The shape of the lesson
By the end of the lesson, learners can
- Set the current turning: Heat a fluid from below and watch the warm fluid rise and the cool fluid sink into a circulating current.
- Driven by density: Every part of a convection current comes back to one cause: differences in density.
- Heated fluid expands and so becomes less dense.
- Less dense fluid rises; cooler, denser fluid sinks to replace it.
- The loop this forms is the convection current. It needs a fluid, so not in solids.
One picture every idea returns to
Convection is the transfer of thermal energy through a fluid by the movement of the fluid itself. Heated fluid expands, becomes less dense and rises, while cooler, denser fluid sinks, setting up a convection current.
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, Hot fluid rises, cool fluid sinks.. Watch dust drift above a radiator and you are seeing air on the move. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Set the current turning | Heat a fluid from below and watch the warm fluid rise and the cool fluid sink into a circulating current. Project tab 5 of the lab (Convection) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Driven by density | Every part of a convection current comes back to one cause: differences in density. | Jigsaw |
| 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 Driven by density, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| Heat does not simply rise on its own. | The heated fluid expands and becomes less dense, and it is pushed upward by the denser, cooler fluid sinking around it. The cause is the difference in density. |
Convection: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Define convection, and state which states of matter it can occur in.
Explain why a region of fluid that has been heated rises.
Explain why convection cannot take place in a solid.
State two everyday situations in which energy is transferred by convection.
A heater for a room is usually placed near the floor rather than near the ceiling. Explain why this helps to warm the whole room.
A student says "hot air rises because heat goes up." Give a more precise physics explanation of why the warm air rises.
Answer key and marking notes
- Convection is the transfer of thermal energy by the movement of the fluid itself. ✓
- It occurs in liquids and gases (fluids). ✓
- The heated fluid expands, so the same mass takes up more volume. ✓
- This makes it less dense than the surrounding cooler fluid. ✓
- Being less dense, it rises (floats up) through the denser fluid. ✓
- In a solid the particles are held in fixed positions and cannot move from place to place. ✓
- So the material cannot flow to carry energy, which is what convection requires. ✓
- Water being heated in a pan or kettle (warm water circulates). ✓
- A room being warmed by a radiator (warm air rises and circulates). ✓
- Air warmed near the floor expands, becomes less dense and rises. ✓
- Cooler air sinks to take its place and is warmed in turn, setting up a convection current. ✓
- This current circulates warm air around the whole room. ✓
- The warm air rises because it has expanded and become less dense than the cooler air around it. ✓
- The denser, cooler air sinks and pushes the less dense warm air upward. ✓
3 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.
Water is heated at the bottom of a beaker. Why does a convection current form?
- AWarm water expands, becomes less dense and rises, while cooler denser water sinks to take its place.
- BWarm water becomes denser and sinks.
- CThe water particles are destroyed at the bottom.
- DCold water rises because it is lighter.
Why is the heating element of an electric kettle placed near the bottom?
- ASo that convection currents carry the warm water upward and heat all the water.
- BSo the kettle looks neat.
- CBecause water is denser at the top.
- DTo stop the water from boiling.
Convection can transfer thermal energy through which of the following?
- Aliquids and gases only
- Bsolids only
- Ca vacuum
- Dsolids and a vacuum
Quiz key
Heated water expands, so its density falls and it rises; cooler, denser water sinks, and the cycle sets up a convection current. Option B has the density change backwards.
Heating at the bottom lets the warmed water rise and cooler water sink, circulating energy through all the water. Heating at the top would leave the lower water cold, since warm water does not sink.
Convection needs a fluid whose particles can move and carry energy, so it occurs in liquids and gases only. It cannot happen in a solid (fixed particles) or in a vacuum (no particles).
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.