Plate 31 · Matter
Why Metal Feels Cold
Leave a metal spoon and a wooden spoon in the same room overnight and they end up exactly the same temperature. Touch them, though, and the metal feels cold, the wood warm. Your hand isn’t feeling their temperature. It is feeling how fast heat flows out of it.
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The Same Temperature
Check with a thermometer: the copper, steel, stone, glass, wood and foam on the table are all at room temperature, say 20°C. They have sat together long enough to even out.
Yet put your hand on each one and they all feel different.
Six materials · all at 20°C
Heat Leaves Your Hand
Your hand is about 33°C, warmer than all of them. The moment you touch one, heat flows from your hand into it.
Copper passes heat along very fast, and it takes a lot of heat to warm it, so its surface stays cool and keeps drawing heat from your hand. Wood passes heat slowly, so your hand soon warms its surface. Foam barely needs any heat to warm up, so it feels warmest of all.
Colour shows temperature · red warm, blue cool
The Temperature Your Skin Feels
Where your hand meets the object, the surface instantly settles at a temperature in between. Which one it lands closer to depends on which can soak up or give off heat faster — a property scientists call thermal effusivity.
Worked out that way, a hand on a 20°C block of wood meets a surface of about 30°C; on steel, about 22°C; on copper just 20.4°C, nearly as cool as the copper itself. On foam it is almost 33°C, and feels almost warm.
The Other Way Round
Even if a metal slide and a wooden bench in the sun are equally hot, the slide feels far hotter, for the same reason: metal pours heat into your hand fast. That is why pan handles are often wood or plastic: heat creeps up them slowly, and what does arrive reaches your hand gently.
Your Turn
Pick a material and set its temperature, and see what temperature your skin meets when you touch it — and how deep into the material the change in temperature reaches.
Try thisAt 0°C, what temperature does your skin feel on copper, and on wood?
What the model leaves out
The contact temperature comes from the formula for two large bodies at the instant they touch; over time, both your hand and the object slowly warm or cool. Real contact also depends on how smooth the surface is and whether your hand is damp. The thermal properties are typical round figures; the same material varies. The hand is treated as plain skin, without blood flow.
A common misreading
Metal isn’t “colder” than wood, and it doesn’t “give off cold.” In the same room they are the same temperature; they feel different because metal carries heat away from your hand faster. Cold isn’t something that flows. Only heat flows.
Sources
Follow the question
Plate 32Why Mountain Water Isn’t Hot EnoughAt the top of Everest, water boils at about 70°C — too cool to hard-boil an egg, however long you wait. The temperature water boils at depends on how much air is pressing down on it.
Plate 17Why Ice FloatsFreeze most liquids and the solid sinks in its own liquid. Ice floats. The secret is in the little hexagons water molecules build.