AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    The temperature of a body increases when the body absorbs radiation faster…

    Full specification wording

    Perfect black bodies and radiation: 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.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 3 of 6
    • 2 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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.

    What earns the marks

    • Stating that the temperature rises when the rate of absorption is greater than the rate of emission.
    • Stating that the internal energy of the body increases.
    • Stating that the rate of emission increases as the body becomes hotter.
    • Concluding that the temperature becomes constant once the two rates are equal.

    Common mistakes and how to put them right

    • 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.

    Exam tips

    • 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.

    Try the lesson

    The start of the Perfect black bodies and radiation: The temperature of a body increases when the body absorbs radiation faster than it emits radiation. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A metal block absorbs infrared radiation at a rate of 40 W and emits infrared radiation at a rate of 25 W. What happens to the temperature of the block?

    Listen

    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. A strong answer earns marks for these points. stating that the temperature rises when the rate of absorption is greater than the rate of emission. stating that the internal energy of the body increases. stating that the rate of emission increases as the body becomes hotter. concluding that the temperature becomes constant once the two rates are equal. Exam tip: Write the chain in three steps: unequal rates, internal energy change, new balance.

    See it

    A vertical flowchart with eight boxes connected by downward arrows. The first box says the body absorbs radiation faster than it emits. An arrow leads to a box saying there is a net transfer of energy into the body, then to a box saying internal energy increases, then to a box saying temperature rises. From there an arrow leads to a box saying the rate of emission increases as the body gets hotter, then to a box saying the gap between the two rates narrows, then to a box saying the rates become equal, and finally to a box saying the temperature settles at a steady value.
    It shows how a body whose absorption rate exceeds its emission rate warms up until the two rates become equal and the temperature settles at a steady value.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Rate of emission increases as the body gets hotter
    2. 2. Temperature settles at a steady value
    3. 3. Gap between the two rates narrows
    4. 4. Body absorbs radiation faster than it emits
    5. 5. Net transfer of energy into the body
    6. 6. Rates become equal
    7. 7. Internal energy increases
    8. 8. Temperature rises

    Key words

    Say what each word means before you turn the card.

    net absorption

    Try answering aloud or in your head before turning the card.

    self limiting heating

    Try answering aloud or in your head before turning the card.

    See every definition
    net absorption
    The overall transfer of energy into a body when it absorbs radiation faster than it emits radiation.
    self limiting heating
    The process in which a body getting hotter increases its own rate of emission, so the gap between the two rates narrows.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart with eight boxes connected by downward arrows. The first box says the body absorbs radiation faster than it emits. An arrow leads to a box saying there is a net transfer of energy into the body, then to a box saying internal energy increases, then to a box saying temperature rises. From there an arrow leads to a box saying the rate of emission increases as the body gets hotter, then to a box saying the gap between the two rates narrows, then to a box saying the rates become equal, and finally to a box saying the temperature settles at a steady value.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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