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Conduction and convection lab – metal rods, layered walls, convection currents

PhysicsThermal Physics & GasesAges 17–18

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Three virtual experiments on heat transfer. A: heat one end of a rod (copper, aluminium, brass, iron, steel or glass), time the wax-held pins as they drop, and watch the temperature profile and the heat flowing in and out until the steady state P = kAΔT/L. B: a wall of up to three layers in series, with thermal resistance R = e/(λA), heat flow rate Φ = ΔT/R and the temperature at each interface. C: convection currents in a beaker of water with a potassium permanganate crystal and in a smoke box with a candle, timing how long the dye or smoke takes to travel round.

Lesson: Thermal conduction (Fourier's law, thermal resistance) and convection

What it shows

Conduction passes energy along a solid as particles vibrate and, in metals, as free electrons move; the rate through a bar or slab is P = kAΔT/L (Fourier's law), so good conductors have a large thermal conductivity k. Layers in series add their thermal resistances R = L/(kA), just like resistors in a circuit. In a fluid, warm regions expand, become less dense and rise while cooler fluid sinks: a convection current. The model solves heat conduction along the rod and a simplified fluid flow in a 2D slice.

How to use

In A. Conduction along a rod, choose a Material, press Start heating and record when each pin drops; use New run to compare materials, Length L or Diameter d, and untick Lagged rod to see heat lost from the sides. In B. Layered wall, choose each layer and its thickness. In C. Convection, press Burner on and Drop KMnO₄ crystal, or switch Fluid to the smoke box and press Light candle and Add smouldering paper.

Parameters you can change

  • Experiment A. Conduction along a rod, B. Layered wall, C. Convection
  • Rod material Copper, Aluminium, Brass, Iron, Stainless steel, Glass
  • Rod length 20–50 cm
  • Rod diameter 0.5–3 cm
  • Hot-end temperature 40–100 °C
  • Lagged (insulated) rod
  • Thickness of wall layer 2 (insulation) 0–30 cm
  • Fluid Water in a beaker, Air in a smoke box
  • Heat source position Left, Centre, Right
  • Heat source power 20–200 W

Questions to explore

  1. Why does the 5 cm pin drop first, and why do some pins never drop?
  2. When the rod reaches a steady state, why is the heat flowing out equal to the heat flowing in?
  3. Why does the purple dye rise above the flame and sink on the far side of the beaker?

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