5
Evaluate
Particle Physics Lab
Duration
50 minutes
Type
Evaluate
Standards
HS-PS2-1, HS-PS2-6, HSF-IF.C.7
Learning Objectives
Students will be able to:
- Design and conduct experiments to measure particle settling velocities
- Compare experimental results to theoretical predictions from Stokes' law
- Quantify particle decay rates in enclosed spaces
- Evaluate the effect of air movement on particle suspension time
- Analyze sources of experimental error and model limitations
Lab Overview
"Measuring Particle Settling Velocity and Validating Stokes' Law"
Background Theory
This lab tests the predictions of Stokes' law for particle settling and applies the well-mixed box model to particle decay in enclosed chambers.
Key Equations
Stokes settling velocity:
vs = (rhop - rhoair) * g * d2 / (18 * eta)
First-order decay:
C(t) = C0 * exp(-lambda * t)
Deposition rate from settling:
lambdad = vs / H (for well-mixed chamber of height H)
Materials
Equipment
- Clear settling column (1-2 m tall)
- PM2.5 sensor (PurpleAir or similar)
- Stopwatch
- Ruler or meter stick
- Enclosed chamber (aquarium or box)
- Small fan (optional)
- Video camera (optional)
Consumables
- Lycopodium spores (~30 um diameter)
- Chalk dust or talc (~10 um)
- Incense or smoke source
- Data recording sheets
- Graph paper
Safety Considerations
Important Safety Notes
- Conduct settling experiments in well-ventilated areas
- Use minimal amounts of test particles
- Avoid inhaling any dust or particles
- Wear dust masks if handling fine powders
- Clean up all particles after experiments
Procedure
Part A: Direct Settling Velocity Measurement
- Set up the settling column vertically with good lighting
- Mark distance intervals on the column (e.g., every 10 cm)
- Release a small puff of lycopodium spores at the top
- Time how long individual particles or the "front" take to fall measured distances
- Calculate vs = distance / time
- Repeat with different particle types (chalk, etc.)
- Compare to Stokes' law predictions
Part B: Chamber Decay Experiment
- Place PM2.5 sensor in enclosed chamber
- Record background concentration
- Introduce particle source (e.g., brief incense burn or particle puff)
- Seal chamber and record PM2.5 every minute for 30+ minutes
- Repeat with small fan running inside chamber
- Compare decay rates between still and mixed conditions
Data Analysis
Required Analysis
Part A: Settling Velocity
- Calculate average settling velocity for each particle type
- Calculate theoretical vs using Stokes' law (assume rhop = 1000 kg/m3 for lycopodium)
- Calculate percent error: |vmeasured - vtheory| / vtheory x 100%
Part B: Decay Rate Analysis
- Plot ln(C/C0) vs. time
- Fit a linear trendline; slope = -lambda
- Calculate half-life: t1/2 = ln(2) / lambda
- Compare lambda between still and fan conditions
Model Validation
- From chamber height H, calculate predicted lambdad = vs / H
- Compare to measured lambda
- Discuss reasons for any discrepancies
Lab Report Requirements
Required Sections
- Abstract (200 words max)
- Introduction with hypothesis
- Materials and Methods
- Results with figures and tables
- Discussion
- Conclusion
- Error Analysis section
Discussion Questions
- How well did Stokes' law predict your measurements?
- Why did the fan change the decay rate?
- What particle sizes would settle too slowly to measure?
- How do these results apply to real indoor environments?
- What are the limitations of the well-mixed assumption?
Assessment Rubric
| Criterion | Excellent (4) | Proficient (3) | Developing (2) | Beginning (1) |
|---|---|---|---|---|
| Experimental Technique | Careful measurements, multiple trials, proper controls | Good technique with minor issues | Some systematic errors | Poor technique, unreliable data |
| Data Analysis | Correct calculations, proper linearization, thorough error analysis | Correct calculations, some error analysis | Some calculation errors | Major calculation errors |
| Model Comparison | Insightful comparison to theory, explains discrepancies | Compares to theory, limited explanation | Basic comparison only | No comparison to theory |
| Scientific Writing | Clear, professional, well-organized | Clear with minor issues | Unclear in places | Difficult to follow |
Extension Activities
Advanced Investigations
- Temperature effect: Compare settling in cold vs. warm conditions (viscosity changes)
- Particle density: Compare settling of particles with different densities (e.g., pollen vs. glass beads)
- Brownian motion observation: Observe milk fat droplets or smoke particles under a microscope
- CFD comparison: Use free CFD software to model air flow in your chamber
Unit Summary
This unit has explored the physics of aerosol particles, from the molecular-scale random motion of Brownian diffusion to the macroscopic fluid dynamics governing air flow in buildings. The lab experiment demonstrates how fundamental physics equations translate to measurable, real-world behavior. Understanding these physical principles is essential for anyone working to improve indoor air quality, design filtration systems, or model aerosol exposure.