Things you always wanted to know the answer to [Vol. 4]
Discussion
Flibble said:
Derek Smith said:
I was asked by a grandchild, "Why does tea get colder when you blow on it?"
It's not conduction, convection or radiation, the three methods of losing heat I was taught at school. Is there a fourth method that was deliberately hidden from me?
I can see that you blow the heated air away and get it replaced by, well, heated air from your breath. It will interrupt the convection, as described at school, so slowing that process, albeit by a few seconds.
I tested it, just in case it was one of those accepted norms that don't work, and no such luck. It gets colder.
I was equal to the question. I said, "You blow the heat away." Not a lot gets past me. However, it would be nice to know the physics of the phenomenon.
Any ideas?
It's forced air convection. You're blowing the warmer air just above the tea surface away and replacing it with cooler air (from your breath, unless your breath is circa 90C). Same effect as blowing air through a radiator with a fan.It's not conduction, convection or radiation, the three methods of losing heat I was taught at school. Is there a fourth method that was deliberately hidden from me?
I can see that you blow the heated air away and get it replaced by, well, heated air from your breath. It will interrupt the convection, as described at school, so slowing that process, albeit by a few seconds.
I tested it, just in case it was one of those accepted norms that don't work, and no such luck. It gets colder.
I was equal to the question. I said, "You blow the heat away." Not a lot gets past me. However, it would be nice to know the physics of the phenomenon.
Any ideas?
glazbagun said:
At a guess you've locally decreased the air pressure over your tea.
Low pressure air is both cooler and more dense than the surrounding air so will remove heat via conduction at a faster rate than the surrounding air would.
The same as blowing soup or fanning yourself on a hot day will cool you.
Guess again, it's not really to do with pressure, you're just blowing the warmer moister air away thus allowing for increased cooling and better evaporation.Low pressure air is both cooler and more dense than the surrounding air so will remove heat via conduction at a faster rate than the surrounding air would.
The same as blowing soup or fanning yourself on a hot day will cool you.
Also low pressure air is less dense than high pressure air (assuming the same temperature).
Edited by Flibble on Tuesday 3rd March 12:24
'Nature' (physics) tries to level the temperature gradient between the air and the tea. If you're rapidly replacing warm air above the mug with cooler air, then the temperature gradient between the two is greater, so the rate of transition of heat between them is greater.
It's just the same as we experience in wind-chill.
FiF said:
Film credits, we're used to seeing long lists of behind the scenes and technical bods.
What's the difference between the jobs.
Director of Photography. J Smith.
Directed by F Bloggs.
Where the second name is the person usually recognised as THE director.
Director of Photography will be in charge of the camera and lighting crew What's the difference between the jobs.
Director of Photography. J Smith.
Directed by F Bloggs.
Where the second name is the person usually recognised as THE director.
https://en.wikipedia.org/wiki/Cinematographer
Director is the person who is at the helm of the whole film
SpeckledJim said:
Flibble said:
Derek Smith said:
I was asked by a grandchild, "Why does tea get colder when you blow on it?"
It's not conduction, convection or radiation, the three methods of losing heat I was taught at school. Is there a fourth method that was deliberately hidden from me?
I can see that you blow the heated air away and get it replaced by, well, heated air from your breath. It will interrupt the convection, as described at school, so slowing that process, albeit by a few seconds.
I tested it, just in case it was one of those accepted norms that don't work, and no such luck. It gets colder.
I was equal to the question. I said, "You blow the heat away." Not a lot gets past me. However, it would be nice to know the physics of the phenomenon.
Any ideas?
It's forced air convection. You're blowing the warmer air just above the tea surface away and replacing it with cooler air (from your breath, unless your breath is circa 90C). Same effect as blowing air through a radiator with a fan.It's not conduction, convection or radiation, the three methods of losing heat I was taught at school. Is there a fourth method that was deliberately hidden from me?
I can see that you blow the heated air away and get it replaced by, well, heated air from your breath. It will interrupt the convection, as described at school, so slowing that process, albeit by a few seconds.
I tested it, just in case it was one of those accepted norms that don't work, and no such luck. It gets colder.
I was equal to the question. I said, "You blow the heat away." Not a lot gets past me. However, it would be nice to know the physics of the phenomenon.
Any ideas?
glazbagun said:
At a guess you've locally decreased the air pressure over your tea.
Low pressure air is both cooler and more dense than the surrounding air so will remove heat via conduction at a faster rate than the surrounding air would.
The same as blowing soup or fanning yourself on a hot day will cool you.
Guess again, it's not really to do with pressure, you're just blowing the warmer moister air away thus allowing for increased cooling and better evaporation.Low pressure air is both cooler and more dense than the surrounding air so will remove heat via conduction at a faster rate than the surrounding air would.
The same as blowing soup or fanning yourself on a hot day will cool you.
Also low pressure air is less dense than high pressure air (assuming the same temperature).
Edited by Flibble on Tuesday 3rd March 12:24
'Nature' (physics) tries to level the temperature gradient between the air and the tea. If you're rapidly replacing warm air above the mug with cooler air, then the temperature gradient between the two is greater, so the rate of transition of heat between them is greater.
It's just the same as we experience in wind-chill.
A gas or vapour is in a higher energy state than a liquid and therefore it takes energy to change from a liquid to a gas.
This energy is taken from the liquid, cooling it down.
You can experinece this by using the alcohol hand gel that people will right now be panic buying.
as it evaproates, you feel your hands go cold.
Blow on them and you will feel it go cold.
The evaproatrion is facilitated by the moving air taking away the moist air.
Strangely if there is higher humidity, evaporation rates increase, for the same temperature and wind speed, as the water molecules like to latch on to other water molecules in the air.
FiF said:
Film credits, we're used to seeing long lists of behind the scenes and technical bods.
What's the difference between the jobs.
Director of Photography. J Smith.
Directed by F Bloggs.
About £3million a film!What's the difference between the jobs.
Director of Photography. J Smith.
Directed by F Bloggs.
The Director is responsible for defining and managing the creative theme of the movie (mood, tempo, lighting) as well as maintaining the flow of the story. The Director of Photography is responsible for articulating that creative theme via the use camera, lens, etc.
glazbagun said:
Are school busses still a thing? Have been reduced to public transport for a week and every bus at peak time seems rammed with squabbling children blocking every aisle.
Definitely still a thing around here as well - but they serve areas which are otherwise not served by regular public transport. Zarco said:
classicaholic said:
What are the green flashing lights on some construction vehicles and diggers? I assume its 1st aiders but thats just a guess.
It indicates that the driver is wearing the seat belt (or he has at least done the buckle and sat on it!) Shakermaker said:
glazbagun said:
Are school busses still a thing? Have been reduced to public transport for a week and every bus at peak time seems rammed with squabbling children blocking every aisle.
Definitely still a thing around here as well - but they serve areas which are otherwise not served by regular public transport. Shakermaker said:
glazbagun said:
Are school busses still a thing? Have been reduced to public transport for a week and every bus at peak time seems rammed with squabbling children blocking every aisle.
Definitely still a thing around here as well - but they serve areas which are otherwise not served by regular public transport. Slightly tangential off topic, one thing that pisses me off greatly, the local train station morning and afternoon has loads of kids travelling to the private grammar school, fees 4.5k per term, plus extras. Number of them who have no tickets is fuzzing ridiculous, it's one service I'd have revenue protection on every day in term time until the entitled w
kers, both parents and kids, got the sodding message. Still no doubt then they'd just drive the brats and make the traffic even worse, any day that school is shut traffic into/out of the city just flows properly, despite the perennially pissing empty bus lanes./minirantGIYess said:
Zarco said:
classicaholic said:
What are the green flashing lights on some construction vehicles and diggers? I assume its 1st aiders but thats just a guess.
It indicates that the driver is wearing the seat belt (or he has at least done the buckle and sat on it!) The green light is just a management tool to check the driver is following the rules.
FiF said:
Shakermaker said:
glazbagun said:
Are school busses still a thing? Have been reduced to public transport for a week and every bus at peak time seems rammed with squabbling children blocking every aisle.
Definitely still a thing around here as well - but they serve areas which are otherwise not served by regular public transport. Slightly tangential off topic, one thing that pisses me off greatly, the local train station morning and afternoon has loads of kids travelling to the private grammar school, fees 4.5k per term, plus extras. Number of them who have no tickets is fuzzing ridiculous, it's one service I'd have revenue protection on every day in term time until the entitled w
kers, both parents and kids, got the sodding message. Still no doubt then they'd just drive the brats and make the traffic even worse, any day that school is shut traffic into/out of the city just flows properly, despite the perennially pissing empty bus lanes./minirantZarco said:
classicaholic said:
What are the green flashing lights on some construction vehicles and diggers? I assume its 1st aiders but thats just a guess.
It indicates that the driver is wearing the seat belt (or he has at least done the buckle and sat on it!) Zarco said:
The amber light signifies the engine is running. That is the warning to others if anything.
The green light is just a management tool to check the driver is following the rules.
Thanks, always wondered why you would need another light for that! Still a bit crazy isn't it!The green light is just a management tool to check the driver is following the rules.
Blib said:
How do musical notes, arranged in a particular sequence, cause such a range of emotional response in humans?
This might go some way to explain the reasonshttps://ledgernote.com/blog/interesting/musical-ke...
GIYess said:
I had heard it was to warn that the driver was in it so to stay clear sort of thing. But nearly the same thing so could see how it could be misunderstood one way or the other.
as i am regularly travelling through some roadworks at the moment, I've had more time than I'd like to sit and look at this stuff in more detail than I should. One of the main contractors doing the work has a symbol and sticker on their mini diggers and dumper trucks with a "thumbs up" icon and it says "Get the "Thumbs Up!" from the operator before approaching this vehicle"Gassing Station | The Lounge | Top of Page | What's New | My Stuff