Perfect black bodies and radiation — AQA GCSE Physics
Test yourself on Perfect black bodies and radiation with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Perfect black bodies and radiation explained
Every body emits radiation across a whole range of wavelengths, not at a single one, and a graph of intensity against wavelength for a black body is a curve with a peak.
Read the full explanation
Temperature controls that curve in two ways. Raise the temperature and the intensity increases at every wavelength, so the whole curve lifts and the area beneath it grows. Raise the temperature and the peak also shifts towards shorter wavelengths. That is why an electric hob glows dull red before it glows orange, why blue-white stars are hotter than red ones, and why objects at room temperature look dark to the eye: their peak lies in the infrared, so we detect them with a thermal camera rather than by sight.
(HT only) A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation.
This is Higher tier only. A body at a steady temperature has not stopped exchanging radiation; it is absorbing and emitting at equal rates, so there is no net transfer and the internal energy stays constant. Both rates are large, not zero. A mug of tea left on a table cools at first because it emits faster than it absorbs, and as it cools its rate of emission falls until the two rates match at room temperature. From then on the mug still emits infrared and still absorbs infrared from the room, in equal measure. This is what is meant by being in thermal equilibrium with the surroundings, and answers that say the object stops emitting score nothing.
The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
The direction of a temperature change is set by which rate is bigger. If a body absorbs radiation faster than it emits it, there is a net transfer of energy into the body, its internal energy increases and its temperature rises. The process is self limiting: as the body gets hotter its own rate of emission increases, so the gap between the two rates narrows until they are equal and the temperature settles at a new steady value. That is why a car parked in sunlight becomes hot but does not keep heating for ever, and why a metal cube in front of an infrared heater reaches a steady temperature. Reverse the inequality and the temperature falls instead.
(HT only) The temperature of the Earth depends on many factors including: the rates of absorption and emission of radiation, reflection of radiation into space.
This is Higher tier only. The Earth absorbs mostly short wavelength radiation from the Sun and emits longer wavelength infrared out to space, because the Earth is far cooler than the Sun. Its mean temperature is steady only while those two rates are equal. Three factors therefore matter. First, the rate at which radiation is absorbed by the surface and the atmosphere. Second, the rate at which infrared is emitted back into space. Third, the fraction reflected straight back into space without ever being absorbed, mainly by clouds, ice and snow. Greenhouse gases such as carbon dioxide (CO₂) and methane absorb outgoing infrared and re-emit some of it downwards, cutting the rate of loss to space.
(HT only) Students should be able to explain how the temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted, using everyday examples to illustrate this balance, and the example of the factors which determine the temperature of the Earth.
This is Higher tier only, and it is one argument applied to many contexts. Always name the two rates, say which is larger, deduce the direction of the temperature change, and finish by saying what happens as the temperature changes. A person standing in sunlight warms because they absorb faster than they emit; a parked car reaches a steady temperature when the rates match; a hot drink cools because it emits faster than it absorbs. For the Earth, the mean temperature is steady while absorbed solar radiation equals emitted infrared. Add more greenhouse gas and less infrared escapes, so absorption temporarily exceeds emission and the surface warms until the balance is restored at a higher temperature.
(HT only) Students should be able to use information, or draw/ interpret diagrams to show how radiation affects the temperature of the Earth’s surface and atmosphere.
This is Higher tier only. The standard diagram shows arrows whose widths represent rates of energy transfer: short wavelength radiation arriving from the Sun, part of it reflected straight back to space by clouds, snow and ice, the rest absorbed by the surface and the atmosphere. The warmed surface then emits long wavelength infrared, some of which escapes and some of which is absorbed by greenhouse gases and re-emitted in all directions, including back down. To interpret a diagram, check each arrow for its direction, its wavelength label and its size, then total the energy in and the energy out: equal totals mean a steady temperature, and an excess arriving means the surface warms.
Your focus
- Describe the two changes to a black body intensity against wavelength curve when the temperature rises.
- Identify the hotter of two bodies from their emission curves and justify it.
- Explain why a heated filament glows red before it glows white.
Show all 18 objectives
- Explain why a flask of hot water loses energy fastest in the first minute of cooling and more slowly later.
- Explain why an object sitting at room temperature is still emitting radiation.
- Predict the temperature change of a body when its two rates are given as numbers.
- Predict the temperature change of a body from its stated rates of absorption and emission.
- Explain why a body warming in sunlight eventually reaches a steady temperature.
- Link a rise in internal energy to an increase in the rate at which a body emits radiation.
- Name three distinct factors that determine the mean surface temperature of the Earth.
- Explain how an increase in the concentration of carbon dioxide raises the Earth's temperature.
- Distinguish radiation reflected into space from radiation absorbed and re-emitted by the atmosphere.
- Explain the temperature change of an everyday object by comparing its rates of absorption and emission.
- Apply the radiation balance to the Earth to account for a rise in mean surface temperature.
- Predict what happens to a body's temperature after the incoming radiation is suddenly reduced.
- Label an energy flow diagram of the Earth with incoming short wavelength, reflected and outgoing long wavelength radiation.
- Use the values given on such a diagram to decide whether the Earth's temperature is rising.
- Explain from a diagram how greenhouse gases raise the surface temperature without any change in the Sun's output.
Perfect black bodies and radiation exam tips
Marking Points
- one mark for stating that all bodies emit radiation over a range of wavelengths
- one mark for stating that the intensity emitted at every wavelength increases as the temperature increases
- one mark for stating that the peak of the distribution moves to shorter wavelengths as the temperature increases
- one mark for identifying the hotter of two plotted curves and justifying the choice
- one mark for stating that the rate of absorption of radiation equals the rate of emission
- one mark for stating that there is therefore no net transfer of energy, so the temperature stays constant
- one mark for stating that the object continues to emit and to absorb rather than stopping
- one mark for applying the balance to a named situation, such as an object left at a steady room temperature
- one mark for stating that the temperature rises when the rate of absorption is greater than the rate of emission
- one mark for stating that the internal energy of the body increases
- one mark for stating that the rate of emission increases as the body becomes hotter
- one mark for concluding that the temperature becomes constant once the two rates are equal
- one mark for naming the rate of absorption of radiation from the Sun as a factor
- one mark for naming the rate of emission of infrared radiation from the Earth into space as a factor
- one mark for naming the fraction of radiation reflected back into space, by clouds or by ice, as a factor
- one mark for stating that carbon dioxide is a greenhouse gas that absorbs infrared, so the Earth's temperature rises
- one mark for comparing the rate of absorption with the rate of emission in the situation described
- one mark for deducing the direction of the temperature change from that comparison
- one mark for explaining that the temperature becomes steady again when the two rates become equal
- one mark for applying the argument to the Earth, naming greenhouse gases absorbing the outgoing infrared
- one mark for identifying incoming short wavelength solar radiation and outgoing long wavelength infrared on the diagram
- one mark for identifying radiation reflected back into space without being absorbed
- one mark for showing greenhouse gases absorbing infrared and re-emitting some of it back towards the surface
- one mark for using the values on the diagram to state whether the temperature will rise, fall or stay constant
Examiner Tips
- 💡Describe both changes, the height of the curve and the position of its peak, for full marks.
- 💡Use 'intensity' for energy emitted per second per unit area rather than vague words like amount.
- 💡When sketching a hotter curve, keep it above the cooler one everywhere.
- 💡Use the word rate in both halves of the sentence; 'absorbs as much as it emits' is not enough.
- 💡This statement is Higher tier only, so expect it in an explain question worth two or three marks.
- 💡Finish by saying what that means for the internal energy of the body.
- 💡Write the chain in three steps: unequal rates, internal energy change, new balance.
- 💡Say which rate is greater rather than simply saying the object gets hotter.
- 💡Use 'net' when describing the overall transfer of energy.
- 💡A list of factors must contain genuinely different factors, not the same idea twice.
- 💡Name the greenhouse gas rather than writing 'gases in the atmosphere'.
- 💡This statement is Higher tier only; expect it alongside the radiation balance argument.
- 💡The word balance in a question is a signal to compare rates, not amounts.
- 💡Finish the answer by saying what happens once the balance is restored.
- 💡This is Higher tier only, and usually carries three or four marks, so write the full chain.
- 💡Label every arrow with its direction and whether it is short or long wavelength.
- 💡Add up the values entering and leaving before writing any conclusion.
- 💡This is Higher tier only; diagrams are often marked for arrow direction as much as for wording.
Common Mistakes
- saying a hotter body emits only shorter wavelengths, rather than more at every wavelength
- reading the peak as the only wavelength the body emits
- claiming that objects at room temperature emit no radiation because they do not glow
- comparing two curves by their height at a single wavelength only
- saying an object at constant temperature has stopped emitting radiation
- comparing total energies rather than rates of emission and absorption
- assuming equal rates require the object and its surroundings to be different temperatures
- confusing a constant temperature with there being no energy transfer of any kind
- predicting that the temperature will rise without limit
- comparing the total energy absorbed and emitted instead of the rates
- saying the body emits nothing while it is warming up
- reversing the condition and claiming the temperature rises when emission exceeds absorption
- saying greenhouse gases reflect the outgoing infrared rather than absorbing and re-emitting it
- confusing the greenhouse effect with damage to the ozone layer and ultraviolet
- treating the Earth's temperature as set only by the output of the Sun
- claiming that more reflection into space would warm the Earth
- describing temperatures rather than rates of absorption and emission
- forgetting that the body keeps emitting radiation while it is warming
- asserting that the Earth's temperature rises without saying which rate exceeds which
- using the word heat loosely instead of naming the radiation absorbed and emitted
- drawing the outgoing radiation at the same wavelength as the incoming radiation
- drawing re-emission from greenhouse gases only downwards, never upwards
- leaving the reflected arrow out when totalling the energy arriving and leaving
- concluding the temperature is constant when the numbers on the arrows do not balance