Leslie cube: emitters and absorbers of infrared sim
Pour hot water into a Leslie cube, read a thermopile 10 cm from each face over three rounds as it cools, then time which waxed pin drops first beside a radiant heater.
Thermal processes: radiation · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Every hot surface gives out infrared radiation, but how much depends on the surface. The net power from each face follows the Stefan–Boltzmann law, P = εσ(T⁴ − T_room⁴), with T in kelvin, σ = 5.67 × 10⁻⁸ W/(m² K⁴), room 20 °C, and emissivity ε = 0.95 (matt black), 0.90 (matt white), 0.30 (dull silver), 0.05 (shiny silver). The thermopile, 10 cm from a face, reads 1 mV for each 100 W/m². At 80 °C the faces read 4.40, 4.17, 1.39 and 0.23 mV: matt surfaces are good emitters and shiny ones are poor, and matt white is nearly as good as black, because colour in visible light does not decide how well a surface emits infrared. The water cools by Newton's law (time constant 25 min), so every reading falls: matt black reads 2.66 mV at 60 °C, a fall of 40 %, much more than the 33 % fall in excess temperature, because P rises with T⁴. Good emitters are also good absorbers: 15 cm from a 2 kW heater, the wax (melts at 55 °C) on the matt black plate lets its pin go after about 28 s, and on the shiny plate only after about 233 s (3 min 53 s). Real results differ: each reading scatters by up to 1 %, the meter shows 0.01 mV, the walls take about 30 s to warm after the pour and the thermopile about 3 s to respond. Model limits: infrared is invisible, so the dashed lines are only a drawing aid; the faces are taken to be at the water temperature once settled; the clock stands still while you read, and moves only when you press Wait 2 minutes; the shiny plate absorbs 12 % of the heater's shorter-wavelength infrared but emits only 5 % at its own temperature; warm air from the heater is ignored; in the absorber test time moves only when you press Wait. By the time the shiny pin drops, the matt black plate is near 165 °C, hot enough to burn: switch the heater off and let the plates cool before touching them.
- Millivoltmeter: thermopile output, 1 mV per 100 W/m²
- Infrared (invisible; lines thicker for more power)
- Wooden stops: sensor exactly 10 cm from each face
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