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Extended · Practice questions · Energy balance

In equals out, and it holds.

Six original Cambridge-style Extended questions on the condition for a steady temperature, why temperature does not run away, and how Earth's energy budget keeps its average temperature in check.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
The one idea behind every answer

Constant temperature means absorbing rate equals emitting rate.

01
[2 marks]

State the condition, in terms of energy, for an object to stay at a constant temperature.

  • The rate at which it absorbs (gains) energy equals the rate at which it emits (loses) energy. ✓
  • So there is no net energy change, and the temperature stays constant. ✓
02
Analysis
[3 marks]

An object is absorbing energy faster than it is emitting it. Describe what happens to its temperature, and explain why the temperature does not keep rising without limit.

  • It absorbs more than it emits, so its temperature rises. ✓
  • As it gets hotter, it emits radiation more quickly. ✓
  • The temperature stops rising once the emitting rate has risen to equal the absorbing rate (a new balance). ✓
03
[2 marks]

For the Earth, state the main source of the energy it absorbs, and the form of the energy it emits to space.

  • Energy absorbed: radiation from the Sun. ✓
  • Energy emitted: infrared radiation, given off to space. ✓
04
Analysis
[2 marks]

Explain why the Earth's average temperature stays roughly constant over long periods.

  • The rate at which the Earth absorbs radiation from the Sun equals the rate at which it emits infrared to space. ✓
  • With energy in equal to energy out, there is no net change, so the average temperature stays roughly constant. ✓
05
Analysis
[3 marks]

Suppose the rate at which infrared radiation escapes from the Earth to space is reduced, while the radiation arriving from the Sun is unchanged. Explain what happens to the Earth's temperature.

  • Less energy now leaves than arrives, so the Earth absorbs more than it emits. ✓
  • So the Earth warms up. ✓
  • As it warms it emits more, until the outgoing radiation again equals the incoming, at a higher temperature. ✓
06
Analysis
[2 marks]

A black object is left in steady sunlight and reaches a constant temperature. Explain, in terms of absorbing and emitting, why its temperature becomes steady.

  • At first it absorbs energy faster than it emits, so it warms and emits more as it heats up. ✓
  • It becomes steady once it is emitting energy just as fast as it absorbs it. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.