The shape of the lesson
By the end of the lesson, learners can
- Follow the random walk: Watch a large visible grain being knocked about by the unseen molecules of the fluid.
- What the jiggle tells us: The motion you can see is indirect evidence for the molecules you cannot.
- Air particles are small, fast, and move randomly.
- They strike a larger smoke particle unevenly from all sides.
- The net push keeps changing, so the smoke particle moves in a random, jerky path.
One picture every idea returns to
Brownian motion is the random, jerky movement of small visible particles suspended in a fluid. It happens because they are constantly struck, unevenly, by the much smaller and faster moving molecules of the fluid around them.
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, Jiggling proof of particles.. Look at smoke under a microscope and the bright specks dance about in tiny, random zig-zags. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Follow the random walk | Watch a large visible grain being knocked about by the unseen molecules of the fluid. Project tab 2 of the lab (Brownian motion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: What the jiggle tells us | The motion you can see is indirect evidence for the molecules you cannot. | Mix-Pair-Share |
| 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, 13 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 What the jiggle tells us, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| The visible grain is not moving under its own power. | It is pushed around by the unseen molecules, and because the collisions arrive unevenly from all sides, the resulting path is random rather than smooth. |
Brownian motion: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
State what the kinetic particle model says about the particles in a gas.
Smoke is placed in a small glass cell and viewed through a microscope. Describe what is seen.
Explain, in terms of particles, why the smoke particles move in this random, jerky way.
In this experiment, state what is actually seen through the microscope, and explain why the air particles themselves are not seen.
State how the motion of the particles in a gas changes when its temperature is increased, and what this means for their kinetic energy.
Explain how the observation of Brownian motion provides evidence for the existence of fast-moving particles in the air.
Answer key and marking notes
- A gas is made of tiny particles that are far apart. ✓
- They are in constant, rapid, random motion in all directions. ✓
- Small bright specks of light (the smoke particles) are seen. ✓
- They move with a random, jerky, zig-zag motion. ✓
- The smoke particles are constantly bombarded by much smaller air particles. ✓
- The air particles move fast and hit from random directions, so the hits are uneven. ✓
- The changing net force pushes the smoke particle a different way each moment, giving a random jerky path. ✓
- What is seen is the larger smoke particles (as points of light). ✓
- The air particles are far too small to be seen, even under the microscope; their presence is only inferred from the motion they cause. ✓
- The particles move faster on average. ✓
- Their average kinetic energy increases. ✓
- The smoke particles could only be pushed about randomly if something kept striking them unevenly. ✓
- This requires many small, fast-moving particles in the air, so the motion is evidence that such particles exist. ✓
2 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.
Large smoke particles viewed under a microscope are seen to move in small, random, jerky paths. What does this provide evidence for?
- ATiny, fast-moving air molecules are colliding with the smoke particles.
- BAir molecules are large and slow.
- CThe smoke particles are alive.
- DThe smoke particles repel one another.
In a Brownian motion experiment, why do we observe the smoke particles rather than the air molecules themselves?
- AThe smoke particles are large enough to see and are pushed about by the unseen air molecules.
- BThe air molecules do not move.
- CThe smoke particles are smaller than the air molecules.
- DThe air molecules glow under the microscope.
Quiz key
The random jerks come from many collisions with unseen, fast-moving air molecules, which is evidence that molecules exist and move randomly. The molecules are far too small to see directly (ruling out B).
Smoke particles are large enough to scatter light and be seen, and their jiggling reveals the impacts of the much smaller, invisible air molecules. The molecules are far too small to observe directly.
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.