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How does your question relate to life?
Animals cool off from: sweat (water has high heat capacity -> takes alot of energy to evaporate -> is a good cooler); increasing the blood being exposed at the surface of the body (increase in thermal radiation; dogs do this by panting; chickens have their comb; we have specialized structures between veins and arteries of the skin called anastomoses which activate when it gets too hot)
All these examples stem from the fact that if your internal heat is too high, you try to dump it in the environment (through interaction with the surrounding air, like Jockmcplop said)
I think cases where it's ultra wam and either ultra humid or dry are very dangerous though, because you can't cool down effectively any more (can someone correct me on this if wrong?), both causing profuse but ineffective sweating, making dehydration a real threat.
Also, humans have the most elaborate cooling system of all (sweat), making our endurance in extreme conditions far above any other fairly large animal (I can't give an underlimit here)
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Its possible to experiment on this type of thing yourself.
Sweating is an evolutionary trait, meaning humans have developed it as a more efficient way of losing heat.
Try putting two identical trays in a heated place for a while, then remove them. Cover the surface of one of them with water, and then point identical fans at them, one fan for each. Measure the rate of heat loss, and you might notice that liquid is more efficient at transferring energy from a solid body into the surrounding atmosphere.
Maybe.
Again I'm just guessing.
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On November 23 2018 08:12 Ayaz2810 wrote:
Stupidest question of the day:
Do inanimate objects cool the same way living things do? What I mean is, I have always found it odd that if you point a fan at a surface in your house, that surface cools off. How does this happen? Does the fan evaporate whatever small amount of moisture is on the surface, reducing the temperature? If you were to take two of the same vehicles and have one sit in the sun idling and drive the other one (assuming both cars produced the same amount of heat whether at rest or moving), will the driven car be cooled by the wind passing over it? If so, how? I know this is asinine, but it's been on my mind more than once lol.
As a mechanical engineer, I'd explain it like so:
The rate of temperature change is related to the difference in temperature between the two bodies, the surface area of the two bodies, and the heat transfer coefficient. The surface area and temperature difference are the same in both scenarios, so the only thing that differs is the heat transfer coefficient.
In a stationary object, you have a warm surface against cold air, this cold air heats up as it interacts with the higher kinetic energy (higher temperature) atoms of the solid surface. Now these warmer air atoms nearby interact with nearby air atoms, and what you get is a warm body with a shell of warm air that gets cooler the further you go out. There's little to displace these warm air atoms, and soon you get interaction with pretty warm air atoms and a warm surface, so very little heat transfer takes place... Thus the surface doesn't lose heat quickly. Through natural mechanisms like diffusion and the fact that hot air rises you'll eventually have warm atoms leave and be replaced with cold atoms, which will allow more heat to be transferred and the body to be cooled.
In the stationary case however, these warm atoms essentially create a heat barrier with these warm air atoms that have a similar temperature to the solid... If you blow the air over the surface however, these atoms will interact with the solid, get warm, get blown away, and in a much faster way will new cold atoms arrive that can take the heat through the interaction with the solid. The more cold air atoms near the surface of the object, the faster that transfer of heat will take place, which means the faster the air blows the faster heat will be lost.
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On November 23 2018 08:12 Ayaz2810 wrote:
What I mean is, I have always found it odd that if you point a fan at a surface in your house, that surface cools off. How does this happen?
Air might be a poor conductor, but it still conducts and air always contain moisture. If you blow hot air into a cooler surface in a house like during a cold winter or into a cellar or using a hair dryer, that surface will warm up will it not? So yes a hot moving car will be cooled by the moving air around it, until it is the same temperature. It also works the other way round, that a cooler moving car will heat up faster if it is moving into hotter air.
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At some point these simplified versions don't work anymore cause there will be friction which will heat up the moving object but not the standing object.
Otherwise your explanations are on point 
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Sure, if your car is experiencing non-negligible aerodynamic heating from supersonic speeds. Or your fan is blowing air at supersonic speeds. Adds new meaning to fan death.
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On November 23 2018 15:40 FiWiFaKi wrote:Show nested quote +On November 23 2018 08:12 Ayaz2810 wrote:
Stupidest question of the day:
Do inanimate objects cool the same way living things do? What I mean is, I have always found it odd that if you point a fan at a surface in your house, that surface cools off. How does this happen? Does the fan evaporate whatever small amount of moisture is on the surface, reducing the temperature? If you were to take two of the same vehicles and have one sit in the sun idling and drive the other one (assuming both cars produced the same amount of heat whether at rest or moving), will the driven car be cooled by the wind passing over it? If so, how? I know this is asinine, but it's been on my mind more than once lol. As a mechanical engineer, I'd explain it like so: The rate of temperature change is related to the difference in temperature between the two bodies, the surface area of the two bodies, and the heat transfer coefficient. The surface area and temperature difference are the same in both scenarios, so the only thing that differs is the heat transfer coefficient. In a stationary object, you have a warm surface against cold air, this cold air heats up as it interacts with the higher kinetic energy (higher temperature) atoms of the solid surface. Now these warmer air atoms nearby interact with nearby air atoms, and what you get is a warm body with a shell of warm air that gets cooler the further you go out. There's little to displace these warm air atoms, and soon you get interaction with pretty warm air atoms and a warm surface, so very little heat transfer takes place... Thus the surface doesn't lose heat quickly. Through natural mechanisms like diffusion and the fact that hot air rises you'll eventually have warm atoms leave and be replaced with cold atoms, which will allow more heat to be transferred and the body to be cooled. In the stationary case however, these warm atoms essentially create a heat barrier with these warm air atoms that have a similar temperature to the solid... If you blow the air over the surface however, these atoms will interact with the solid, get warm, get blown away, and in a much faster way will new cold atoms arrive that can take the heat through the interaction with the solid. The more cold air atoms near the surface of the object, the faster that transfer of heat will take place, which means the faster the air blows the faster heat will be lost.
I meant to reply last night but got distracted. Just wanted to say thank you very much for the effort you put forth here. It was an excellent explanation, and more in depth than I had expected.
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On November 23 2018 06:00 Fecalfeast wrote:
I'm pretty concerned about internet privacy and take steps to ensure my online habits are hard to track. This has worked so far for basic stuff like googling my real name. Still, I have the suspicion that if a powerful entity (google/government agencies etc) or 1337 h4x0r truly wanted to, they could find out basically everything about me.
I use a (paid) VPN 80% of the time, I use ghostery and ublock extensions in my browser, I have a fake name facebook that I hardly use except for event planning, and I have no other social media profiles.
However, I do have a google account tied to my real name and I don't use a VPN on my smart phone.
Are all of these efforts for naught, or would it be sufficiently difficult for someone to bring up everything I do on the internet with minimal skilled effort?
If someone/some entity wants to know past any trick you apply, they just will. You can only make it harder, but it's anything but impossible in any case.
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How much is cannabis (weed) going for around the world nowadays?
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Canada11355 Posts
On December 15 2018 16:57 GreenHorizons wrote:
How much is cannabis (weed) going for around the world nowadays?
Black market around here (BC, canada) ranges from 3.50 to 9 per gram. Not sure what the new legal dispensaries are selling at but the ones that were around prior to legalization were 9-12 per gram with various specials as low as 5 per gram.
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Does it take as much energy to warm up 20 m³ of air from 10°C to 15°C than it is from 15°C to 20°C?
Does maintaining the same temperature of 20°C of those 20 m³ cost more, equal or less energy than cooling it to 15°C, maintaining that and then warming it to 20°C.
I guess insulation is an important facet here, but my knowledge on thermodynamics seem quite lacking here to answer this question for myself in any decent way. Also, question is based on heating of spaces in a home (so making it more complicated would be to take into account the heating of the pipes etc., but let's just skip that for now)
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If perfectly isolated:
The energy cost of heating 1kg of air by 1°C is very,very similar, not matter what temperature you are currently at. Thinks change a lot once you get to temperatures that allow for a change of states, or some chemical interactions. (Example: It takes a lot of energy to take water from being ic at -1°C to being water at 1°C, if you heat wood to a certain temperature it actually starts heating itself for a while (because it is burning)
But as a general simplified idea, the amount of energy used is proportional to both the change in temperature and the mass you are heating, and independent of the temperature you are currently at.
Maintaining a temperature does not use any energy, if you are completely isolated. If you are not completely isolated, the energy it takes is the amount of energy that is lost to the surroundings. That amount is dependent on how well it is isolated, and the amount of temperature difference to the surroundings. This is very hard to generalise, because isolation can be very different depending on the situation, and might also depend on the stuff that surrounds you (Humidity)
Generally speaking, it will use up less energy if you let the space cool down, and then heat it up when you use it instead of keeping it at the same temperature at all times. The reason for that is if your space cools down, the difference in temperature to the outside is reduced, and you lose less energy to the surroundings.
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Alright. That's what I thought. Seems like my intuition isn't that bad after all.
I guess that's why passive houses and EPC values are such a thing. Get your isolation up so high that the heat that's in stays in and you need very little energy heating.
I'll be happy once I move in with my girl, her apartment is at least isolated twice as well (I don't have double glazing where I currently live, which will required by law in 2019 lol)
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For the last point (20 -> 15 -> 20 or keeping 20) also keep in mind the system you are using to do so. From a thermodynamic point of view the 20 -> 15 -> 20 is always better indeed, but the system you are using (here to warm up, in my case it was to cool down), could be more efficient if running at low power (keeping a temperature), than powering up (making a larger change). So in my case it was more interesting to keep the A/C at home at 27 during the day then 25 at night, rather than going up to 30+ and all the way back down daily.
Same goes with heaters, if you have an inertia electric radiator, leaving it running (maybe lowering it a bit) during the day is better than doing on/off all the time (which is also why many of them can be programmed now). However, if you are using a global heating system that is always on and you just chose to let it in or not, or a chimney, or something like this, then let it cool down.
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How do underwater brine lakes exist if salts are supposed to dissolve into an area of water of lesser concentration.
Edit: I’m watching blue planet ii. Tried googling it but still no simple e enough answer for me to understand.
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On January 02 2019 08:55 Emnjay808 wrote:
How do underwater brine lakes exist if salts are supposed to dissolve into an area of water of lesser concentration.
Edit: I’m watching blue planet ii. Tried googling it but still no simple e enough answer for me to understand.
In a closed system, long term, you would end up with a slightly higher salinity in a uniform ocean.
For the moment, there is diffusion from the brine lake to the water above, but there is also salt coming in the lake from the salt layer under it.
(no idea how long it is expected to last, but the salt layer under the lake in the Gulf of Mexico is 8km deep)
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There are two things which will mix the salt into the surroundings: Random molecule movement due to the temperature, and currents moving large amounts of water.
The salty water is heavier than the less salty water, which actually counteracts the mixing if the salt layer is at the bottom (which it usually is)
So if the temperature is low, and there are not a lot of outside currents, there is little exchange with the surrounding, less salty water. So while the salt will eventually move into the surroundings, this can be a very, very slow process.
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On January 02 2019 21:12 Simberto wrote:
There are two things which will mix the salt into the surroundings: Random molecule movement due to the temperature, and currents moving large amounts of water.
The salty water is heavier than the less salty water, which actually counteracts the mixing if the salt layer is at the bottom (which it usually is)
So if the temperature is low, and there are not a lot of outside currents, there is little exchange with the surrounding, less salty water. So while the salt will eventually move into the surroundings, this can be a very, very slow process.
I think this skips over some essential variables.
For starters, the difference in chemical potential between the brine and fresher water should cause currents to form, pushing salt water into the fresher water. This is what Oshuy pointed out. It is balanced out by salt dissolving into the water from the ground.
The question then is why you have a sharp boundary, rather than a gradual decline.
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EPT
