperature)
B1 place the detector a fixed distance from one face and record the meter reading
B1 turn the cube so the other face is the same distance from the detector and record the reading
B1 any one from: readings taken quickly so the water temperature hardly changes / detector pointed at the centre of each face / same detector for both faces / the larger reading shows the better emitter

(b) Predict which face gives the larger reading. 1 mark No calculator

Mark scheme

B1 the dull black face

Question 2. Two identical metal plates stand upright, one on each side of an electric radiant heater and at the same distance from it. The face of one plate towards the heater is dull black; the face of the other is shiny silver. A drawing pin is fixed to the back of each plate with a small blob of wax.

(a) The heater is switched on. Explain which drawing pin falls first. 2 marks No calculator

Mark scheme

B1 the pin on the dull black plate
B1 dull black is a better absorber of infrared (radiation), so that plate's temperature rises faster and its wax melts first; ORA for shiny silver

(b) Suggest why the plates are placed at the same distance from the heater. 1 mark No calculator

Mark scheme

B1 so that both plates receive infrared at the same rate, and any difference is caused only by the surface, owtte

(c) Identify one other variable that must be kept the same for a fair comparison. 1 mark No calculator

Mark scheme

B1 any one from: amount of wax / size or thickness of the plates / metal the plates are made of / starting temperature of the plates / type of drawing pin

Question 3. A space probe is in orbit around the Sun, far from any planet. It absorbs radiation from the Sun and emits infrared radiation into space.

(a) Explain why energy can be transferred to and from the probe only by radiation. 2 marks No calculator

Mark scheme

B1 space is a vacuum / has no particles (accept almost no particles)
B1 conduction and convection need particles (a medium), but (infrared) radiation does not need a medium

(b) At one moment the probe absorbs energy at a rate of $600$ W and emits energy at a rate of $450$ W. Assuming these rates stay constant, calculate the net energy the probe gains in $2.0$ minutes. 2 marks

Mark scheme

C1 (600 − 450) × 120 or 150 × 120 seen
A1 18 000 J (accept 1.8 × 10⁴ J or 18 kJ); unit required

(c) The temperature of the probe rises at first and then becomes constant. Explain why. 2 marks No calculator

Mark scheme

B1 as its temperature rises, the probe emits radiation at a greater rate
B1 the temperature becomes constant when the rate of emission equals the rate of absorption (600 W)

Question 4. The average temperature of the Earth depends on the balance between the radiation it absorbs from the Sun and the radiation emitted from its surface that escapes into space.

(a) Explain how an increase in the concentration of carbon dioxide in the atmosphere leads to an increase in the Earth's average temperature. 3 marks No calculator

Mark scheme

B1 the Earth's surface emits infrared (radiation)
B1 carbon dioxide / greenhouse gas absorbs (some of) this infrared and re-emits it, some back towards the surface; 'reflects' is insufficient
B1 so energy is radiated into space at a lower rate than it is absorbed (from the Sun) / less infrared escapes into space, and the temperature rises, owtte

(b) Large areas of ice melt, uncovering darker sea and land. Suggest how this affects the Earth's average temperature. 2 marks No calculator

Mark scheme

B1 less of the Sun's radiation is reflected / the darker surface absorbs more radiation
B1 so the Earth absorbs energy at a greater rate than it radiates energy away, and its temperature increases

How this is examined

Describing an experiment that compares two surfaces as emitters, or as absorbers, as a fair test: a 4-mark Describe built around named apparatus — a Leslie cube and an infrared detector for emitters, or two plates either side of a radiant heater for absorbers — which may be followed by a 1-mark Predict of the result One mark for each step a reader could repeat: (1) how the surfaces are set up — for emitters, at the same temperature (hot water in the cube); for absorbers, the same distance from the heater; (2) what is measured — the detector reading at the same distance from each face, or the time for the wax to melt and the pin to fall; (3) a further variable kept the same, by name; (4) the result — dull black gives the higher reading, or its pin falls first. 'Make it a fair test' with no variable named is insufficient.
Why the Earth's temperature rises when the balance of radiation is tipped: a 3-mark Explain about more greenhouse gas, or a 2-mark Suggest about a change such as melting ice Three links in order: the Earth's surface emits infrared; greenhouse gases (carbon dioxide, methane, water vapour) absorb some of it and re-emit it, some back towards the surface; so energy is radiated into space at a lower rate than it is absorbed, and the temperature rises. For melting ice: less radiation is reflected, so more is absorbed. Write 'absorb and re-emit', not 'reflect'.
Comparing the rate at which an object receives energy with the rate at which it transfers energy away: a 1-mark State of what happens to the temperature, a 2-mark Calculate of net energy using $P = Δ E / t$ , or a 2-mark Explain of why a temperature levels off Equal rates: constant temperature. Receives faster: temperature rises. Transfers away faster: temperature falls. In a calculation, C1 for (rate in − rate out) × time seen, A1 for the value with the unit J. For levelling off: 'a hotter surface emits radiation at a greater rate', then 'until the rate out equals the rate in'.
Choosing a surface for a job and saying what it does to infrared: a 2-mark Explain or Suggest about a real object: a roof, a solar water heater, a vacuum flask, a cooling fin Both halves: the surface (dull black, or shiny/white) and its effect on infrared — 'good absorber of infrared', 'poor emitter', 'good reflector'. Naming the colour alone is insufficient; 'black attracts heat' is wrong science.
Why radiation is the way energy crosses a vacuum: a 1- or 2-mark State or Explain about the Sun's energy reaching the Earth, a space probe, or the gap in a vacuum flask 'Thermal radiation is infrared, an electromagnetic wave, and does not need a medium', together with the contrast 'conduction and convection need particles, and a vacuum has none'.

Words to know

Thermal radiation: Transfer of thermal energy by infrared radiation, an electromagnetic wave; it does not need a medium.
Infrared radiation: The region of the electromagnetic spectrum that all objects emit; a hotter surface emits it at a greater rate.
Medium: A material — solid, liquid or gas — that energy or a wave passes through.
Emitter: A surface giving out radiation. A good emitter gives it out at a high rate; dull black is the best.
Absorber: A surface taking in the radiation that falls on it. A good absorber is also a good emitter.
Reflector: A surface that sends radiation back off it rather than absorbing it; shiny and white surfaces are good reflectors.
Leslie cube: A hollow metal cube with faces of different surfaces, filled with hot water so all faces are at the same temperature; used to compare emitters.
Greenhouse gas: A gas in the atmosphere, such as carbon dioxide, methane or water vapour, that absorbs some of the infrared emitted by the Earth's surface and re-emits it in all directions.
Rate of energy transfer: Energy transferred per second, measured in watts (W).

All Physics lessons Next: 2.3.4 Consequences of thermal energy transfer