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Let me explain.

My shower is inconsistant in how hot it gets. Whenever I turn on the shower, I always put my hand really close to the water, but not touching it, so I can feel the heat radiate from the shower. That way, if I can feel it from a distance, I know it is too hot.

I also notice that if the water is very cold, I can feel it as well. Do I feel the absence of radiating heat? Am I actually touching the water and not noticing it?

Sorry if the questions isn't technical enough; it's more of an everyday physics question.

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2 Answers 2

up vote 7 down vote accepted

I think your question is perfectly fine, I don't think this forum is only for advanced research level questions.

Assuming you are not actually receiving small droplets of water, the air around the droplets is cooled by the water, which will then cool your skin. This is strongly accentuated by the fact the water create air currents.

While the effect you suggest of feeling an absence of radiating heat is physically sound, my intuition would tell me it is negligible in this case. Radiation of objects at room temperature wouldn't be very strong compared to the diffusive and convection effects induced by your cold shower.

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Dear Olli, OK question and OK answer but could you please try to supplement some numbers (and approximate calculations) about the contributions to the heat and/or temperature increase of the skin that is due to 1) radiation and 2) convection, 3) heat diffusion? –  Luboš Motl Jun 6 '12 at 6:08
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I'm afraid I won't be able to do that. I don't think diffusion and convection could be considered independently. Diffusion alone over a few centimetres of air is incredibly slow (that's the whole point of double glazings) but in that case, there is convection in the open air so both effects get coupled. I don't think I could come up easily with a model to quantify the radiation and convection-diffusion transfers of heat. –  Oli Jun 6 '12 at 6:26
    
I'd say Oli is correct though modelling the effect would be hard. A quick Google suggests it might be similar to a "shower cooling tower". see for example ht.energy.lth.se/fileadmin/ht/Kurser/MVK160/Project_08/… –  John Rennie Jun 6 '12 at 7:26
    
@Oli: This isn't a forum. –  NikolajK Jun 6 '12 at 8:24
    
@LubošMotl It is possible to calculate convective, diffusive and radiative components by using Fourier's law, Newton's law of cooling and linearized law of radiation. The problem is to get the right coefficients for the latter two laws. –  Pygmalion Jun 6 '12 at 8:41

There are Three Types of Heat Transfer:

  1. Convective heat transfer is what most of us are familiar with. This is how our forced air heating system or our baseboard system transfers energy (heat) to a space. Air moves over a heating element, becomes warmer and expands into the space. In a forced air environment, most of the hot air is at the ceiling, much the same way the hot air balloon rises, so will the warm air in a room heated with forced air. Convective heat transfer is the least efficient means to transfer energy. In terms of insulation convection happens when thee is a crack in the window or a hole in the insulation. Hot and cold air passed through the space via convection. Stop convection by sealing all holes in the house, aka seal the envelope.

  2. Conductive heat transfer refers to two surfaces touching each other. Imagine a metal pan on the stove. If your hand is positioned an inch above the hot handle, you really won’t feel much from the handle, and you can keep your hand there as long as you wish. But, when the handle is touched, your hand instantly begins to feel hot. This is conductive heat transfer. The pot is giving off the energy (heat) in the handle to your hand in a very fast, efficient manner. Conduction is one of the more efficient modes of heat transfer. In home insulation you reduce it by putting bad conductors between good ones. Wood conducts heat well so you would put a material like foam that conducts poorly over the wood studs to reduce heat loss.

  3. Radiant heat transfer is the best because it isn’t slowed down by air. Radiant energy is only felt when the energy wave strikes another surface. This means the surrounding surfaces all reach set temperature. By enclosing your body by warm surfaces, we can better control how our bodies lose heat. Radiant floor heat means better comfort with higher efficiency. To reduce radiant heat loss the best materials are ones that literally reflect the heat. These are foil covered insulation and types of silica that also reflects heat.

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