Represent vibrating particles in a solid
Connect close particles and fixed relative positions to solid shape and volume.
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
- Recall a solid's characteristic shape and volume.
- Distinguish whole-object motion from internal particle motion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which property normally distinguishes a solid from a liquid?
What you will learn
- Describe solid particles as close together and vibrating about fixed positions.
- Interpret a simple solid particle diagram without treating the particles as motionless.
Fixed positions do not mean no motion
In the simple solid model, particles are close together and vibrate about fixed positions. A particle moves to and fro near its position rather than travelling freely through the sample. The particles normally keep their relative positions, so the solid retains its own shape and approximately fixed volume under the stated ordinary conditions.
The entire solid can move while its particles still vibrate relative to their positions within it. A rolling ball therefore does not need the internal random travel of a gas. Likewise, a stationary block on a table is not evidence that all of its particles are motionless. Describe both the scale and the kind of motion.
A simple diagram is a model
Draw close circles in a compact arrangement and use small two-way arrows to suggest vibration near selected positions. A regular arrangement is a useful simple example of a crystalline solid. It is not a claim that every solid has one perfect regular lattice; the required account is close particles whose relative positions are retained in the solid model.
Circles are symbols for particles, not magnified photographs or measured atom sizes. Do not draw long arrows carrying each particle through the whole container when depicting solid vibration. The diagram's circle count and gaps illustrate a model sample and are not a calibrated density or a complete population count.
Pause and explain
What does 'particles vibrate about fixed positions' mean?
Worked example
A diagram shows close particles with short two-way arrows near fixed positions. Describe its state and motion.
Show the worked solution
- The particles are close together.
- They vibrate near fixed relative positions rather than travelling freely.
- This represents a simple solid model retaining shape and volume.
Answer A solid with particles vibrating about fixed positions.
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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.
Describe a solid's arrangement, separation and motion in the simple particle model.
Give me a hint
Give a separate description for position, closeness and movement.
Compare my reasoning
- Particles are closely arranged.
- Separation is small relative to the widely separated gas model.
- They vibrate about fixed positions rather than moving freely through the sample.
Close particles retaining relative positions and vibrating about them.
Look for these in your work
- Include motion as well as arrangement.
- Do not call the particles motionless.
Sketch a simple crystalline solid using twelve circles and annotate its motion. State one limit of this diagram.
Give me a hint
Use a compact arrangement and short vibration indicators.
Compare my reasoning
- Place twelve close circles in a regular compact group.
- Label short to-and-fro motion about fixed positions.
- State that circles and gaps are schematic, and not all solids have a perfect regular lattice.
A close regular example with fixed-position vibration; a schematic rather than a universal lattice or scale drawing.
Look for these in your work
- Represent the selected solid model.
- State a diagram limitation.
A block is resting on a table. A student erases its vibration arrows because it is not travelling. Correct the reasoning.
Give me a hint
Separate the block's overall rest from internal motion.
Compare my reasoning
- The block has no overall travel relative to the table.
- Its particles can still vibrate about their positions.
- Vibration arrows remain appropriate for the particle model.
Macroscopic rest does not remove particle vibration.
Look for these in your work
- Name both scales.
- Preserve the solid motion model.
Common mistakes
- Fixed positions do not mean motionless particles.
- A simple regular diagram does not describe every solid lattice.
- A diagram is not a photograph or a scale measurement.
What can you explain now?
Can a stationary solid still contain moving particles?
Compare with the explanation
Yes.
Its particles can vibrate even though the object has no overall motion.
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Make it stick
Describe a solid in three particle terms and explain why fixed positions do not mean zero 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.