How far away can you feel something hot? - Page 1
Forum section: electronics / general

author: corrado33
author_rank: Frequent Contributor
timestamp: June 21, 2017, 03:51:47 am
content: Was faced with an interesting question the other day.Say I have a 1000 C object with no insulation in air. How close would my hand have to be to it to feel a temperature change. (Assuming a human hand can distinguish +- 1 C.)What equations would one use to calculate that?

author: helius
author_rank: Super Contributor
timestamp: June 21, 2017, 04:31:00 am
content: TheStefan-Boltzmann equationand the inverse-square law would do it.See alsohttps://en.wikipedia.org/wiki/Radiance

author: Vipitis
author_rank: Frequent Contributor
timestamp: June 23, 2017, 06:22:21 am
content: I think the sun is really far away, and I can definitely feel it's heat

author: WastelandTek
author_rank: Frequent Contributor
timestamp: June 23, 2017, 06:34:56 am
content: Quote from: Vipitis on June 23, 2017, 06:22:21 ambeat me to it

author: hamster_nz
author_rank: Super Contributor
timestamp: June 23, 2017, 07:27:40 am
content: It must be proportional to apparent size (their must be a more correct term for this).If the 1000C object is as big disc, as thin as paper, then side on it will have minimal heating, however front on it will be very hot indeed.Reminds me of a 'proof' the the universe and stars are not unchanging and infinite. It it was, then everywhere you look in the sky would be looking directly at a star, and each would be as hot as the sun. Hence the dome of the sky would be as bright as the sun, so surface of the earth would be this temperature too.

author: bap2703
author_rank: Regular Contributor
timestamp: June 25, 2017, 11:25:17 am
content: Quote from: hamster_nz on June 23, 2017, 07:27:40 amThis apparent size is called the "solid angle" (it's just an angle but in 3D).

author: grumpydoc
author_rank: Super Contributor
timestamp: June 25, 2017, 12:07:53 pm
content: Quote from: WastelandTek on June 23, 2017, 06:34:56 amMe threeI've often thought that it is pretty impressive to be stood 150 million kilometres from something and to still feel warmThe total solar output is slightly mind blowing - the average distance to the sun is 149597870700m, the surface area of a sphere of that radius is 2.8123x1023m2. I think that the figure of average insolation 1362W/m2at the top of the atmosphere given by Wikipedia is at the equator so that is 1361x2.8123x1023m2or 3.83x1026W.To put it another way that is (if I have it right) approx 383 trillion TW over the whole solar surface in the visible and IR spectrum.No wonder it feels a bit warm.

author: jwm_
author_rank: Frequent Contributor
timestamp: June 25, 2017, 02:27:14 pm
content: Its crazy how much energy is all locked up sitting around being matter in stars. My favorite order of magnitude space fact is that a supernova 100 million miles away will be brighter than a nuclear bomb pressed against your eyeball. By 9 orders of magnitude.https://what-if.xkcd.com/73/

author: StillTrying
author_rank: Super Contributor
timestamp: June 25, 2017, 02:36:35 pm
content: Just to be awkward, I've always thought that the detection of heat or cold should be considered a separate sense from touch.

author: komet
author_rank: Unknown
timestamp: June 25, 2017, 04:01:23 pm
content: Quote from: StillTrying on June 25, 2017, 02:36:35 pmIt is. The "sense of touch" is a concept taught in primary school and not used afterwards.