Can you feel the cold?

By Marcus Wilson 21/01/2010

Writing the last piece about fridges has reminded me about a comment I heard from a fellow student while I was an undergraduate. I can’t remember the exact circumstances, but it quite possibly had something to do with objects in liquid nitrogen.  Anyway, the comment was something along the lines of ‘The temperature’s so low you can feel the cold radiating from it’.

Hmmm. Yes, we know what you mean, but it’s not quite right, is it?  It is heat that radiates. Hotter things radiate more. Cold is the lack of heat.   If we hold our hand close to something very hot, we can feel the heat radiation. But the hot thing isn’t the only thing that’s radiating, our hand radiates heat as well. What matters is the difference between the heat it receives and the heat it gives off. In my office at the moment, my hands feel to me neither cold nor hot, because what they are pointing at (namely the keyboard) is pretty well the same temperature as my hands themselves.  The amount of radiation arriving on them is roughly balanced by the amount leaving.

So when we ‘feel the cold radiating from something’, what we are feeling is that not enough heat arrives on our hands to balance the heat that is leaving. (Plus probably we are feeling the cold air too, due to convection currents).

But, before I treat my fellow undergraduate too harshly here, I should point out that physicists are quite adept and speaking about the absence of something as being something itself.   When we describe semiconductors (e.g. silicon, as in chips), we talk about n-type and p-type material. ‘n’ stands for negative, and in n-type silicon we consider electrical conduction happening because electrons move. That is a conventional way to think about electrical conductivity.  But in p-type (p is for positive) the mechanism for conduction is slightly different – we talk about the moving of positively charged holes.   A hole is really the absence of an electron, but we can treat a hole as an entity itself – even to the point of assigning it a mass. It’s a bit like those slidy puzzles – slide one tile into the square gap to create a gap where the tile was – the gap (hole) appears to move, though of course it is really the tiles (electrons) that do the moving. When you’ve worked with semiconductors for a while you can forget that a ‘hole’ isn’t a real thing.

So is it then really wrong to say you can feel the cold radiating off something?


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