Explain Brownian motion as particle-model evidence
Distinguish a suspended microscopic grain from the gas or liquid particles bombarding it.
Core and Extended; Physics 0625, 2026–2028, section 2.1.2. Describe particle arrangement, separation and motion; relate temperature to motion and absolute zero; explain gas collisions and Brownian motion. Forces, force per unit area and precise atom/molecule distinctions are developed in the labelled Supplement lessons. Written practice is self-checked.
Before you begin
- Describe random motion of fluid particles.
- Distinguish a microscopic suspended grain from the surrounding fluid.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does suspension in a liquid mean here?
What you will learn
- Describe irregular random motion of microscopic particles suspended in a fluid.
- Explain it through unequal random collisions and identify what provides evidence.
Observe the suspended object
Microscopic particles suspended in a gas or liquid can show an irregular, continually changing motion called Brownian motion. A smoke particle or suitably small suspended grain is much larger than an individual particle of the surrounding fluid. In an appropriate observation, the microscope shows the suspended object moving, not each individual gas or liquid particle striking it.
Random motion of the suspended particles is evidence for the kinetic particle model because it is consistent with a surrounding fluid made of moving particles. A schematic drawn here is an illustration of the proposed explanation, not a newly measured observation. Actual observation and interpretation require suitable apparatus and teacher supervision; no practical competence is certified by this written exercise.
Unequal collisions produce changing motion
Surrounding gas or liquid particles strike the suspended object from different directions. At a particular moment the impacts are not exactly balanced, so their combined effect can change the object's motion. The imbalance changes unpredictably, giving irregular changes in direction. It is not necessary for the grain to be alive, to propel itself or to be pushed by a visible hand.
A steady current or convection can also move suspended material, but directed bulk drift alone is not the required random Brownian-motion evidence. If a supplied record shows all grains travelling persistently in one direction, investigate bulk flow rather than automatically labelling that record Brownian motion. Random jitter and an overall drift can coexist; distinguish the two described components.
Pause and explain
All grains drift steadily right in a flowing liquid. Is that directed drift alone evidence of Brownian motion?
Worked example
A microscopic grain in still liquid moves irregularly and changes direction. Explain its motion and what the observation supports.
Show the worked solution
- Surrounding moving liquid particles collide with the grain.
- The instantaneous combined impacts are unequal and continually change.
- The grain's random motion supports the kinetic particle model; the grain is not one liquid molecule.
Answer Changing unequal collisions cause random motion, supporting the moving-particle model.
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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.
Explain random motion of a microscopic grain suspended in still liquid.
Give me a hint
Identify the object observed and the surrounding particles that collide with it.
Compare my reasoning
- The microscopic grain is the observed suspended object.
- Moving liquid particles collide with it from different directions.
- Changing unequal impacts produce irregular changes in its motion.
Random collisions by fluid particles produce changing unequal effects on the grain.
Look for these in your work
- Distinguish grain and fluid particles.
- Explain changing direction.
State why observed random grain motion supports the kinetic particle model, and whether a pre-drawn zigzag is itself a new experiment.
Give me a hint
Separate an observation from a model illustration.
Compare my reasoning
- Observed irregular motion is consistent with surrounding fluid particles moving and colliding.
- The observation therefore supports the kinetic model rather than showing every fluid particle directly.
- A pre-drawn zigzag illustrates the account but is not newly recorded experimental evidence.
Observed random motion supports moving fluid particles; the schematic is not a new experiment.
Look for these in your work
- Name the evidence and mechanism.
- State the drawing's limit.
A suspension record shows steady downward drift with irregular sideways jitter. Discuss which part could support Brownian motion and what else needs interpreting.
Give me a hint
Separate the directed component from the changing random component.
Compare my reasoning
- The irregular jitter is consistent with random unequal collisions.
- The steady downward drift needs a separate bulk or settling explanation.
- Do not call all motion Brownian or claim this description alone rules out every other effect.
Random jitter can support Brownian motion; steady drift is a separate component needing interpretation.
Look for these in your work
- Distinguish the components.
- Avoid stronger claims than the record supports.
Common mistakes
- The observed grain is not an individual fluid molecule.
- A drawn random path is not a measured observation.
- Directed drift alone is not Brownian-motion evidence.
What can you explain now?
Why can a suspended grain change direction unpredictably?
Compare with the explanation
The unequal combined effects of surrounding collisions change.
Collisions arrive randomly and need not balance at each instant.
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Make it stick
Describe Brownian evidence, explain unequal collisions and distinguish random jitter from directed bulk motion.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.