That shouldn’t be related. AFAIK the infrared sensors detect movement, not heat.
I hate those company infrared group photos - I usually try to make sure I have the day off.
Hmm, let’s think about what this means. Presumably if they don’t ‘detect heat’, it means that they’re not passive infrared detectors, but shine out a faint IR light and sense of something solid is bouncing it back to them, like a small-scale radar system. (Moving to a smaller wavelength makes sense to scale down a radar set, because radar waves would be too large for human hands.)
So, does the reflection of infrared light have anything to do with the warmth or coolness of the object it falls upon? Could KneadToKnow have such cold hands that they suck up the IR light and don’t reflect any back? Or is it more like the absorbtion/reflection of ordinary light, so that you’d have to have hands that are very ‘black’ to not trigger it, not cold hands?
>That shouldn’t be related. AFAIK the infrared sensors detect movement, not heat.
It is more complicated than that. Most of these are pyroelectric sensors for the thermal infrared, with polyethylene Fresnel lenses. Pyroelectric sensors give a signal that is proportional to the rate of change of the radiation falling on them (the radiation changes the temperature of a piezoelectric material, which changes their stress and generates voltage). They have a sort of a sawtooth sensitivity pattern by the design of the sensor and lens. When something moves across this pattern, and radiates in the thermal infrared, the signal out of the sensor fluctuates, and that’s what is exploited downstream. So, movement and heat are both part of the process.
Italics added for emphasis. Your words.
It is one thing to use an alternative projection with inversed signs in order to make a useful application. Think of computing satellite paths from geocentric coordinates*, or analyzing circuit paths using the positive flow convention.** Same thing applies to your example of light ray tracing. Why waste time tracing light rays from the source that will go somewhere else that aren’t relevant? Instead, trace from end to beginning. Think of it like solving a maze. Most people start at the beginning, but one could just as well start at the end and work backwards. The final path is still the final path.
But when you are trying to explain how things work, it is important to stress the actual nature of things rather than the useful projection. Heat is energy. Cold is a lack of energy. Heat flows. Cold does not.
This is particularly important in cases like the OP. One needs to know the nature of heat and cold in order to understand the answer to why heat is emitted but cold is not.
- Geocentrists always claim NASA does this, because the math is easier.
** Electrical current in circuits is the flow of electrons, which is a negative current. However, in the early days of studying electricity, the scientists didn’t know that, so they assigned positive and negative charges, and then created the convention that positive charges flow. Circuit analysis still uses this convention, even though we now know it is the negative charges flowing. As long as you are clear on the convention, the actual answer is clear and correct.
Let me just ask a question here, as long as we’re on the subject. Is it possible to imagine a living, biochemical organ akin to one of those first-aid cold paks? Could a creature devise some a means of creating what is essentially a chemical heat sink?
With that kind of “what if” question there is no definite answer, but my WAG would be “Not really”. Producing chemical compounds for such organ would be prohibitive from energy balance point of view. Simply put, to cool yourself you’d need to use energy to produce enough cooling chemicals (which would produce heat), store it for unknown time, use energy to mix’em, and then use energy to expel byproduct… I think you would be producing heat with everything summed up, and without obvious advantage of short time local cooling, there is no way to evolve something like that.
Oh, there probably is. Pumping energy into the production of various things happens all the time - all it would take is for the starter chemicals for the cooling process to be useful in some other way.
The dissolution of glucose in water is endothermic - it’s not too hard to imagine a biological system evolving that used glucose as an energy store, then modified it for some kind of transient heat management.
If I understand you correctly (meaning no one else does), you’re asking if the human body can actively cool things down. I’m pretty sure the answer is no, other than cooling itself down through sweat. When sweat (or any liquid) evaporates, it absorbs a lot of heat energy. But internally, it would require pumping heat, and there’s just no such mechanism in the body. So no ice breath, or even remotely cool breath, unless you just drank something cold.
Or, more accurately, the human body can’t cool something down unless that something is warmer than the human body. Heat moves to places where there’s less heat.
Of course, CC. But suppose some creature had found some use for alternating heat and cooling cycles — natural distillery, perhaps, or condensation. Is it theoretically possible?
Funny that you think that’s ironic, in light of the old adage “cold hands, warm heart,” I’d expect her personality to be the norm.
For normal every day conversational purposes you are probably correct. It’s easier to say “hey it’s cold in here” than “hey, there is a lack of thermal energy in here”.
And you can certainly project “cold”. Really what you are doing is spraying something like liquid nitrogen or helium or freon or some other refridgerant that is much cooler than your target and will suck heat out of it. You can do this at home by turning an ordinary aerosol keyboard cleaner upside down and spraying your friend with the cooled liquified gas. Don’t worry, he won’t shatter into a million pieces (I’ve tried).
>But when you are trying to explain how things work, it is important to stress the actual nature of things rather than the useful projection.
What things are useful to the OP depends on what he includes in “the same or similar manner we give off heat”. Heat and cold are not completely symmetric, in the sense that heat can be defined as a certain stuff while cold is only the lack of it. But you can contribute a cooling effect of so many watts to a room by turning on an air conditioner, in a similar manner as you can contribute a heating effect by turning on a space heater.
A detail: whether heat flows from one object to another doesn’t depend on whether there is more heat in the first or the second. It depends on whether the temperature is higher in the first or the second. Temperature is therefore a “potential function” for heat. The amount of heat in an object is a function of its temperature, how big it is, and what the specific heat capacity of its material is. Roughly, the heat in an object is its absolute temperature times its volume times its volumetric specific heat capacity. Most solid materials have volumetric specific heat capacities of around a couple million Joules per cubic meter Kelvin.
And a much finer detail, more of a technical nitpick at this level: technically, heat is defined as the thermal energy that flows through some surface, and not as any thermal energy that might not be flowing. Why this is a better way to define heat is lost on me. I think for the purposes of this OP it is more useful to use the more common understanding of heat as just plain thermal energy, flowing or not.