Extra · Thermodynamics · Classes 7–9
Convection currents
Objective: To observe convection currents in water using a coloured crystal.
🧰 Apparatus
- Large beaker of water
- Potassium permanganate crystal (or food colour)
- Straw to place the crystal
- Spirit lamp or low-voltage heater
- Tripod and gauze
📐 Formulas and SI units
warm fluid: lower density → rises
Δρ/ρ ≈ βΔT (β_water ≈ 2.1 × 10⁻⁴ /K)
💡 Theory
Heated water near the flame expands and becomes less dense, so it rises; cooler, denser water sinks to take its place. This sets up a circulating convection current. Convection only happens in fluids (liquids and gases) and explains sea breezes and room heaters.
🪜 Procedure
- Fill the beaker with cold water and let it settle.
- Drop a crystal near one edge at the bottom using a straw.
- Heat gently just under the crystal.
- Watch the coloured stream rise, spread and sink on the other side.
- Sketch the path.
📋 Observations Simulated values
✅ Result and conclusion
A 20 K difference makes the warm water about 0.42% less dense; the colour rises above the heat and sinks on the cold side in a loop.
🔬 In a real lab
Uneven heating and stirring mix the colour quickly, so the loop is clearest in the first minute.
⚠️ Safety
Use a spirit lamp only with a teacher; keep sleeves and hair away from flames. Potassium permanganate stains skin.
❓ Quick check
1. Warm water rises because it is…
2. Convection cannot happen in…
3. A room heater is best placed…
🏋️ Practice
EasyWhy is the freezer at the top of a fridge?
Answer: Cold air sinks and cools the whole fridge
MediumWhat causes a sea breeze?
Answer: Land heats faster; warm air rises, cool sea air moves in
ChallengeName the three ways heat travels.
Answer: Conduction, convection, radiation