Cooling curves – thermal insulation, Newton's law of cooling and freezing
PhysicsThermal Physics & GasesAges 17–18
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Sign in to playA virtual practical that records temperature against time. A: a beaker of hot water wrapped in cotton wool, bubble wrap, newspaper, felt or polystyrene with a chosen number of layers, with or without a lid, and with a black, white or shiny outer surface; read the thermometer every 30 s for 20 minutes, plot the cooling curves and fit T − Tₐ = (T₀ − Tₐ)e^(−t/τ) to compare insulators by their time constant τ. B: cooling curves of stearic acid, salol and paraffin wax in a boiling tube, with a flat section at the freezing point and an estimate of the specific latent heat of fusion.
Lesson: Thermal insulation, Newton's law of cooling, cooling curves and specific latent heat
What it shows
A hot object loses energy faster when it is much hotter than its surroundings. If the rate of energy loss is proportional to the temperature difference, the temperature difference falls exponentially: this is Newton's law of cooling, with a time constant τ that is larger for better insulation. Insulating materials trap air, a poor conductor, and a lid stops evaporation and convection from the surface. When a pure liquid freezes, its temperature stays constant because latent heat is released as the particles bond into a solid.
How to use
In A. Beaker and insulation, choose Wrap the beaker in, Layers, Outer surface and Lid, then press Start; a reading is taken every 30 s. Press New run to test another set-up, and use Graph to switch between the cooling curves, ln(T − Tₐ) against time and the cooling rate against T − Tₐ. In B. Cooling curve of a pure substance, choose a Substance and Mass and press Start to see the flat section.
Parameters you can change
- Experiment A. Beaker and insulation, B. Cooling curve of a pure substance
- Insulating material No insulation, Cotton wool, Bubble wrap, Newspaper, Felt, Polystyrene
- Number of layers 1–5
- Lid on
- Outer surface As it is, Matt black paint, White paint, Shiny aluminium foil
- Starting water temperature 50–95 °C
- Room temperature 10–30 °C
- Substance in the tube Stearic acid, Salol, Paraffin wax
- Mass of substance 5–25 g
- Cooled in Air, Beaker of water
Questions to explore
- Which variables must you keep the same so that the insulators are compared fairly?
- Why does a lid reduce the cooling rate so much more than one layer of newspaper?
- Why does the temperature of stearic acid stay at about 69 °C while it freezes?
