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HEAT · B1 · 011 · Heat flow

Metal feels colder than wood. They're the same temperature.

Heat flow: your skin doesn't sense temperature, it senses how fast heat leaves it. Where your finger meets a block, the surface settles at T = (e_skin × T_skin + e_block × T_block) ÷ (e_skin + e_block), where e is how hard each material grabs heat (its effusivity).

Try itTwo fingers, two blocks, one room

Both blocks have sat in the same room all night, so they are at room temperature. Press a finger on each. The number that matters is the temperature right under your fingertip, and that depends on the material.

Room temperature 20 °C

Your skin 33 °C

Test block (effusivity) 4.38

Under finger, test block–

Under finger, wood–

Heat out of 1 cm² of finger–

Test block feelsFREE

NumbersHow big is it?

The sim treats both blocks as thick and uses effusivity (e = √(k × density × specific heat)): skin about 1360, wood about 400, aluminium about 24,000. Real fingers also have blood flow, so treat the readouts as the first second of touch.

≈ 33 °C

Normal temperature of your skin. Your hand is warmer than the room, so heat always leaves it when you touch something at 20 °C.

> 1000×

Aluminium carries heat about 200 W/m·K, hardwood about 0.16 W/m·K. Over a thousand times better.

21 vs 30 °C

Surface under your 33 °C finger on aluminium vs wood in a 20 °C room. Our own maths with skin effusivity 1360 (from the source).

HowWhat happens, step by step

Everything in one room ends up at the same temperature.
Leave a metal and a wooden spoon out overnight and both settle at about 20 °C.
Your finger is warmer, so heat flows out of it.
At about 33 °C, your skin is always losing heat to a 20 °C spoon. Your nerves feel how fast.
Wood holds the heat right under your finger.
It barely carries heat away, so the spot you touch warms up to near skin temperature and feels warm.
Metal pulls the heat away and spreads it out.
The surface stays close to room temperature, so your finger keeps cooling. Same temperature, different heat flow.

Same trick in winter: a metal railing and a wooden bench outside are the same temperature. Only the railing makes your hand ache, because it drains heat so fast.

Check3 quick questions

One more thing

That's why sauna benches are wooden. At 80 °C the wood is fine to sit on, but a metal nail head at the same temperature would burn you.