Distinguish Brownian grains from atoms and molecules
Name the visible suspended object separately from the light fast particles striking it.
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028, section 2.1.2. Relate state properties to forces, distances and motion, describe collision pressure as force per unit area, and distinguish atoms or molecules from suspended microscopic particles. Core particle descriptions should be studied first. Written practice is self-checked. Human subject-teacher review, supervised practical experience and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Describe Brownian motion and its collision explanation.
- Distinguish a model illustration from recorded experimental evidence.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement distinguishes the two scales in smoke in air?
What you will learn
- Distinguish a microscopic suspended particle from individual atoms or molecules.
- Explain how many light fast molecules can cause an irregular grain motion.
Two different scales share a word
In a smoke-in-air example, the observed microscopic smoke particle is a small grain containing many atoms or molecules. The surrounding air molecules are much smaller and are not individually resolved in the usual school observation. Calling the visible smoke grain 'one air molecule' confuses the suspended object with the fluid particles that strike it.
Use atoms for a supplied monatomic gas model and molecules for a supplied molecular gas or liquid model. 'Microscopic particle' in a suspension description refers to the larger suspended object in this context. The same broad word particle can describe both scales, so clear naming is essential. The drawn large and small circles illustrate this distinction without supplying a measured size ratio.
Many light fast molecules can change grain motion
The surrounding light, fast-moving molecules bombard the suspended grain from many directions. Individual molecules are much lighter than the grain, but their many collisions need not balance at each instant. A changing unequal combined force produces the observed irregular changes in the grain's motion. The grain does not need to be a living object or to carry an internal engine.
Opposite impacts do not necessarily cancel instant by instant; their changing imbalance is the important explanation. Conversely, a whole group drifting steadily due to fluid flow is not sufficient Brownian evidence by itself. This qualitative drawing uses representative impacts and an illustrative grain path, not a count of all collisions, a calibrated molecular-speed distribution or a newly recorded trajectory.
Pause and explain
Why can many light molecules move a much heavier suspended grain irregularly?
Worked example
A microscope shows an irregular smoke-grain path in air. Identify what is observed, what strikes it, and why equal cancellation cannot be assumed at every instant.
Show the worked solution
- The visible object is a microscopic smoke grain containing many constituents.
- Much smaller fast air molecules strike it from varied directions.
- Their momentary combined effects can be unequal and change, giving random grain motion.
Answer Observe the grain; air molecules bombard it with changing unequal combined effects.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Label the observed and colliding objects for smoke Brownian motion in air.
Give me a hint
Use 'grain' for the suspension and 'molecules' for the surrounding molecular gas.
Compare my reasoning
- The observed microscopic object is a smoke grain.
- It contains many microscopic constituents.
- Small air molecules collide with it and are not individually resolved in the usual observation.
Observed smoke grain; surrounding colliding air molecules.
Look for these in your work
- Use distinct names.
- Identify which object is observed.
Explain why a heavier grain can move randomly when struck by many light fast molecules, and why its visible path does not show each molecule directly.
Give me a hint
Combine unequal impacts without inventing a molecular observation.
Compare my reasoning
- Molecules strike repeatedly from different directions.
- Their instantaneous combined effects can be unequal and continually change.
- The observed grain path is indirect evidence for those collisions, not an image of every molecule.
Changing combined impacts move the grain; the grain path is indirect molecular-motion evidence.
Look for these in your work
- Explain the combined effect.
- State the observational limit.
A drawing's large dot is labelled 'one air molecule', and its small dots are labelled 'smoke grains'. Correct the labels and a claim that the drawn path is measured data.
Give me a hint
Match the suspension object and surrounding fluid to their correct scales.
Compare my reasoning
- The large suspended dot should represent a smoke grain.
- The small surrounding dots represent air molecules in this molecular-gas example.
- The illustrative path is not measured data unless an actual observation record is supplied.
Large grain, small air molecules; the schematic path is not a measured trajectory.
Look for these in your work
- Correct both scales.
- Separate illustration from evidence.
Common mistakes
- The visible grain is not one air molecule.
- Small molecule mass does not mean no combined effect.
- Opposing random impacts do not balance at every instant.
What can you explain now?
Are air molecules and the microscopic smoke grain the same object?
Compare with the explanation
No.
The grain is a larger suspended object, while much smaller surrounding molecules collide with it.
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Make it stick
Name both Brownian scales and explain unequal bombardment by light fast molecules without claiming direct molecular observation.
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