—
Heat Transfer · interactive
Everything warmer than absolute zero glows. Heat one surface and watch its emission spectrum brighten, sharpen, and slide from infrared toward visible — the same march that takes an iron bar from black to red-hot to white-hot. Then aim two surfaces at each other and see who wins the photon exchange.
Spectral emissive power · Planck's law
—
Surface 1 uses T & ε above. Large parallel plates: F₁₂ → 1.
Conduction and convection scale roughly with ΔT. Radiation scales with T⁴ — so it's negligible at room temperature and utterly dominant in a furnace, a rocket nozzle, or on the Sun. Double the absolute temperature and you radiate 16× as much. Drag T from 300 K to 6000 K and watch the total (the area under the curve, and the σT⁴ readout) explode while everything else feels linear.
The peak of the curve slides left as T rises: λ_max = 2898/T µm·K. At 1000 K the peak is deep in the infrared — you feel the heat but see nothing. By ~1000 K a fraction leaks into the red (a stove element). At 3000 K (an incandescent bulb) it's orange-white; at 5800 K (the Sun) the peak sits in visible green and the light looks white. The "Visible glow" readout tracks how much of the curve has crossed into the 0.4–0.7 µm band.
Real surfaces aren't perfect emitters; ε < 1 scales the whole curve down. When two gray surfaces face each other, the net exchange is q = εσ(T₁⁴ − T₂⁴)·F₁₂ in the simple limit — driven by the difference of fourth powers, so a small temperature gap between two hot surfaces still moves enormous power. Lower the view factor (turn the surfaces away from each other) and the exchange falls off proportionally. Set T₂ above T₁ and the sign flips — surface 1 now gains heat.
EngineeringCandy · Planck, Wien & Stefan–Boltzmann, integrated live · heat it, break it, learn