Trapped!
Learning Objectives
Students will be able to:
- Explain how filters trap particles from the air
- Compare filters with different-sized holes
- Predict which filter will catch smaller particles
- Explain why we need filters with very small holes for PM2.5
The Big Question
"How do filters trap tiny particles from the air?"
Show students a dirty furnace filter or vacuum filter. "Look at all the stuff that got trapped! Where do you think this came from? All of this was floating in the air!"
The dark, dusty stuff on a used filter is proof that there are particles floating in our air all the time. The filter caught them before they could get into our lungs!
A filter is like a net that catches particles while letting air pass through.
The key is the SIZE of the holes:
- Big holes = catches big particles, misses small ones
- Medium holes = catches medium and big particles
- Tiny holes = catches almost everything!
Screen Door
Big holes - stops flies and bugs, but not dust or pollen
Regular Filter
Medium holes - stops dust and lint, but not tiny particles
HEPA Filter
Tiny holes - stops 99.97% of particles, including PM2.5!
Materials:
- Different "filter" materials: window screen, paper towel, coffee filter, fabric
- Different sized particles: dried beans, rice, sand, and flour
- Funnels or cut-off bottles
- Containers to catch what passes through
Steps:
- Predict: Which filter will stop which particles?
- Test: Pour each particle type through each filter material
- Observe: What got caught? What passed through?
- Record: Fill in the data table
Data Table
| Filter Type | Beans | Rice | Sand | Flour |
|---|---|---|---|---|
| Window Screen | ||||
| Paper Towel | ||||
| Coffee Filter | ||||
| Fabric |
Mark: Stopped / Passed Through / Some Passed
MERV Ratings
Real air filters have a rating called MERV (Minimum Efficiency Reporting Value). Higher MERV = catches smaller particles!
| MERV Rating | What It Catches | Where It's Used |
|---|---|---|
| MERV 1-4 | Pollen, dust mites, spray paint | Basic home filters |
| MERV 5-8 | Mold spores, pet dander | Better home filters |
| MERV 9-12 | Fine dust, auto emissions | Good quality filters |
| MERV 13-16 | Bacteria, smoke, PM2.5! | Hospitals, clean rooms |
HEPA Filters
HEPA (High Efficiency Particulate Air) filters are even better! They catch 99.97% of particles 0.3 micrometers or larger. That's tiny PM2.5 particles and even some viruses!
Filters Have a Problem:
Better filters (smaller holes) also make it harder for air to pass through. It's like trying to drink a milkshake through a coffee stirrer vs. a regular straw!
Solution: We use fans to push or pull air through the filter. More fan power = more air through a tight filter!
- Why didn't any filter stop the flour? What would we need?
- If you had allergies, what MERV filter would you want in your home?
- Why do we need to replace filters sometimes?
- What happens if a filter gets too clogged?
Record and answer:
- Draw how a filter works (show particles getting caught)
- Which filter in our experiment caught the most particles? Why?
- Why do we need filters with very small holes to catch PM2.5?
- What's the trade-off between catching more particles and airflow?
Key Takeaways
- Filters trap particles while letting air pass through
- Smaller holes catch smaller particles
- MERV ratings tell us how well a filter works (higher = better)
- HEPA filters catch 99.97% of particles - including PM2.5!
- Better filters need more fan power to push air through
Filter
A material with small holes that catches particles while letting air or liquid pass through.
MERV Rating
A number that tells how well an air filter works. Higher = catches smaller particles.
HEPA
High Efficiency Particulate Air - a very fine filter that catches 99.97% of tiny particles.
Airflow
The movement of air through a space or filter.