The shape of the lesson
By the end of the lesson, learners can
- Find the balance point: Adjust how much energy flows in and watch the object settle at the temperature where emission matches absorption.
- Steady, but not still: Equilibrium is a balance of two flows, not the absence of them.
- Warms when energy absorbed per second is greater than energy emitted per second.
- Cools when energy emitted is greater than energy absorbed.
- Hotter emits more, so a disturbed object drifts to a new balance rather than running away.
One picture every idea returns to
An object stays at a constant temperature when the rate at which it absorbs energy equals the rate at which it emits energy. This balance, not a halt in energy flow, is what fixes the temperature. The same idea sets the average temperature of the Earth.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, When in equals out.. An object left in the Sun does not heat up forever. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Find the balance point | Adjust how much energy flows in and watch the object settle at the temperature where emission matches absorption. Project tab 7 of the lab (Energy balance) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Steady, but not still | Equilibrium is a balance of two flows, not the absence of them. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Steady, but not still, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A balanced object has not stopped exchanging energy. | At a constant temperature it is still absorbing and emitting all the time; the two rates are simply equal. Equilibrium means matched flows, not zero flow. |
Energy balance and equilibrium: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State the condition, in terms of energy, for an object to stay at a constant temperature.
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.
For the Earth, state the main source of the energy it absorbs, and the form of the energy it emits to space.
Explain why the Earth's average temperature stays roughly constant over long periods.
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.
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.
Answer key and marking notes
- 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. ✓
- 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). ✓
- Energy absorbed: radiation from the Sun. ✓
- Energy emitted: infrared radiation, given off to space. ✓
- 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. ✓
- 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. ✓
- 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. ✓
4 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which surface emits thermal radiation at the greatest rate when hot?
- AA large matt black surface at a high temperature.
- BA small shiny silver surface at a high temperature.
- CA small white surface at a low temperature.
- DA large shiny surface at a low temperature.
A good emitter of thermal radiation is also a good ......?
- Aabsorber of thermal radiation
- Breflector of thermal radiation
- Cthermal insulator
- Delectrical conductor
An object is hotter than its surroundings. What is true about the thermal radiation involved?
- AIt emits radiation faster than it absorbs it, so it cools down.
- BIt absorbs radiation faster than it emits it, so it warms up.
- CIt neither emits nor absorbs radiation.
- DIt only absorbs radiation.
The Earth stays at a roughly steady average temperature over time. What does this imply about radiation?
- AThe power the Earth absorbs from the Sun equals the power it radiates back into space.
- BThe Earth absorbs much more than it radiates.
- CThe Earth radiates much more than it absorbs.
- DThe Earth does not radiate at all.
Quiz key
Rate of emission is greatest for a hot surface that is large and matt black, since all three factors increase it. Shiny, small or cooler surfaces (the other options) all emit less.
Matt black surfaces are both the best emitters and the best absorbers of thermal radiation. Shiny surfaces are the good reflectors and poor emitters, which is the opposite case (ruling out B).
A hotter object emits more radiation than it absorbs from its cooler surroundings, so there is a net loss of energy and it cools. It still absorbs some radiation, just less than it emits (ruling out C and D).
A constant average temperature means energy in equals energy out, so the solar power absorbed balances the infrared power radiated back to space. A persistent imbalance (B or C) would steadily heat or cool the planet.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.