Are heat radiation and absorption symmetric?

AI Thread Summary
The discussion centers on the symmetry of heat radiation and absorption, particularly in the context of clothing color on hot days. While black clothing absorbs and radiates heat more effectively than white, this does not mean the processes are equivalent in practical terms. A black shirt will typically result in a higher temperature due to greater heat absorption, despite the symmetry in emissivity and absorptivity coefficients. Factors such as clothing fit and material can influence heat management, as seen with Bedouin robes, which allow for effective heat dissipation. Overall, wearing black in hot climates is generally disadvantageous.
Pythagorian
Messages
4
Reaction score
0
A friend of mine heard a popular science show on the radio. A caller asked what is better to wear on a hot day, white clothes or black clothes. The answer given was that it did not matter because although black absorbs more readily it also radiates it more readily. My friend said of course that is true, because these things should be symmetric.

My thoughts were that although I do not know about rates of heat radiation, it does not seem to me that processes must be symmetric. For instance, in thermodynamics, entropy always increases, which does not seem symmetric to me.

My question has two parts
  • In this specific case, are the two processes symmetric?
  • In general, are physical processes symmetric?
 
Science news on Phys.org
You're not going to get any answers by posting this in the New Member Introductions thread.
 
Borg said:
You're not going to get any answers by posting this in the New Member Introductions thread.

Thanks
 
They are symmetric in a certain sense; however, not in the way that the radio program suggests. They are just wrong. A black shirt will generally make you hotter, if you are out in the sunlight.

So, in what sense are they symmetric? The emissivity and absorptivity coefficients are the same. The coefficients are functions of wavelength (or frequency) with value between 0 and 1. The thermal emission and absorption are both reduced by this same factor. So, a very shiny material will have a coefficient of close to 1 for visible wavelengths, so it will absorb only a small fraction of the incoming light (and reflect the rest). It will also emit a small fraction of what a black shirt does. A black shirt will absorb most of the incoming light, but will also emit close to a black body spectrum. (Look that up). But the Sun puts out way more thermal radiation than your shirt, even when far away, so decreasing the radiation transfer of the Sun and your shirt by the same fraction will decrease the overall temperature of the shirt.
 
In this sense of symmety - i.e., heat absorbed = heat radiated, the situations are indeed symmetric, under certain assumptions (thermodynamic equilibrium).
Not all processes need to be symmetric, though. For example, before reaching equilibrium, and object will emit/absorb more radiation than it absorbs/emits.

However, symmetry does not mean the situations in this case are equivalent. While a black stone and a white stone left on the sun will end up emitting the same amount of energy as each of them absorbs, these won't be the same amounts for both stones. The black stone will reach higher equilibrium temperature (the amount of emitted radiation depends on temperature) than the white one.

This would suggest that wearing black is disadvantageous in hot climates. But there are more factors at play. For example, in loose-fitting clothes (think Bedouin robes), where the material does not touch the skin directly, the absorbed heat can be reradiated without much conduction.
Additionally, white material allows deeper penetration of radiation, which offsets its higher reflectivity.

Khashishi said:
A black shirt will generally make you hotter, if you are out in the sunlight.
However, see here:
Why do Bedouins wear black robes in hot deserts?
http://www.nature.com/nature/journal/v283/n5745/pdf/283373a0.pdf
(paywalled)
 
Last edited by a moderator:
Problem: You’re an Uber driver with a Tesla Model 3. Today’s low: 30F, high: 65F. You want to reach a USD$ profit target in the least number of hours, but your choices could have added cost. Do you preheat the battery only when you are headed to the charging station (to increase the charging rate by warming the battery — however the battery might not be “warm enough” when your reach the charger and thus slower charging rates), or do you always “navigate to the charger” the entire day (which...
Thread 'Is Callen right in claiming dQ=TdS for all quasi-static processes?'
Hello! I am currently reading the second edition of Callen's Thermodynamics and an Introduction to Thermostatistics, and I have a question regarding Callen's definition of quasi-static. On page 96, Callen says: Another way of characterizing Callen's definition is that a process is quasi-static if it traces out a continuous curve in the system's configuration space. So far it's all well and good. A little later, Callen claims that the identification of $$TdS$$ as the heat transfer is only...
Been around 40 years since I took basic physics in college and while I remember doing some examples of insulation values / energy conduction, I doubt I could to the math now even if I could find the formulas. I have some some corrugated plastic sheet (think of the plastic signs you see on the side of the road) that is used in bee hives. Also have some used in a green house though a bit different in dimensions than this example but the general approach should still apply. Typically, both...

Similar threads

Back
Top