After my 5 minute experiment shows IR heats water from above, I didn’t believe there could be any argument that IR heats from above. But apparently I was wrong. Some IR is sneeky and doesn’t get absorbed by the top layer, so it was argued that it was this sneeky IR that was doing the heating.
So, then I thought – let’s just see what happens if you heat all the water from above with IR without letting any IR into the water at all!
So, I simply revised the experimental set up as as follows:
Oven dish with cold water and covered with silver foil “floating” on top.
Now, because the silver foil is floating on the surface, all the heating is to the very top of the water. Now, if the theory that IR is only absorbed in the top few um of water, then clearly very soon after applying the heat I would see rapid warming of the surface, it would boil and then the silver foil would fly off.
I filled the dish with water from the tap (at 16.4C). The silver foil was pushed down onto the water so that it was in contact. I then left it to stand a while to let all water movement cease. I turned on the heat – and as expected, the silver foil did not rise up with the super heated steam.
30mins later I carefully took the dish out and lifted the lid. The IR thermometer read 39C. I then stirred put my fingers in and there was a distinct temperature gradient. I then used pre-warmed stirrers (my fingers) to quickly mix the water and found that it fell to 35C.
Conclusions
- The top did not fly off as predicted by the “it only heats the top few um” argument.
- Water does heat from above when the heat is only applied at the very surface and it quickly conducts downward.
- Even around 4-5cm of water is enough to create a significant temperature gradient when heated under a grill.
- Even if all the radiation from the previous experiment (~1cm depth over 5min) had been absorbed at the surface (i.e. the part in contact with the silver foil), this experiment proves it would have heated the water.
- And … Silver foil does absorb IR (i.e. whilst it has a lower emissivity, it is clearly not zero). This is why you can cook with it!
Using the squared law of heat penetration, I would get roughly the same effect over twice the distance if the experiment ran for four times the time. So, in an eight hour day if the heat penetrated 4cm in this experiment, less power over a longer period of 8 hours, would penetrate something like 10cm. So over 16 days it would heat the top 40-60cm cm of sea. Over a year around 2-3m of sea. Of course, waves and currents would stir it up long before this, but in a perfectly calm sea, this will be typical of the distance and time it takes conduction to heat from the surface.
I know they people will now argue that the water was not evaporating … but using the squared law of heat penetration how quickly does it take for heat to travel 1um?
And the answer is …. 30 x 60 x (0.000001/0.04)2 = 1us
So, basically, whilst a massive and instantaneous IR source (nuclear bomb?) could create a significant temperature gradient over the top 1um of water for around 1us, the heat flow through the water stops a significant temperature gradient for this ~1kw grill heater element, and with solar being an order of magnitude lower, the heat gradient would be around 10cm per day.
For info, with absolutely no mixing in the top 10cm, and assuming all the heat from the sun (1000W/m2 tangent to sun, sunny day) is absorbed in only the top 10cm, that means the heat will rise by:
Temperature rise = 1000 (w/m2) x 8 (hr) x 3600 (s) /(100 (kg/m2) x 4200)
= 69C
Interestingly that suggests that if there is no evaporation and no waves or wind to mix the water, that the top of the sea could reach boiling point in a day (from 30C). However, because of the squared law of heat penetration, over 1/4 of the time, the heat penetrates 1/2 the distance so the average temperature rise in 2hours is 34.5C. And so the shorter the time, the lower the heat rise. Therefore the heat rise when only the top 1um has been heated is ~4 x 10-4C. In other words there is no possibility of the top “boiling away” because IR only penetrates 1um.
And I think that is by far the best position to leave this subject but …
If you want to go a bit further …
The ultimate experiment would be to find an IR absorbing material that wicks water and to float it on the surface. I was going to use a sheet of office paper – but I knew people would argue it wasn’t opaque (hence the 100% opaque aluminium). However, I’ve done enough to convince myself that ordinary physics works as expected.
Can you please express your intent for an experiment? Are you are trying to demonstrate (something) or you are trying to refute (something else)! Your description includes every form of energy transfer known to earthlings. How do you distinguish between the various forms? All you have shown is that adding electrical energy, adds energy!
Will, the experiment was set up to determine if water can be heated by raising the temperature of the surface layer.
In other words, even if IR could not penetrate beyond one atomic thickness (as per conduction) can it heat the bulk of the water.
The answer, is that water is heated from the surface and so IR will heat water irrespective of the penetration.
(The other thing was I wondered is whether I would get a significant thermal gradient in such a shallow container – if so, it might be a nice demonstration for kids.)
Mike,
Your answer is correct It makes no difference the path length for absorption of EMR at any frequency, If the energy is converted to sensible heat the temperature must increase if that energy is converted latent heat of evaporation no temperature change is required. But it is still EMR heating at any frequency. EMR energy conversion to carbohydrate via photosynthesis, and the electrical current generated by PV cells, is not heating.
The big thing is the missing definition of “to warm”. Dr. Roy insists that “to warm” means to increase temperature! This is a major part of the lukewarmer scam. Boiling water depends on a continuous application of energy (to warm) with no change in temperature. Of course the sheeple don’t know that. Yet!!
“(The other thing was I wondered is whether I would get a significant thermal gradient in such a shallow container – if so, it might be a nice demonstration for kids.)”
Freeze a layer of ice on the bottom add water, then add surface energy.
any surface temperature above 0o Celsius with ice on the bottom, is your thermal gradient. Beware water has a higher thermal conductivity than most liquids.